IntroductionTop

Forests and other woodlands cover 27.7 million hectares of the Spanish land (MAPAMA, 2011; INE, 2017), and provide important services such as carbon storage, habitat for fauna, wood and non-wood products, as well as societal benefits like education, recreation, and conservation (Montero & Serrada, 2013). Spanish forests are variable in composition, comprising more than 150 tree species and have an overall complex structure (Alberdi et al., 2017). Forests in mountain areas are generally dominated by Pinus, Quercus, Fagus, Abies or Betula species. Many of these forests are structurally complex and considered natural. Natural forests coexist with very homogeneous coniferous reforestations from the middle 20th century in the Mediterranean region and with fast growing plantations of Pinus and Eucalyptus in the Atlantic region. In the plains open woodlands (named dehesas) and dense forests (often as coppices) dominated by Quercus and Fraxinus are spread over the Mediterranean area, along with pinewoods managed for production of timber, fruit and resin, and productive plantations of Populus and Eucalyptus (MAPAMA, 2011). Forests may be difficult to access, especially in the mountains, making field work inconvenient and giving added value to remote sensing (RS) technologies. Under a multi-functional and sustainable forest management paradigm (Cubbage et al., 2007) monitoring forests poses specific reporting requirements. A traditional field-sampling-based long rotation (e.g., 10 years) inventory of wood products followed by statistical generalization does not cover current information needs for multipurpose sustainable management, which requires more frequent data acquisition to fulfil national and international reporting obligations, especially where fast-growing species are planted (Diaz-Balteiro & Romero, 2008). Carbon and biodiversity reports demand frequent, specific, and detailed characterizations based on systematically acquired data that enable comparable and harmonized information as required by global policies. Moreover, understanding forest dynamics and drivers of change at various spatio-temporal scale is essential for preservation and management in a context of rapid change, and requires up to date data to be regularly acquired.

Remote sensing technology provides an exceptional source of data acquired with overview perspective, and powerful tools for monitoring forest dynamics and the drivers of change. RS provides data at a variety of spectral, spatial, and temporal resolutions enabling modelling forest condition and change under different scenarios. Forestry applications have benefited from RS data since Earth observations were available in the early 1970s (Cohen & Goward, 2004). Applications have become more detailed and specific with the improvement of data quality, storage capacity, and analysis techniques, and also as result of the information needs imposed by society, going from simple characterization to complex measure and modelling. As forest management policies intensify preservation, and international agreements on forest monitoring begin to include forest degradation (Kissinger et al., 2012) there are greater demands on RS to provide a range of detection capabilities (Cohen et al., 2018). Applying RS methods in Mediterranean forests may pose a different set of challenges to those found in temperate, boreal, or tropical forests, related to the low canopy density and the presence of shrubs and understory vegetation in some forest types. Likewise, RS application in the Spanish Atlantic region requires attention to the complexity of the landscape, which results from fire regimes and impacts the forest structure.

Current international Earth Observation programmes such as the European Copernicus with the Sentinel satellites, or the USA Landsat and MODIS provide huge amounts of data accessible online (Table 1), including their processing standards to facilitate use. Although data access policies are variable, there is an increasing trend towards data free of economic cost to all users (e.g., Sentinel, Landsat), and some programs facilitate the use for research with reduced costs (e.g., the Advanced Land Observation Satellite, Phased Array type L-band Synthetic Aperture Radar—ALOS PALSAR) (Table 1). The frequency of available and useable observations depends on mission characteristics, cloud regime (for optical data), and sometimes historical management (Wulder et al., 2016). MODIS acquires daily observations with various spatial resolutions (250-1000 m), whereas Landsat OLI/ETM+ and the Sentinel-2A/B MSI observe the entire Earth with 8 and 5-days intervals respectively (Li & Roy, 2017) providing optical data of medium to high spatial resolution (10-60 m). Sentinel-2 and Landsat-OLI optical sensors are highly compatible and constitute a virtual satellite constellation (Wulder et al., 2015; Claverie et al., 2018). In Spain, the National Territory Observation Program (Plan Nacional de Observación del Territorio, PNOT) (Arozarena et al., 2006) which coordinates the acquisition and sharing of national geographic information, encompases SIOSE (Sistema de Información sobre Ocupación del Suelo en España), PNT (Plan Nacional de Teledetección), and PNOA (Plan Nacional de Ortofotografía Aérea). PNOT supplies RS data covering the entire country, including aerial multispectral orthophotography updated every 3 years (http://pnoa.ign.es/) and airborne LiDAR coverage intended to be updated every 6 years. The first LiDAR acquisition (density of 0.5 point × m-2) was acquired between 2009 and 2015. A second LiDAR acquisition with variable pulse density dependent on regional government co-funding (0.5-14 pulse × m-2) is being acquired since 2015 and expected to be completed by 2020, promising important opportunities to assist forest monitoring.

Table 1. Examples of currently operational satellites providing data applicable in forest monitoring.

This communication reviews the RS technologies employed to monitor the Spanish forest ecosystems during the last decades, and identifies opportunities offered by the currently available data and analysis techniques. In the next section an overview of RS technologies is presented, followed by a review section of RS applications in Spanish forests. We then wrap-up with a synthesis of the current needs and opportunities offered by RS to monitor the Spanish forests.

Remote sensing techniquesTop

Remote Sensing involves a range of technologies including acquisition of data from a certain distance and their analysis. The platform type (i.e., satellite, aircraft, unmanned aerial vehicle—UAV) and on-board sensor (i.e., optical, thermal, LiDAR, radar) determines the characteristics of the data acquired, which in turn influences the potential applications. Sensors may be active or passive, according to whether they emit energy toward the target object, or just detect sun radiation reaching the sensor. Passive sensors (e.g., optical, hyperspectral) take advantage of the sun energy, whilst active sensors (e.g., radar, LiDAR) beam their own energy pulses. RS synergically combines with Geographic Information Systems (GIS) and machine learning for spatial data analysis and modelling (Figure 1). Herein we provide an overview of RS techniques commonly used in forest applications in Spain, grouped by the characteristics of the data acquired.

Figure 1. Example of a typical flowchart for application of RS technology in forest monitoring.

Aerial photogrammetry

Aerial photographs have been used as base for developing forest maps and resource inventories since the 1930s (Moessner, 1953) and these images are frequently used as reference or validation data. Photogrammetric techniques for analysis are well established, and interpretation is also intuitive. A very high spatial resolution (0.1-0.25 m) is the strongest trait of aerial photography, and when accurately geo-referenced it facilitates precise identification of objects on the ground. Its low temporal repetition and limited spatial coverage—both related to high cost of acquisition—limit a more generalized use of aerial photography. Tree top displacement in overlapping photo acquisitions and tree shadows have traditionally been used for estimation of tree heights using photogrammetry. Nowadays, digital aerial photography (DAP) provides a source of 3D information enabled by recent improvements in sensor technology and image matching algorithms (Leberl et al., 2010) like Structure from Motion (SfM) and Multiview-Stereo (MVS). These image matching algorithms facilitate producing point clouds to reconstruct forest three dimensional structure in near real time (Smith et al., 2016). Radiometric data captured by DAP can be employed for stand delineation and characterization, as well as for identification of forest species (Packalén & Maltamo, 2006; Packalén et al., 2009). Point clouds derived from DAP provide limited information about vertical distribution of vegetation within the canopy, and they lack capacity to provide information about the position of the ground (Lisein et al., 2013). Alternatively, DAP can be efficiently combined with LiDAR (see below) (Packalén et al., 2009; Valbuena et al., 2011) for measurement of vegetation heights whilst informing spectral features (Manzanera et al., 2016), posing new opportunities for improving the certainty of forest estimations (Valbuena et al., 2013a; 2017a).

Satellite optical remote sensing

Optical remote sensing is the most commonly used RS technique for monitoring forests (Wulder, 1998), due to an intuitive interpretation of the visual spectrum and to the wide range of spatial (i.e., from a few cm to some km) and temporal resolutions offered. Optical sensors are passive sensors that record values of the returned sun radiation from targets on the Earth, enabling relative comparison of spectral response in space (single date observations) and time (multi-temporal observations). Strong relationships are found between forest reflectance at different wavelengths—visible (0.4-0.7 µm), near-infrared (0.7-1.5 µm), and shortwave infrared (1.5-3.0 µm)—and forest parameters, thus enabling the construction of direct models (e.g., biomass and species diversity), as well as identification of landscape disturbances and recovery (White et al., 2017) and drivers of change over time (Kennedy et al., 2015; Oeser et al., 2017). When related to biomass and other forest parameters, optical sensors are limited by the saturation of spectral values (Turner et al., 1999; Duncanson et al., 2010), that is, given a threshold value of e.g., biomass, reflectance response does not change.

