Prescribed underburning (PB) is a silvicultural treatment widely used worldwide and has multiple objectives (Pyne et al., 1996), including reduction of the forest fuel load below the canopy in Pinus stands in order to prevent forest fires (e.g. Espinosa et al., 2020). Although the implementation of PB for forest management in Spain was first suggested in the 1980s, it was not until the last decade that Autonomous Communities and Spanish Central Administration made efforts to train forest firefighters in planning and executing PB (Fernandes et al., 2022). The technique is used in Pinus pinaster Ait. stands in Portugal, whereas in Spain it is used as management tool in maritime pine stands in some autonomous regions. The National Programme for experimental prescribed underburning (González-Sancho et al., 2020; García-Feced et al., 2022), funded by the Ministry for Ecological Transition and Demographic Challenge (MITECO), was launched in 2012 with the aim of fostering application of the treatment across Spain. This programme has also focused on PB in P. pinaster stands (Rodríguez-García et al. 2018). Nevertheless, studies on the effect of PB on trees are scarce in Spain (Valor et al., 2015; Espinosa et al., 2018, 2020, 2021; Madrigal et al., 2023). To our knowledge, only the previously mentioned study, carried out by our research group, has assessed the effect of PB on resin canals in tapped P. pinaster stands (Rodríguez-García et al., 2018).

Prescribed underburning tries to minimize the impact of fire on trees (Espinosa et al., 2020, 2021) and it can be used to prune and thin young stands. Nevertheless, some studies have shown that PB increases the exudates and resin-producing capacity of trees and leads to alterations in the resin composition (Lombardero et al., 2006; Perrakis et al., 2011), as well as altering the resin ducts (Perrakis et al., 2011; Hood et al., 2015). An increase in resin production in the year of execution of the PB and in the following years may therefore be expected due to the synthesis of terpenes as a response mechanism to a wound or stress conditions for the plant (Knebel et al., 2008), potentially improving the resin yield (Rodríguez-García et al., 2018) and resilience to potential outbreaks (Vilà-Vilardell et al., 2024).

Resin sector was a main activity in the Iberian Peninsula until the second half of the past century (Palma et al., 2016), but almost disappeared in the late 1970′s. In the last 20 years, the increasing demand of natural renewable resources, and the raising of resin price were favouring a progressive resurgence of the resin sector (Rodriguez-García et al., 2016, 2018). In the context of revitalising resin activity due to the current market situation (Clopeau et al., 2022), the extensive monospecific stands of P. pinaster in the Northern Plateau of the Iberian Peninsula have a strong potential to revitalize the bioeconomy in rural areas (Rodríguez-García & Martínez-Carreira, 2022). These products have multiple uses in different industrial sectors and also provides relevant complementary ecosystem services such as fire protection and rural employment (Soliño et al., 2018). This includes areas in the province of Soria (Central Spain) where tapping pines trees for resin production has traditionally been carried out (Gil, 1991) and now emerges as an option to add value where timber profitability is compromised, or low timber prices and a high risk of wildfires could be present. However, managed stands using traditional resin tapping methods are very vulnerable to wildfire. Among adaptations to the high recurrence of fires, P. pinaster generates thick bark that protects living tissues from high temperatures (Fernandes & Rigolot, 2007). The tapping operation makes trees very vulnerable to fire, reducing their resistance to burning. For the same reasons, carrying out PB before tapping in P. pinaster provenances with thick bark has been proposed as a suitable treatment for managing tapped stands (Rodríguez-García et al., 2018).

Analysis of the effect of PB before tapping enables evaluation of the potential of the procedure for use in P. pinaster stands. Use of PB would also improve accessibility, both for resin collectors and firefighting teams and would also reduce fire hazard due to reduction in fuel load. Rodríguez-García et al. (2018) verified that burning generates a significant increase in the number of resin canals but does not produce an increase in resin yield in the same year of tapping or one year after. However, secondary defences are not usually induced in trees until the third or fourth year after disturbance (Knebel et al., 2008). With the aim of testing this hypothesis, a new experimental design was applied to evaluate the resin yield four years after PB. The aim of this study was therefore to answer the following question: Does the resin yield increase in a Pinus pinaster stand when subjected to PB before tapping? The underlying study hypothesis is that PB increases the opening of the resin canals in the trees, which may be associated with an increase in resin yield (Rodríguez-García et al., 2018).

Prescribed underburning led to the consumption of 32.73±15.45% of litter biomass and 77.82±13.09% of fine fuel (<6 mm). Low intensity burning (average flame length 1±0.5m) and a conservative prescription (rate of spread higher than 1.41±0.07 m/min, fuel fine moisture content 12%) generated moderate consumption of heavier fuels (>6mm), of between 18 and 33% (Table 1). Mean consumption of the total fuel load from 1 to 100-h was 40.44±22.44% (Table 1). These values are similar to those reported for Pinus nigra and mixed P.nigra - P.pinaster stands (Espinosa et al., 2018 and 2020). Similar values were also reported by Rodriguez-García et al. (2018) for experimental plots close to those in the present study, burned in 2015, and values obtained by the National Programme for experimental prescribed underburning (González-Sancho et al., 2020, García-Feced et al., 2022). The results confirm the efficacy of PB in reducing fire hazard under canopy in Mediterranean ecosystems (Fernandes et al., 2022). Resin yields differed between the four years of study (within-subject factor) (p<0.001), with mean production levels of 1.46±0.24 kg in 2018, 1.90±0.35 kg in 2019, 1.77±0.30 kg in 2020 and 2.73±0.88 kg in 2021 (Fig. 2). However, no significant differences in production were detected within the same year (inter-subject factor) between the burned, tapped stands relative to the tapped stands (p=0.93). The GLMM revealed a significant effect of random variables (tree and plot) (p<0.01). A high level of variability among resin yield in trees and sites is common in tapped stands (Rodríguez-García et al., 2018).

We expected a cumulative effect of resin yield between 2018 and 2021 (Fig. 2) related to the generation of “traumatic” resin ducts (secondary defence in response to wounding) after treatment (Ruel et al., 1998, Lombardero et al., 2000). Nevertheless, the absence of significant differences for inter-subject factor indicates that the significant increase in the resin yield over time (Fig. 2) may be more closely related to the variability in weather and the increase in resin canals for both treatments. Therefore, the stress produced by wounds opened up during tapping operations may mask the potential effects of PB (Rodríguez-García et al., 2018).

The present study findings confirm those previously obtained by (Rodríguez-García et al., 2018), in which no increase in resin yield was detected in the short term (two years after burning). The experiment was specifically designed to include a longer duration of resin yield monitoring. Robust results would therefore be obtained with two independent experiments used in the same area.

The findings indicate that prescribed under canopy burning in P. pinaster stands does not increase resin yield. However, the findings also indicate that PB is an effective tool to reduce fire hazard compatible with the first tapping in tapped stands managed for resin production, as the reduction in fuel load will reduce the forest fire risk and vulnerability of tree trunks to fire (Madrigal et al., 2019; Espinosa et al., 2020). The results suggest that PB would increase resin yield if a higher intensity fire (Vilà-Vilardell et al. 2024) is prescribed. Thus, burn prescriptions compatible with tapped P. pinaster stands could improve the local bioeconomy.