Numerous satellite missions are equipped with optical sensors to monitor the environment (Belward & Skøien, 2015) providing data with diverse characteristics to meet a range of information needs. However optical data is frequently hindered by the presence of clouds and clouds´ shadows, reducing the amount of usable observations. Pixel-based image compositing has become a common practice to produce complete representations of a territory using clear observations from various dates (White et al., 2014). Pixel-based compositing technique combines data from a user-restricted period (e.g., a month, year, various years) and quality level into an image composite representing a specific time. Open data policies have facilitated the development of optical data analysis techniques (Wulder et al., 2012) incorporating the temporal dimension into long (e.g., Landsat archive with 45 years of data) or dense (e.g., MODIS daily data) records. Since forests are highly dynamic systems, our understanding and assessment of resources benefit from analysis and interpretation of time series of data (Banskota et al., 2014). Optical data are used by themselves or enhanced in combination with other data sources for estimation of land cover attributes, including forest distribution, condition, structure, and composition.

Hyperspectral remote sensing

Hyperspectral sensors acquire data in many—typically hundreds—very narrow bands along the electromagnetic spectrum (from the visible, near- and mid-infrared, to thermal infrared) facilitating identification of Earth surface features. Hyperspectral imagery is unique for identification of vegetation species (e.g., Clark et al., 2005) through spectral libraries or field references (Xie et al., 2008), for monitoring forest health (Fauzi et al., 2013) and environmental stressors (Schlerf et al., 2010). Processing hundreds of bands and identifying the most informative ones is not straightforward (Axelsson et al., 2012). To date hyperspectral data has been mainly collected from airborne platforms (e.g., AVIRIS, with 224 bands) with just a few satellite missions carrying hyperspectral sensors (Transon et al., 2018), among which the Earth Observing-1 (EO-1) satellite launched by NASA in 2000 carried the Hyperion sensor. Hyperion was an instrument equipped with two spectro-radiometers acquiring VNIR to SWIR (i.e., from 0.43 to 2.40 µm) data with 30 m spatial resolution and an average spectral resolution of 0.010 µm for each of its 220 functional channels (Datt et al., 2003; Ungar et al., 2003). EO-1 acquired data by request and was decommissioned on February 2017, with its 16 years of archived imagery remaining accessible online (https://earthexplorer.usgs.gov/). Other satellite hyperspectral missions are in study stage (e.g., HyspIRI from NASA) or about to be launched (e.g., EnMap from Germany).

Synthetic aperture radar (SAR)

Radar (radio detection and ranging) is an active RS technology emitting microwave pulses (1 mm-1 m) and recording the radiation backscattered from the surface. Radar has the capacity to provide data in nearly all-weather conditions, day and night (Henderson & Lewis, 1998). The radar instrument configuration—wavelength and polarizations—determines its capacity to acquire information from the ground. Different wavelengths (X-, C-, L-, and P-bands) have been used in forestry applications with success depending on objectives and analysis methods. Most synthetic aperture radar (SAR) systems have the capacity to measure the phase—related to the distance between the sensor and the target—and the backscatter coefficient—related to the target scattering properties. In forestry applications the phase information is often used to derive forest height, through interferometric (InSAR) or polarimetric-interferometric (PolInSAR) processing (Askne et al., 2003; Garestier et al., 2008) or to provide information on the dominant scattering mechanism using polarimetric decomposition techniques (Cloude & Papathanassiou 1998; Hajnsek et al., 2003). Recently, multiple SAR observations acquired with a certain platform separation (baseline) are used to resolve the vertical structure of the forest using SAR tomographic processing (Tebaldini & Rocca, 2012).

The relationships between radar backscatter coefficient and forest structure were demonstrated almost three decades ago (Le Toan et al., 1992). The sensitivity of radar backscatter to forest parameters increases with increasing wavelength, with P-band recognized as the most sensitive due to its greater penetration through vegetation (Dobsonet al., 1992; Le Toan et al., 1992; Rignot et al., 1994). Also, stronger relationships between the radar backscatter and forest structural properties are generally found for the cross-polarized (HV and VH) channels when compared to the co-polarized (HH and VV) channels (Le Toan et al., 1992; Pulliainen et al., 1994; Sandberg et al., 2011; Cartus et al., 2012; Shimada et al., 2014). The scarcity of historical and consistently acquired radar data, especially when compared with the optical archives, precludes long retrospective analysis. However, operational satellite programmes carrying SAR instruments ensure data continuity over the next decades at least for some wavelengths. For example, the Sentinel-1 C-band mission is guaranteed until 2030 through the European Space Agency (ESA) agreements for the procurement of replacement satellites. Of particular interest for forestry, the European BIOMASS satellite mission is due for launch 2021, promising unprecedented capabilities for global assessment of forest biomass and carbon accounting from P-band data (Le Toan et al., 2011), while the Japan Space Exploration Agency (JAXA) L-band PALSAR programme would continue past the current mission (i.e., PALSAR-2). Among SAR missions currently in feasibility phase, the Tandem-L would provide global data for implementation of space-borne L-band PolInSAR with single-pass acquisitions, enabling forest height and height change assessment. Tandem-L could be launched in 2022 with a 10-year operational life span (Moreira et al., 2015).

Light Detection and Ranging (LiDAR)

LiDAR (light detection and ranging) is an active remote sensing technology with high capacity to assist in mapping, monitoring, and assessment of forest resources (White et al., 2016). RS LiDAR instruments measure the time a laser emitted beam, usually near infrared (NIR) takes to travel forth and back from the target, as one or multiple returns in the case of a discrete return system, or as a continuous return waveform in the case of a full-waveform system. The high positional accuracy granted by the Global Navigation Satellite Systems (GNSS) combined with an Inertial Measurement Unit (IMU) on LiDAR aircrafts allows the generation of three dimensional point clouds representing the spatial distribution of canopy elements, thus providing accurate measures of the vegetation’s structure (Lefsky et al., 1999).As any other sensor, LiDAR can be mounted on a variety of platforms: ground-based, UAV, airborne or satellite. Discrete return LiDAR systems on-board airplanes are commonly known as Airborne Laser Scanning (ALS). A key property of discrete LiDAR data is the pulse density or number of pulses reaching the surface unit. LiDAR pulse density may differ from number of points returning from the surface unit, as a function of the sensor configuration and the surface complexity. To date LiDAR is the most accurate RS technique to measure forest structure (Valbuena et al., 2013b; Bottalico et al., 2017) and it is typically used for predicting forest inventory attributes (González-Ferreiro et al., 2012; Montealegre et al., 2016; Mauro et al., 2017a; Valbuena et al., 2017b) and probability density functions (e.g., Arias-Rodil et al., 2018). The 3D structural information obtained from LiDAR can also be employed to characterize forest areas (Valbuena et al., 2013c; 2016b) and a combination of LiDAR and optical data can efficiently complement the capabilities of each sensor (Manzanera et al., 2016; Valbuena et al., 2017a). The wealth of information provided by LiDAR enables forest managers to make informed and dynamic decisions at small management units (Pascual et al., 2016). Despite relatively high costs of data acquisition, LiDAR is operationally used in forest inventories in some countries (Tomppo et al., 2008; Hilker et al., 2008).

Satellite LiDAR is expected to provide important opportunities for forest applications in the near future. From 2003 to 2009 the Ice, Cloud, and Land Elevation Satellite (ICESat) carried the Geoscience Laser Altimeter System (GLAS) sensor, providing waveform data from space with a 170 m footprint (Schutz et al., 2005). Although conceived to study the evolution of land and sea glacial masses, GLAS potential and application to analyse large scale forest structure was relevant (Lefsky, 2010; García et al., 2012). ICESat-2 was launched on 15th September 2018 carrying ATLAS, an improved sensor with smaller footprint (70 m). Another LiDAR sensor launched at the end of 2018 (5th December) is the Global Ecosystem Dynamics Investigation (GEDI) which will orbit the Earth on-board the International Space Station (ISS) scanning forests between 52°S and 52°N. GEDI will provide high-resolution full-waveform LiDAR data aimed to measure vegetation height, vertical structure, and bare ground elevation (Qi & Dubayah, 2016). GEDI is the first LiDAR on space mainly created to study the carbon cycle and biodiversity in forest ecosystems.

Unmanned Aerial Vehicles (UAV)

Commonly known as Unmanned Aerial Vehicles (UAV), the small Remotely Piloted Aircraft Systems (RPAS) constitute an innovative means to assist civilian applications including forest monitoring (Pajares, 2015). UAV flying space and civilian use regulations are under development worldwide. In Spain UAVs of less than 25 kg are subject to simple specific regulations by the national Agency of Aerial Safety (Agencia Estatal de Seguridad Aérea, AESA) and the latest regulatory framework was established in 2017 (RD 1036/2017). UAVs can typically fly under 120 m height and no more than 500 m from the remote pilot (Visual Line Of Sight, VLOS flying mode), although this distance can be protracted with observers (Extended Visual Line Of Sight, EVLOS). Remotely piloted vehicles may carry a number of sensors (Gómez & Green, 2017), among which conventional photographic cameras are most popular for easiness in data processing and interpretation, as well as for their low cost. Complex sensors (e.g., LiDAR, hyperspectral) are generally heavier and require more power supply, restricting the number of vehicles that can carry them. Fixed-wing platforms are adequate for monitoring larger areas with a pre-defined flight plan and need space for landing, while multi-rotor platforms are better suited for manoeuvrability, having easier take-off and landing. Both types of platforms are well suited for forestry applications (Torresan et al., 2017). For example, a fixed-wing vehicle equipped with multispectral (MS) sensor can repeatedly fly over the same area providing information of the forest health at different dates. A rotary wing vehicle would be more efficient and better suited to observe plots in difficult areas. Like the piloted counterparts, UAVs require very accurate location information, which is provided by an IMU and GNSS receivers. UAVs flexibility enables optimal time data acquisition, provides very high spatial resolution data, and are relatively low-cost. Photogrammetric matching algorithms mentioned before (e.g., SfM, MVS) have found in UAV-based photogrammetry an extensive field for application. In comparison to ALS, UAV-based digital aerial photography is inexpensive and the point cloud can easily match ALS densities. However, the larger point density does not necessarily yield greater vertical accuracy, since it cannot penetrate vegetation (Guerra-Hernández et al., 2017). Current limitations to the use of UAVs are imposed by battery duration, payload weight and local regulations (Manfreda et al., 2018), as well as massive data processing capability. Although the sensor on-board a UAV defines the RS technology, we have considered UAV separately as the flying conditions impose specific characteristics to the data and processing required.

Remote sensing applications in Spanish forest ecosystemsTop

The range of techniques outlined above, together with an increasing amount of data available and the improved storage and computing capacity offer myriad opportunities for monitoring forest ecosystems. As summarized in this section, many of these techniques have been used to monitor the Spanish forests.

Landscape characterization

Land cover (LC), land use (LU), and their changes over time are fundamental information for many environmental applications, including assessment of carbon budgets and diversity, and characterization of forest structure and dynamics. RS offers spatially explicit and comprehensive data to get valuable insights about the land cover and use at different scales. The overall monitoring of Spanish landscapes is supported by three projects employing some form of RS: the Forest Map of Spain (Mapa Forestal Español, MFE), the Spanish Land Use Information System (Sistema de Información de Ocupación del Suelo en España, SIOSE), and CORINE (Coordination of the Information on the Environment) Land Cover.

The main objective of MFE is to support the national forest inventory. MFE has mapped the national LC three times since 1990, at scales ranging from 1:200000 to 1:25000. The most recent MFE versions are derived by photointerpretation of aerial photography and digitization of polygons with minimum mapping unit (MMU) from 0.5 ha in treed areas to 2 ha in agricultural areas. Polygons are characterized and classified according to the vegetation present in the area. MFE has a temporal frequency enough to support decadal forest inventories but too scarce for assessment of forest dynamics. SIOSE is generated to fulfil national information needs of land cover and use, with photointerpretation of satellite images and orthophotos at 1:25000 scale. The main source of data for the first SIOSE version was SPOT HGR (fusion of MS 10 m and panchromatic (PAN) 2.5 m data) complemented with aerial photography, Landsat images, and other cartographic data sources available. SIOSE is produced by manual digitization of polygons of MMU 0.5-2 ha, labelled according to a descriptive data model: polygons are not given a single label but a set of descriptors, providing flexibility for advanced interpretations. The first SIOSE was carried out in 2005, and has been updated in 2009 and 2011. CORINE LC is a continental project to map Europe from Landsat imagery. Abiding to some general guidelines, each country maps its territory with its own resources. CORINE was first developed in 1990 and has been updated in 2000, 2006, 2012, and 2018. The latest Spanish versions of CORINE are produced by generalization of SIOSE maps (García-Álvarez & Camacho-Olmedo, 2017; Martínez-Fernández et al., 2019) representing a change in methodology and making the comparison with previous versions troublesome. Despite the completeness of the three mapping projects (MFE, SIOSE, CORINE) changes in very dynamic landscapes may remain undetected. However, data acquired by optical Sentinel-2 or radar Sentinel-1 could support national scale LC maps and drastically increase their frequency, enabling detailed monitoring of landscape dynamics. Although just at the scene level, the capacity of Sentinel-2 data to map land use has already been explored in Spain by Borrás et al. (2017) with better results obtained when compared to using SPOT images.

At the landscape level, habitat mapping is required for the European Natura 2000 conservation commitments and assessment of habitat connectivity and fragmentation is a following challenge (Hernando et al., 2017). Regional efforts ongoing in Castilla y León (Bengoa et al., 2017) or Cantabria (Álvarez-Martínez et al., 2017) combine optical, LiDAR, and ancillary data to classify and map vegetation types with machine learning techniques. Gastón et al. (2017) recently compared the performance of PNOA LiDAR, MFE data, and CORINE data to assess forest habitat suitability for brown bears across the Cantabrian Range employing canopy cover variables. Object-based image classification techniques—in which the basic unit is a group of spectrally similar pixels rather than the pixel itself—combining aerial multispectral imagery and LiDAR data from PNOA were used by Hermosilla et al. (2012) to characterize forest abandoned lands. In addition, texture information from spectral bands may improve accuracy in land cover classification (e.g., Ruiz et al., 2005). Data fusion combining SPOT 6, Landsat 8, and Terra MODIS data was also crucial in describing spatial landscape heterogeneity to identify forested and human modified areas by Silveira et al. (2018).

Evidence on species composition is needed to inform silvicultural prescriptions, biodiversity or other management needs. Although traditional RS approaches to characterize tree species dominance have had variable success (Fassnacht et al., 2016; White et al., 2016), improved results were obtained with multi-date or time series analysis. Gómez et al. (2018) have recently mapped the distribution of Fagus sylvatica L. (European beech) in the Central Range, based on a multi-date classification of Landsat OLI data. Beech species, considered relict in the area, is expanding as indicated by the comparison of current and previous cartographic records as well as field verification measurements. To estimate changes in species dominance in Ordesa National Park a 33 year annual series of Landsat data classified with support vector machine was used by Gómez et al. (2016a), corroborating trends in F. sylvatica L., Abies alba Mill., and Pinus sylvestris L. recent dynamics (Camarero et al., 2011; Sangüesa-Barreda et al., 2015). Combining field data and time series of Tasseled Cap Wetness values (a linear combination of spectral bands which is indicative of water content) in a geostatistical model Aulló-Maestro et al. (2017) confirmed a change in species dominance in Pinar de Hoyocasero (Ávila) that will affect local biodiversity (Rubio et al., 2011). Coarser spatial resolution data from MODIS has been employed to discriminate pine species by differences in phenology (Aragonés et al., 2017). The authors modelled 368 16-day composites of data acquired in 2000-2016—spatially stratified by field data from the National Forest Inventory—and characterized curve patterns corresponding to five pine species classified with >70% accuracy.

Quantification of resources

In Spain as in many other countries the National Forest Inventory (NFI) is an effort to keep forest resources (e.g., volume, biomass) assessed periodically, providing base information for decision making, forest management, and research. The Spanish NFI (SNFI) is based on a 1×1 km network of permanent field plots measured every ten years. The high cost of measurements precludes more frequent updates, making the sole use of SNFI data imperfect for current reporting needs. SNFI represents a robust database reliable as reference for calibration and validation of forestry studies and applications based on RS datasets. For instance, González-Alonso et al. (2006) estimated biomass at national level calibrating their models with data from the SNFI 2nd rotation, and Gómez et al. (2014) modelled and assessed biomass and change of biomass in pines of the Central Range with Landsat time series calibrated with data from the 2nd (ca. 1990) and 3rd (ca. 2000) SNFI rotations. Other authors have found useful the integration of SNFI and SAR data for estimation of biomass (Joshi et al., 2017) and SNFI and LiDAR data for estimation of canopy fuel (González-Ferreiro et al., 2017) and structural parameters (Fernández-Landa et al., 2018).

The nationally available LiDAR data from PNOA has been operationally used for forest inventory from management unit to forest scale (100-10000 ha), and some online tools have been developed to facilitate access to volume estimates or fire models. Some examples are GINFOR for Castilla la Mancha (Blanco-Martínez et al., 2017) or Forestmap, which is currently available for 11 provinces (Fernández-Landa et al., 2017; Tomé et al., 2017). This kind of tool requires basic input from the user, like selecting an area of interest, and facilitates rapid estimations for decision making. LiDAR allows extraction of individual tree attributes through individual tree crown (ITC) approaches (Hyyppä & Inkinen, 1999) and estimation of stand-level variables using the area based approach (ABA) (Næss et, 2002; White et al., 2013) or Empirical Best Linear Unbiased Predictors (EBLUPs) (Mauro et al., 2016). The PNOA LiDAR dataset was tailored for topographic applications, and its low point density may limit forestry applications such as structural characterization of dense forests (Adnan et al., 2017). Nonetheless, many important inventory variables (e.g., height, density) can be estimated with sufficient accuracy for certain purposes when there are enough ground returns to retrieve an accurate DTM, by choosing the appropriate relation between LiDAR pulse density and plot size (Ruiz et al., 2014). Although high density (> 3 pulse × m-2) LiDAR is expensive, some regional administrations in cooperation with the National Geographic Institute have acquired this quality of LiDAR data (e.g., Navarra: 14 pulse × m-2; Basque Country and La Rioja: 2 pulse × m-2)—superior to densities typically found in national programmes in countries with highly productive forest resources, such as Finland (Valbuena et al., 2016a)—that may provide more accurate estimates in dense forests.

When forest inventories require estimates of structural attributes at stand or sub-stand level (0.5-50 ha) with relative errors below 5-10% (e.g., for management purposes; Pascual et al., 2018b), ABA LiDAR assisted methods become economically unaffordable due to the need of sufficient field data. To address this problem Mauro et al. (2016) implemented small area estimation approaches to a LiDAR-assisted inventory in a Pinus pinaster Ait. forest in Burgos. Mauro et al. (2016, 2017a, 2017b) based their estimations on EBLUPs using LiDAR data as auxiliary information, and demonstrated this approach is more accurate than traditional inventories over small areas. Additionally, with this approach area level models just require identification of the plot/stand correspondence and an accurate location of plots is not needed. Thus, the SNFI plot positioning difficulty no longer applies (Mauro et al., 2011; Valbuena et al., 2012; Pascual et al., 2018a), enhancing the EBLUP methods the value of SNFI and PNOA LiDAR for operational forest inventories.

Focussing on biomass and carbon budgets—necessary for monitoring management practices and for reporting to international commitments (Montero et al., 2005; Ruiz-Peinado et al., 2011)—a host of RS techniques and data types have been employed in Spanish forests during the recent decades (Table 2). González-Alonso et al. (2006) estimated forest biomass over the entire country at the province level with Normalized Difference Vegetation Index (NDVI) composites from SPOT VEGETATION and NOAA-AVHRR, and SNFI plots. This approach is useful for overall reports but lacks enough detail for management or local assessment. In a more detailed scale, optical images from the Advanced Spaceborne Thermal Emission and Reﬂection Radiometer (ASTER) images were used by Fernández-Manso et al. (2014) to estimate biomass of pines in Segovia. A combination of the red and SWIR bands with the green fraction obtained applying Linear Spectral Mixture Analysis (LSMA) yielded the strongest relationship with biomass (R = 0.63). LSMA was applied to lessen the effect of mixed pixels and showed a positive contribution in the modelling. Gómez et al. (2012a) described changes in carbon content at the landscape level in pines of the Central Range employing a time series of Landsat images (8 images for a 25-year period). Through interpretation of the temporal derivative of the time series—named Process Indicator (PI)—the rates and directionality of change (i.e., increase or decrease) were characterized. The same Landsat series served a 2D wavelet transformation model calibrated with SNFI plots for estimation of biomass dynamics (Gómez et al., 2014), whereby changes in biomass were mapped with 70% accuracy. In general, the biomass of Spanish forests has proven difficult to characterize with spectral traits (Vázquez de la Cueva, 2008) in part due to their heterogeneity and location in rugged areas. Such factors, added to the saturation of optical and radar sensors, preclude accurate estimation of high values of biomass. LiDAR technology has become key for assessment of aboveground forest biomass, enabling estimation of its distribution among crowns, trunks, branches and leaves, and quantification of biomass loss and CO2 emissions (Table 2).

Table 2. Examples of works for estimation of biomass and carbon fluxes with remote sensing in Spanish forest ecosystems.

National Parks Administration (Organismo Autónomo de Parques Nacionales, OAPN) currently monitors the net primary production (NPP) of ecosystems in National Parks with REMOTE, an application for analysis of MODIS NDVI and EVI (Enhanced Vegetation Index) time series (Cabello et al., 2016). Information of the NPP contributes to inform about the National Parks state of conservation. The high frequency of continuous data and accumulated reference data facilitates an alarm system for identification of anomalies as well as characterization of tendencies. Cicuéndez et al. (2015) demonstrated that the NASA derived MODIS Gross Primary Production (GPP) product (MOD17A2, 1 km spatial resolution) underestimates dehesa GPP due to ecological parameters such as soil moisture and precipitation. For such finding the authors compared 5 years (2004-2008) of MOD17A2 with a MODIS-based locally calibrated GPP in a 600 ha holm-oak dehesa in Cáceres.

Shrub ecosystems—18.4 million ha in Spain, MAPAMA 2011—have attracted efforts for estimation of biomass and volume. Estornell et al. (2011a) employed high density LiDAR (average 8 point × m-2) to evaluate biomass of a Q. coccifera dominated area in Chiva (Valencia) and obtained accurate results (R2 = 0.73) in plots of 1.5 m radius when a highly accurate Digital Terrain Model (DTM) (RMSE < 0.2 m) was employed. Biomass estimates over the same area were improved by combining LiDAR with spectral data from an airborne flight and when assessing results in squared plots of 100 m2 (Estornell et al., 2012). Trassierra et al. (2017) estimated Cistus laurifolius L. aerial biomass in experimental plots (11.3 m radius with subplots of 2 m radius) in Soria and compared models based on PNOA LiDAR or Landsat variables. The authors found better results when building parametric models with LiDAR data, but Landsat spectral information was considered as an acceptable alternative.

Although information from SAR images is particularly complex to retrieve in fragmented landscapes with steep topography, as frequently found in Spanish forests, SAR images have demonstrated potential for estimation of aboveground biomass (Tanase et al., 2014a; Joshi et al., 2017). Tanase et al. (2014a) used SNFI plots to evaluate parametric and non-parametric modelling retrieval of biomass as a function of dual-polarized (HH, HV) ALOS PALSAR backscatter of coniferous forests in Aragon. The study concludes that observed errors obtained with non-parametric models are similar and that within the sensitivity interval of the L-band wavelength (10-100 T × ha-1) biomass estimates are relatively accurate (RMSE = 20-35%). Considerably larger errors were observed outside this interval since at low biomass levels (<10 T × ha-1) backscattering largely depends on surface properties while at high biomass levels (>100 T × ha-1) signal saturation sets in. Méndez et al. (2016) used ALOS PALSAR for estimating eucalyptus and pine forest biomass in Huelva by modelling the relationship between the backscatter coefficients and wood volume. Correlations were high (R = 0.7-0.8) but so was the relative error (RMSE = 39.8-51.6%). The signal saturation point was identified at 100 T × ha-1 suggesting that improved modelling approaches are needed to meeting forest management needs, a conclusion also reached in other studies over similar environments (Tanase et al., 2014b). Joshi et al. (2017) found that the inclusion of forest structural information is crucial to establishing suitable relationships between stand volume or biomass and SAR backscatter, and using that approach mapping forests with SAR images may not need to be restricted to areas with low biomass.

Structural characterization

Characterizing structural parameters like dominant height or basal area at different scales—individual tree, plot, stand, landscape—employing diverse techniques and datasets is a typical RS effort (Table 3). Very high spatial resolution (0.7-2.4 m pixel size) data from single date QuickBird-2 images were employed by Gómez et al. (2012b) to estimate quadratic mean diameter, basal area, and number of trees per hectare in pine areas of the Central Range. In the same areas Gómez et al. (2011) modelled the stand structural diversity and found that image texture variables make a valuable contribution in structural modelling. The advent of LiDAR technology since the beginning of the century has reduced estimation error, marking a milestone change in this field (Table 3). At local scale UAVs can be used for estimation of tree heights (Zarco-Tejada et al., 2014), with an on-board LiDAR or from DAP point clouds. However, the DAP technology is not yet operationally used in the Spanish forest sector, although it has shown valuable for estimation of structural parameters with accuracies similar to those from LiDAR when an accurate DTM is available (Navarro et al., 2018).

Table 3. Examples of forest structural characterization in Spain employing remote sensing.

LiDAR data by itself or in combination with other data sources have demonstrated capacity for assessment of the main forest structural variables (i.e., height, basal area, volume) and also a number of derived forest properties (e.g., complexity, diversity, regeneration) (Table 3). For example, Fernández-Landa et al. (2018) estimated basal area, volume, and number of stems per hectare in pine and beech forests of La Rioja with PNOA LiDAR data and SNFI plots, and Gonçalves-Seco et al. (2011) estimated canopy cover, density, and tree height in dense stands of eucalyptus plantations in Galicia. Estornell et al. (2011b) predicted dominant height of Quercus coccifera L. in Chiva (Valencia) from discrete LiDAR metrics with accuracy (R2 = 0.73), while Crespo-Peremarch et al. (2018) characterized understory vegetation attributes (i.e., mean and maximum height, cover, and volume) at the plot level employing full-waveform LiDAR metrics in Sierra de Espadán (Castellón). ITC approaches have sometimes been used for measurement of individual tree height (e.g., González-Ferreiro et al., 2013b in P. radiata plantations in Galicia), but ITC approaches are more sensitive to pulse density than ABA and there fore less employed in forest inventories. Although discrete LiDAR returns below 1.5-2 m are frequently considered signal noise and dismissed—leaving shrub structure below this height unaccounted for—some studies have focused on the structure of forest lower layers, including regeneration stages. Valbuena et al. (2013c) showed the relationship of under-canopy parameters to other forest structural properties and employed these relationships to unravel the success of natural regeneration in P. sylvestris forests of Valsaín (Segovia). Blázquez-Casado et al. (2015) studied forest dynamics and regeneration after storm damage with 2011 acquired LiDAR (≥ 6 pulse × m-2) and historical (1956/1977/1996) aerial photography, showing how natural disturbances influence forest development. Simonson et al. (2018) explored the effects of phenology on LiDAR metrics in mixed stands of Quercus suber L. and Quercus canariensis Willd. in Los Alcornocales Natural Park (Cádiz). Employing two spring datasets acquired in a six week interval, there was consistency in the maximum and mean height estimations but some differences in standard deviations and skewness. Combining data from multiple sensors usually provides important synergies for the characterization of forest structure (Pascual et al., 2010; Manzanera et al., 2016; Ruiz et al., 2018). However, Valbuena et al. (2017a) obtained mixed results when combining LiDAR with MS information from DAP, suggesting that synergies among sensors may be beneficial in some cases but counterproductive for structural variables that just depend on vegetation heights.

Mapping the structural complexity, that is, structural types and development stages of forests helps decision making. Pascual et al. (2008; 2013) developed a two-stage method for depicting forest structural types of P. sylvestris stands in the Central Range. Attending to an increasing participation of forest management expert opinion, the best classification of structural types was obtained from a fully automatic delineation of stands with LiDAR data—by means of an object-oriented segmentation algorithm—with subsequent k-means clustering of stands into five structural types. Automated methods developed from LiDAR data to describe forest structural types in Spain (Valbuena et al., 2013c) have recently been extended for a more generalized use across ecotypes in Europe (Adnan et al., 2018).

There has been an intense research effort for optimization of methods employing LiDAR data in the assessment of structural properties in Spanish forests. The influence of pulse density—a key variable when acquiring LiDAR data—has received particular attention. In 2012 González-Ferreiro et al. evaluated a range of pulse densities (0.5-8 pulse × m-2) for estimation of height, basal area, and volume of Pinus radiata D. Don. plantations in Galicia, and found similar performances. Varo-Martínez et al. (2017) evaluated the capacity of (0.5/4.0/10.5 pulse × m-2) LiDAR data in the delineation of P. sylvestris stands in Sierra de Los Filabres (Almería) and found no significant difference, but for estimation of height the densest dataset performed best. On the contrary, Marino et al. (2017a) found similar performance in the estimation of P. sylvestris height in Valsaín (Segovia) when comparing 0.5 with /1.5-5.0 pulse × m-2 LiDAR data, but the lower strata was better characterized with denser point clouds. Ruiz et al. (2014) analysed the combined effect of plot size and LiDAR pulse density on estimates of volume, biomass, basal area, and canopy cover in pines of the Central Range (Cuenca). The authors found that the rate of improvement in model estimates decreases when using plot areas ≥ 500-600 m2, while densities >1 pulse × m-2 do not significantly improve predictions. The variety of sometimes apparently opposite results suggests there is no general optimal LiDAR data density, but it rather depends on the work objectives and structure of the target forest. A good choice of LiDAR predictive variables is relevant when modelling forest structure (García-Gutiérrez et al., 2014; Valbuena et al., 2017b), and the estimation and classification methods may also play a significant role (Guerra-Hernández et al., 2016; Valbuena et al., 2016b; Domingo et al., 2017, 2018). Regarding the scale of data aggregation, Mauro et al. (2016) showed that it has important consequences and demonstrated the subsequent trade-offs with the desired accuracy in the estimation of forest structural variables.

Fire assessment

Remote sensing technology has extensively been used for fire related applications in Spain, including identification of area burned and fire severity, characterization of fire drivers, and monitoring regeneration (Table 4). A burned forest area can be determined by classification of a single post-fire image (Quintano et al., 2006) since the spectral signature of burned vegetation has higher visible and SWIR values and lower NIR values compared with non-burned areas. However, differential approaches (i.e., temporal comparison) and active fire information based on thermal anomalies are more reliable for large and heterogeneous areas. Merino de Miguel et al. (2010)

Table 4. Examples of forest fire related remote sensing applications in Spain.

successfully applied a scar detection algorithm based on MODIS active fire data and a single MODIS post-fire infrared reflectance image (500 m) in Galicia, making use of freely available and highly processed products and without needing field data. Deepening on this cost-effective method and working on the same area, Huesca et al. (2013a) demonstrated similar mapping results employing MERIS post fire infrared reflectance data (300 m), and certainly higher accuracies than achieved by global fire products. Overall, these low spatial resolution datasets have great value for regional assessments, although they lack sufficient spatial detail for management. With fine spatial detail Verdú & Salas (2010) compared four pairs of Landsat and SPOT composites for the period 1991-2005 at irregular intervals of 1-5 years and visually identified and mapped fire scars over Spain. As expected the total area burned by fires larger than 100 ha was better correlated with the official fire database in the shortest interval product (1999-2000) than in other 5-year interval products. But mapping fire scars at large scale with fine temporal frequency and spatial resolution using RS techniques requires automatic approaches. In this sense, Bastarrika et al. (2014) developed semi-automatic software named Burned Area Algorithm Software (BAMS) for identification of burned areas based on threshold values of various spectral indices. BAMS supports the use of Landsat TM, ETM+, and OLI images and works on ArcGIS environment. Recently time series approaches are preferred to bi-temporal approaches for their effectiveness and temporal accuracy in identifying fire occurrence. Gómez et al. (2017) tested Composite2Change (C2C), a change identification algorithm based on trend analysis, to reconstruct 31 years (1985-2015) of annual fires in Northern Spain. C2C (Hermosilla et al., 2015, 2016) was developed for analysis of forest change in Canada, and it analyses the Normalized Burn Ratio (NBR) trajectory of individual pixels of Landsat composites, identifying an abrupt decrease of values in the trend as a change, and aggregating neighbouring pixels with similar trend into polygons. Furthermore, the object-oriented approach usually performs better than pixel-based approaches when mapping burned area and severity, as shown by Álvarez-Taboada et al. (2007b) using Landsat TM data.

Burn severity is frequently estimated fitting ground reference data—e.g., the Composite Burn Index (CBI, Key & Benson, 1999) a semi-quantitative index of severity—and RS variables from a range of data sources (De Santis & Chuvieco, 2007). In order to understand the causes of variability in spectral response with variations in burn severity, Chuvieco et al. (2007) simulated factors like soil background, leaf colour, and leaf area index, and compared models of burn severity produced with various sensors (Table 4). Landsat-TM provided the best compromise between spectral and spatial resolution and it best fitted the measured and observed CBI values. Burn severity models are typically more reliable in estimation of high than intermediate or low severity levels, both working at regional (Tanase et al., 2011a) or local scale (De Santis & Chuvieco, 2009). Viedma et al. (2015) used Landsat data to estimate severity in burned pines in Guadalajara (>12600 ha) and identified burning conditions like weather, propagation direction or rate of spread, as more relevant factors driving severity than pre-fire stand structure and directional topography. Temperature measured from a series of post-fire Landsat ETM+ datasets was tested as indicator of burn severity by Quintano et al. (2015) evidencing that surface temperature is strongly related with ground CBI values, thus proving its value to understand fire severity patterns. Fernández-Manso et al. (2016a) employed Sentinel-2A data to discriminate four levels of burn severity in Sierra del Teleno, demonstrating the superiority of the red-edge indices for this purpose, in agreement with Huang et al. (2016) who found the 20 m MSI NIR, red-edge, and SWIR bands best for mapping burned areas in different vegetation formations around the Globe. Aiming to evaluate the capacity of LiDAR data, Montealegre et al. (2017a) modelled and mapped burn severity in four large fires (> 500 ha) in Aragon with PNOA LiDAR data. Correlations between LiDAR and field measured CBI were comparable to those between Landsat-based NBR maps and CBI. Hyperspectral imagery has also demonstrated capacity to estimate burn severity, from the satellite platform Hyperion (Parra & Chuvieco, 2005), and from an aerial platform (Huesca et al., 2013b), but the scarcity of data makes this type of sensor less attractive for the purpose. On the contrary, over the past decade SAR-based retrieval of fire impacts has received significant attention over Spanish forests and the potential of radar sensors has been demonstrated for all wavelengths (X-, C-, and L-bands) available on satellite platforms. The variables and approaches implemented are diverse, including a range of SAR metrics from backscatter coefficient (Tanase et al., 2010a), interferometric coherence (Tanase et al., 2010b) and polarimetric decomposition (Tanase et al., 2014c). A combination of active and passive datasets in a multi-temporal change detection approach was also proposed in an operational framework for rapid fire impact assessment at regional to continental scales (Tanase et al., 2015a, 2015b). The framework was tested in various locations in Spain as well as in Australia and the US and it is based on the Radar Burn Ratio (RBR), an index pre-calibrated with in situ data.

Evaluating fire risk and danger requires knowledge of the fuel type and its moisture content, as well as factors like climate and topography. Certainly at large scale these factors are best estimated or modelled with some RS support. Riaño et al. (2002) generated a fuel type map of Cabañeros National Park with a supervised classification of Landsat data, getting the global accuracy considerably increased—from 67.3% to 79.4%—when illumination and slope were considered. Arroyo et al. (2006) demonstrated the usefulness of very high spatial resolution data to map fuel types by classifying optical data from QuickBird-2 (0.7-2.4 m) and mapping six vegetation structural types in Madrid with an overall accuracy of 80%. To predict the potential type of wildfire (surface, passive-crown, active-crown fire) at large scale, Arellano-Pérez et al. (2018) employed Sentinel-2 data over homogeneous plantations of P. radiata and P. pinaster in Galicia. A main limitation of spectral data for fuel type mapping is the inability to penetrate forest canopies (Keane et al., 2001) and to provide direct estimation of vegetation height. On the contrary, LiDAR airborne data can successfully be used to estimate critical canopy fuel parameters (e.g., Riaño et al., 2004 in pine forests of central Spain) which may be integrated with SNFI data (e.g., González-Ferreiro et al., 2017 in pine forests of Galicia). González-Olabarría et al. (2012) combined fuel type derived from LiDAR data with fire behaviour models to assess fire risk at the landscape level in Urbión (Soria). At regional scale LiDAR and spectral data have been combined to provide fuel type cartography in Natural Park of Alto Tajo (Guadalajara) (García et al., 2011). Likewise, to map forest fuel types in Canary Islands Marino et al. (2016) employed LiDAR data (1 pulse × m-2) after stratification with Landsat images, and Alonso-Benito et al. (2016) fusioned WorldView-2 optical images and LiDAR data into an object-oriented classification approach. In the context of wildfire suppression in the wildland-urban interface, Robles et al. (2016) evaluated the risk of damage in case of a wildfire of buildings and infrastructures in a 36 km2 rural area of Pontevedra. With LiDAR and aerial photographs from PNOA, and an object-oriented approach the authors classified forests into 5 forest fuel types and the buildings next to forests into 3 groups of risk.

Live fuel moisture may be estimated with passive (Chuvieco et al., 2004a) or active sensors (Tanase et al., 2015c), and it is an important parameter to determine fire risk, but also burning efficiency for evaluation of gas emissions from wildland fires (Chuvieco et al., 2004a). Aiming to evaluate fire danger in shrubs and pastures in Cabañeros National Park, Chuvieco et al. (2002) employed seven Landsat images acquired at various dates—spring, summer, autumn—over three years to estimate moisture with a number of spectral indices. The authors indicated the relevance of SWIR data for estimation of vegetation moisture and interpreted spectral variations according to vegetation types. In a later work in the same area Chuvieco et al. (2003) employed NOAA-14 AVHRR—with low spatial resolution (1100 m) and lacking SWIR bands—images acquired during summer time in 1996-1999. A model including spectral and thermal variables was accurate (R2 = 0.8) and helped identifying trends of moisture change in the area and when extended to other Mediterranean areas (Chuvieco et al., 2004a). Comparing the performance of Landsat-TM, SPOT-Vegetation, and NOAA-AVHRR in estimation of fuel moisture, Chuvieco et al. (2004b) demonstrated the synergies of NIR and SWIR combined, and that the NDVI relationship with vegetation moisture over time is stronger in grasslands than in shrubs. Yebra & Chuvieco (2009) employed MODIS 8-day composites, with 500 m pixel size and including NIR and SWIR data, to demonstrate that the retrieval of fuel moisture content is more accurate when species specific conditions are considered. The authors worked in an area dominated by Quercus ilex L. and compared generic and specific reflectance look-up-tables.

Despite an elusive relationship between spectral recovery and vegetation regeneration, recovery after fire is frequently studied with RS time series (e.g., Vicente-Serrano, 2011; Viana-Soto et al., 2017). Also with a time series approach Martínez et al. (2017) employed LandTrendr (Landsat-based Detection of Trends in Disturbance and Recovery, Kennedy et al., 2010) an algorithm designed to characterize landscape changes, for evaluation of recovery processes in a large forest fire (> 7600 ha in Zaragoza/Navarra) characterizing patterns of spectral recovery and classes of recovery magnitude. The trajectory based approach showed there is a relationship between fire severity and recovery magnitude. Prominent among the approaches to retrieve change and regeneration information after fire is using vegetation indices such as NBR and its multi-date approach (dNBR—differential NBR, RdNBR—Relative differential NBR) (Álvarez-Taboada et al., 2007b; Botella-Martínez & Fernández-Manso, 2017; Arellano et al., 2017) or NDVI (Díaz-Delgado & Pons, 1999; Ruiz-Gallardo et al., 2004). However, other approaches may be more informative of forest regeneration. For example, short term recovery from fire was modelled by Fernández-Manso et al. (2016b) in 30 km2 of P. pinaster in Sierra del Teleno using a 13-year series of Landsat MESMA (Multiple Endmember Spectral Mixture Analysis) fraction images. The authors found a recovery period between 7 and 20 years depending on fire severity and indicated interpretation simplicity as an advantage of image fraction over vegetation indices time series. Tanase et al. (2010a) compared the sensitivity of radar (X-, C-, and L-bands) and optical data to post fire forest regrowth in various Pinus halepensis Mill. locations of Spain. They found that L-band backscatter is sensitive to forest structural changes 40 to 60 years past disturbance, whereas optical-based indices reach saturation within 10 to 20 years, representing a reduced monitoring capacity. LiDAR can also be useful to evaluate post-fire regeneration at the landscape level. For example, Marino et al. (2017b) compared metrics derived from < 4 m strata returns of three LiDAR datasets (1 pulse × m-2) acquired pre- (2011) and post-fire (2012, 2014) in Garajonay and characterized vegetation recovery, demonstrating the value of repetitive LiDAR acquisitions. Debouk et al. (2013) employed low density LiDAR data (0.7 pulse × m-2) acquired over 104 km2 of mixed forest (P. halepensis and Q. ilex) in Barcelona five years after fire, and modelled vegetation recovery with an Artificial Neural Network (ANN) for classification and mapping. Martín-Alcón et al. (2015) combined low density LiDAR (0.5 pulse × m-2) acquired in 2009 with MS aerial photography acquired in 2011 to classify a P. nigra forest burned in 1998 into five post-fire regeneration types. Malak et al. (2015) related vegetation regrowth over an area ~2900 km2 in Valencia with Landsat TM EVI time series, and also demonstrated that the number of fires occurred in a short interval have a negative impact on vegetation growth. LiDAR and aerial photography combine well although both sources of data are constrained by the limited frequency of acquisition. Regeneration after a large fire has been recently studied with ultra-high spatial resolution imagery (0.2 m) acquired with UAV technology during a two-month campaign (Fernández-Guisuraga et al., 2018) in a 3000 ha area in León. Despite some banding noise and non-homogeneous radiometry, when compared with high spatial resolution WorldView-2 data (2 m pixel size) the UAV provided more accurate information of structural variability.

Health status

The Spanish forests and plantations host endemic populations of insects like the pine processionary moth (Thaumetopoea pityocampa D. &. Schiff.), the European gypsy moth (Lymantria dispar L.), the beech weevil (Rhynchaenus fagi L.) or the eucalyptus snout beetle (Gonipterus platensis Marelli). These populations cause low level defoliations but eventual outbreaks may occur in years of climate deviations (Cardil et al., 2017). Monitoring is necessary to evaluate the severity and areal extent of pest effects on the health and growth of trees, for management, and to develop effective protection strategies. The Spanish national forest health monitoring system is based on field observations over a network of plots (UNECE, 2016) and provides valuable data for overall assessments, but has inherent limitations for detailed mapping. RS data with complete spatial coverage and periodical observations may enhance the value of in situ measurements, and facilitate modelling and assessment of trends and deviations from normal condition. However, according to Radeloff et al. (1999) monitoring defoliation with RS is hampered by three problems: the short periods when defoliation can be detected, a difference in the scale of affection (leaves) and detection (canopy), and the close interactions between factors and effects on insect populations.

Discerning the canopy reflectance signal from noise in forests slightly affected by a pest or disease requires fine spatial and spectral resolutions coupled with the right temporal acquisitions, ideally at pre-, peak-, and post-defoliation times (Rullán et al., 2013). Rullán et al. (2013) suggested a two level scaled system for regional or national level monitoring of insect defoliation, with an early warning provided by MODIS time series, and Landsat data to assess damage affection. In 2004 Álvarez-Taboada et al. proposed a monitoring system of the health status in Eucalyptus globulus Labill. incorporating modelling, RS, and GIS (Eucalyptus Health Monitoring System, EHMS). Although an optimal application of the EHMS depends on climatic, soil and forest stand data, and validation of some relationships between the radiometric information and eucalyptus stand parameters, damage detection just requires Landsat TM SWIR data, a DEM, and stand density data. When applied in Galicia the EHMS identified damaged stands with leaf loss over 25% with a true positive accuracy of 72.31% and user’s accuracy of 95.92% (Álvarez-Taboada, 2006; Álvarez-Taboada et al., 2007a). SAR-based change detection approaches may be better suited to identify areas susceptible to insect outbreaks or experiencing the initial outbreak phase, as demonstrated for coniferous forests elsewhere (Tanase et al., 2018). In this work the L-band SAR backscatter was sensitive to insect induced changes a year in advance when compared to optical reflectance from high resolution orthophotos. Such differences were explained by the sensitivity of the SAR data to the vegetation moisture content, which decreases during the initial attack phase (green phase) when leaves are still green (i.e., there is little to no change in optical reflectance).

As noted by Carter (1993) discoloured vegetation stressed by a pest or disease increases reflectance in the green and red (VIS), an effect typically first observed in the red edge (0.7 µm), whereas defoliation is identifiable by a decrease in the NIR reflectance (Jensen, 2005). Vegetation indices based on VIS, NIR, and SWIR wavelengths are frequently used to quantify forest defoliation. In particular the ratio between SWIR and NIR, named Moisture Stress Index (MSI, Rock et al., 1986) has been found to be strongly related with defoliation caused by diverse drivers (e.g., pine processionary moth in Sierra de Gúdar (Teruel)—Sangüesa-Barreda et al. (2014); beech weevil in the southern Cantabrian range—Rullán-Silva et al., 2015). In absence of extreme defoliation, modelling damage with MSI becomes more robust for intervals of low and moderate affection (Rullán-Silva et al., 2015). Álvarez-Taboada et al. (2014) developed a multi-sensor and multi-scale system for monitoring forest health in P. radiata stands affected by the European gypsy moth in a study area of 150 ha in Cubillos del Sil, León. At stand level the authors identified three levels of defoliation severity employing pre- and post- outbreak Landsat OLI data and an object oriented supervised approach, achieving an overall accuracy of 97.61%. In the same area Castedo Dorado et al. (2016) tested the UAV technology with RGB and NIR images (Ground Sample Distance, GSD = 0.15 m) acquired with a fixed-wing platform to map defoliation severity at tree level. Overall accuracies were 67.68%, 71.72%, and 92.93% for 4, 3, and 2 severity classes. Also using UAVs Cardil et al. (2017) assessed defoliation by the pine processionary moth in two pine stands in an area of 24.6 ha. The authors classified RGB images captured with a Phantom 3 DJI and validated the results with field estimations at the tree level. The accuracy of detection was 79%, and only a few trees with low level of defoliation (10-20%) were misclassified.

Pests and diseases may have long term effects on trees that are more difficult to notice with RS than temporal defoliation, requiring additional data for interpretation. Sangüesa-Barreda et al. (2014) combined Landsat data with dendrochronological characterization of changes in basal area to estimate loss of growth due to the processionary moth. Cifuentes et al. (2017) classified affections caused by the fungus Cryphonectria parasitica (Murrill) (blight) in chestnut stands in El Bierzo (León). The authors estimated blight severity levels by visual analysis of RGB ortophotography (GSD = 0.08 m) acquired with a fixed-wing UAV and validated its correspondence to 182 field measurements. The overall accuracy for six severity levels was 63%, whereas for 5and 4 levels, was 74% and 77%, showing usefulness of this approach to map blight severity at the tree level.

For an early detection of forest decline photosynthetic activity and pigment content are better indicators than structural degradation. Sun induced fluorescence (SIF), which can be assessed from ultraviolet active laser fluorosensors and from passive multispectral or hyperspectral radiance sensors, has shown to be a proxy of photosynthetic activity. Q. ilex declining condition due to water stress and Phytophtora was explored by Hernández-Clemente et al. (2017) analysing the red and far-red SIF from airborne hyperspectral imagery. The authors found the relationships between SIF and vigour decline depend on spatial resolution, being significant for 0.6 m pixels but not for 30 m pixels. Recently Zarco- Tejada et al. (2018) explored the capacity of red-edge spectral data to assess pine decline in 7000 ha of P. pinaster and P. nigra in Extremadura analysing the temporal responses of Sentinel-2A red edge chlorophyll index and NDVI. Validated with aerial hyperspectral data and field measures of chlorotic and defoliated trees the authors found that declining and healthy pine trees have different NDVI vs. chlorophyll index temporal trajectories, demonstrating the value of the red-edge data to monitor forest decline.

SynthesisTop

Forests and other woodlands cover more than half of the Spanish land and provide important services to society, including economic benefits and recreational opportunities. RS offers options for monitoring the environment and it is increasingly being employed to improve our understanding on the state and dynamics of forest ecosystems in Spain. Applications that benefit from the use of RS techniques include medium to large scale characterization of forest structure, estimation of aboveground biomass, mapping of fire extension and severity, and monitoring of forest health. Certainly optical medium spatial resolution data have been the most frequently used source of data in the past, due to availability and suitability for a range of applications. However, LiDAR and SAR data are increasingly being employed (Table 3), especially for the retrieval of forest structural parameters, due to their capability to penetrate through the canopy. Innovative RS techniques are developed and applied in Spanish forests, being remarkable the use of small aerial platforms (UAVs) for local scale data acquisition and assisting in assessment of forest health, and the application of machine learning for analysis and modelling.

Through this review we have identified some needs and opportunities in the monitoring of Spanish forests where RS techniques can play a significant role (Table 5). In general free access to abundant and frequent data, as well as the increased storing and processing capacity offer unprecedented opportunities for forestry RS applications at spatial scales from local to national and with detailed temporal recurrence. Extending local models to a national level to provide an overall and consistent perspective should be a pursued effort, and understanding dynamics retrospectively would provide baseline information to build knowledge for the future. In this review we mentioned a representation of the most relevant RS applications in Spanish forests found in the scientific literature, with special attention to the most recent ones.

Table 5. Synthesis of the needs and opportunities in the Spanish forestry remote sensing.

Transversal to landscape, structure, fire, and health is the dynamic character of ecosystems. Perhaps the most remarkable current opportunity offered by RS technology resides in its capacity to characterize dynamics at a range of temporal resolutions, facilitated by the amount of free data available from long-life duration satellites like Landsat, MODIS and the Sentinels. There is an opportunity to monitor trends with high temporal frequency and spatial resolution and to retrospectively reconstruct a history of change to learn from patterns, by combining the Landsat records held by the USGS and ESA archives. Integration of data from both archives requires self-implemented standard processing (i.e., geometric alignment and radiometric corrections) until the Landsat Global Archive Consolidation initiative (Wulder et al., 2016) completes efforts to have all images centralized in a global archive and with similar processing. To overcome eventual scarcity of available data due to historical circumstances, compositing data with a per-pixel approach (White et al., 2014) facilitates complete coverage with high frequency. Hence, phenological characterization of forest ecosystems (Pasquarella et al., 2016; Simonson et al., 2018) and identification of species for habitat mapping or characterization of invasive species after fire (Bradley, 2014) are enabled, adding insights to our understanding of global change. The need to understand changes in species dominance and structural dynamics retrospectively, as well as recovery after fire, exists in Spain for reporting and management at national scale. Linking historical records and current insights facilitates prospective modelling in different scenarios for informed decisions.

National scale landscape characterization currently based on SIOSE, CORINE, and MFE products may be improved with more frequent land cover updates. For example, incorporating data from the European Copernicus Programme—optical Sentinel-2 and radar Sentinel-1—and data from the Landsat Programme may update LC products and enable monitoring changes annually (Gómez et al., 2016b; Hermosilla et al., 2016; Hermosilla et al., 2018). Trade-offs between temporal frequency and spatial or spectral resolutions in data acquisition have reduced their relevance thanks to virtual constellations (Wulder et al., 2015) that provide a stream of available and compatible data from different satellite programs. For retrospective monitoring of landscape dynamics Landsat is undoubtedly the most adequate source of data, due to its long–term archive, spatial resolution, and spectral quality. Retrospectively identifying changes at large scale with a time series approach (e.g., C2C, Land- Trendr) and interpreting rates of change (e.g., Gómez et al., 2012a) helps understanding patterns as well as drivers of change (Regos et al., 2015).

For an accurate assessment of resources at national level, forest structural maps including height, canopy cover, and biomass will benefit overall reports, management, and habitat mapping. At regional or national scale characterizing structure with RS requires extensive and reliable continuous data, and there are currently a range of opportunities. Although single-date optical data has typically yielded models with high relative errors, seasonal imagery acquired at key dates over the year have demonstrated higher accuracy in estimation of tree density, basal area, and wood volume in Mediterranean forests (Chrysafis et al., 2017). Undoubtedly the national coverage of PNOA LiDAR data provides a unique opportunity to create a national map of forest structure. With a sampling approach PNOA LiDAR can also be used to calibrate predictive models of forest structure metrics and biomass using optical time series data, an effort successfully implemented at very large scale in Canada (e.g., Zald et al., 2016). Additionally, a second complete coverage acquisition of LiDAR data with comparable density and precision will facilitate structural comparisons over time and assessment of change. However, for a reliable characterization of all kinds of forest structure, it would be beneficial to attain an increase in the scanning density of the national level PNOA LiDAR data (Adnan et al., 2017). Higher point densities would also facilitate the implementation of individual tree methods (e.g., Valbuena-Rabadán et al., 2016). In order to enable the production of updated results PNOA LiDAR data has to be promptly available to users. Combining LiDAR and photogrammetric data might be a cost effective option for regular assessment of change in forest structure (Tompalski et al., 2018; Navarro et al., 2018). With increased temporal frequency, the demonstrated synergies between LiDAR and optical data for large area mapping of structure (Manzanera et al., 2016; Matasci et al., 2018) could provide relevant results in Spain, at least in the most dynamic areas. At detailed scales, species identification and structural analysis at tree level are possible by combining multispectral images and LiDAR data (e.g., González-Ferreiro et al., 2013b), and in the near future multispectral LiDAR will provide an integrated alternative. Radar data has capacity to characterize forest height and height change applying interferometric (Olesk et al., 2016) and Pol- Insar techniques (Xie et al., 2017) over large regions like the Spanish national territory. PolInsar metrics make feasible the retrieval of information on the vertical structure of forests which may overcome saturation effects when estimating biomass or height (López-Sánchez & Ballester-Berman, 2009). Data from the TanDEM-X mission are available for research (Table 1) and data from the Spanish PAZ launched in 2018 will be fully compatible with Tan DEM-X, adding to the stream of data. Sentinel-1, although not optimally configured in polarization and frequency for forestry applications, offers a large amount of frequent data and opportunities still unexplored. Satellite radar missions like BIOMASS, expected to orbit in the near future, and satellite constellations combining multiple sensors may open important opportunities to monitor forest resources. Playing a key role for calibration and verification, UAVs equipped with one or more sensors already enhance the characterization of forest structure (e.g., Sankey et al., 2017). And in the future, unmanned high altitude platforms or pseudo-satellites (HAPS) flying at around 20 km height, will provide a link between data acquisition scales, complementing satellite and aircraft imagery (Gonzalo et al., 2017).

Driven by the relevance of fire as trigger of change in Spanish forests, a great effort was focused in the last decades on fire related RS applications. Still, complete and updated national scale cartographic records of fire at high spatial resolution are missing, and most assessments rely on non-spatially explicit statistics. Developing historical annual cartography of fire in forest areas with high accuracy is feasible with the current availability of data (Gómez et al., 2017), facilitating analysis and interpretation of change patterns and drivers of change (Cohen et al., 2016; White et al., 2017). As a modelling technique the mapping limitations should be reported to avoid misinterpretation or overstating results, providing measures of accuracy and confidence intervals. As the archive of available data gets longer, standardizing data quality to apply novel algorithms is possible (e.g., Hermosilla et al., 2017) and enables the maintenance of maps up to date. Moreover, identification of hot spots and characterization of the wildland urban interface at different scales for operational use in wildfire prevention and suppression, and planning of prescribed fires benefit from the use of time series of Landsat OLI and Sentinel-2 as well as LiDAR data. SAR-based burned area detection algorithms are also developed under the ESA Fire-CCI Phase 2 project (Lohberger et al., 2018; Belenguer-Plomer et al., 2018) and may be applied at national scale. Also relevant for fire management is the capacity of radar data to estimate live fuel under forest canopy demonstrated by Tanase et al. (2015c). LiDAR data can be used to estimate fuel variables of the forest canopy, crucial information used as input in fire behaviour models, while full-waveform systems are proper to provide information of the understory vegetation (Crespo-Peremarch et al., 2018), particularly relevant in Mediterranean ecosystems where shrubs are main drivers of wildfire regime. Future attention should be paid to LiDAR satellites, such as IceSAT-2 and GEDI, possibly coupled with TanDEM-X, since these data will become available from 2019. These new sensors will likely open a new range of operational and research applications.

Biological invasions, pests, and diseases progressively getting more frequent and intense may compromise the health of Spanish forests. To meet the operational needs of timely and accurate forest health monitoring systems nowadays efforts focus on integrating data at various scales. Comprehensive and spatially-explicit data—only feasible from RS—contribute towards increasing our knowledge of the invasions biology and developing more efficient management strategies (Hernández et al., 2014; Pascual et al., 2016). RS techniques also improve the efficiency of sampling for prediction of outbreaks (Wulder & Dymond, 2004). In this sense UAV technologies have emerged as an opportunity offering above canopy perspective of stand condition that can bridge field to satellite scales, and as a source of data for calibration and validation of RS monitoring systems (Hall et al., 2016). Two of the major threats to the chestnut stands in Spain are Cryphonectria parasitica (chestnut blight) and Phytophthora cinnamomi (ink disease) (Melicharová & Vizoso-Arribe, 2012), which eventually can cause the death of trees. Combining data from different sensors mounted on UAVs can provide helpful information (e.g., detection, monitoring of the treatments) about infestations which require treatments at tree or at stand level. Detecting and monitoring Bursaphelenchus xylophilus (a pine wood nematode), and Xylella fastidiosa, the biggest hazards regarding forest health in Spain (Karnkowski & Sahajdak, 2010) remains challenging. Xylella fastidiosa is one of the most dangerous plant bacteria worldwide, causing a variety of diseases with huge economic impact (Sherald, 2007). Due to its severity and economic impact, the European Union has taken emergency control measures for both (EC, 2017), which involves their detection, location, and monitoring. For an early detection with RS high spatial and hyperspectral imagery is needed, being multispectral imagery useful to supervise and monitor whether the affected stands have been removed, and whether the decay is spreading beyond the demarcated areas. Despite the ephemeral character of defoliation, near real time monitoring of this effect is possible with dense time series of multispectral data (Pasquarella et al., 2017) at stand or forest scale, although the defoliation driver may remain unknown. Common pests in forest plantations like defoliators of Eucalyptus spp. (e.g., Gonipterus platensis) or pine engravers like the bark beetle (Ips sexdentatus) which causes decay and even death of Pinus spp. may be monitored in Spain with this approach. Especially in the case of the bark beetle, monitoring the decay will help knowing whether the population is under control or whether pheromone traps or tree removal is needed to prevent its spread. A quick spread of new pests like the chestnut gall wasp (Dryocosmus kuriphilus) is an outstanding example of recent human-aided biological invasion with ecological impacts and economic losses (Bonal et al., 2018). Detecting this type of pest with RS is challenging unless the level of infestation is very high, but in heavily infested areas monitoring the treatment success at stand level could be a suitable task for multispectral high spatial resolution imagery (e.g., Sentinel-2, World View-4).

Overall, RS contributes to our better understanding of the services provided by Spanish forest ecosystems, allowing insights on the forest state and dynamics and this helping towards a better planning and sustainable management. We live a time of opportunities provided by the use of optical, radar, hyperspectral or LiDAR sensors, individually or in combinations that leverage their synergies for forestry applications.

AcknowledgmentsTop

We thank the reviewers and editor for their thorough work and contributions to the final manuscript version.

ReferencesTop

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REVIEW ARTICLE

Remote sensing for the Spanish forests in the 21^st century: a review of advances, needs, and opportunities

IntroductionTop

Remote sensing techniquesTop

Remote sensing applications in Spanish forest ecosystemsTop

SynthesisTop

AcknowledgmentsTop

ReferencesTop

REVIEW ARTICLE

Remote sensing for the Spanish forests in the 21st century: a review of advances, needs, and opportunities

IntroductionTop

Remote sensing techniquesTop

Remote sensing applications in Spanish forest ecosystemsTop

SynthesisTop

AcknowledgmentsTop

ReferencesTop

Remote sensing for the Spanish forests in the 21^st century: a review of advances, needs, and opportunities