Whether or not forest growth can be increased by removing some trees or to what extent volume growth is reduced by heavy thinning are questions that have been addressed by studies for many years ( Zeide, 2001 ). These questions can be answered by testing Wiedemmann’s hypothesis, which states that volume growth is constant among a wide range of stand densities, also expressed by Langsaester’s curve or density growth relationship ( Langsaester, 1941 ; Wiedemmann, 1951). This relationship varies among species, but also depends on stand age and site conditions ( Assmann, 1970 ; Pretzsch, 2005 ), so it is necessary to quantify this variation for each forest species.

The application of thinning throughout the rotation in Scots pine stands reduces the cumulative or total volume, i.e . lower volume increment, although the magnitude of this decrease depends on the thinning regime. The loss in total volume depends mainly on the intensity of the regime, the volume increment decreasing with the thinning intensity. This total yield-thinning intensity pattern has been observed in Scots pine thinning experiments where the extraction method was from below ( Montero et al ., 2001a ; Mäkinen & Isomäki, 2004a ; del Río et al ., 2008 ; Nilsson et al ., 2010 ), a common method for Scots pine since it is particularly suitable for this shade intolerant species. However, reductions in total volume were also found for other methods ( Dittmar, 1991 ; Nilsson et al ., 2010 ), as can be seen in Fig. 1 (right) for crown thinning or thinning from above. When comparing different combinations of the number and weights of interventions, Nilson et al . (2010) found greater yield losses with few heavy thinning interventions rather than with frequent light thinning, highlighting the importance of the thinning intensity on stand volume growth.

According to these results, the pattern of Langsaester’s curve for Scots pine follows an increasing pattern, without an optimum below the maximum density. This pattern was also found by Gizachew & Brunner (2011) through models based on the Norwegian National Forest Inventory. However, some studies have identified the existence of a maximum, Assmann’s optimum basal area ( Assmann, 1970 , pp: 229-232), for light thinning at young stand ages ( Chroust, 1979 ; Kramer & Röös, 1989 ), which indicates the importance of the timing of the first thinning. Accordingly, the critical basal area following Assmann’s definition seems to be lower at younger ages ( Erteld, 1960 ).

The findings concerning the possible effect of site conditions suggest that the thinning intensity-total yield pattern does not differ significantly among site fertilities for Scots pine ( Mäkinen & Isomäki, 2004a ; del Río et al ., 2008 ; Gizachew & Brunner, 2011 ). However, the differences among thinning intensities seem to be higher at richer sites ( Mäkinen & Isomäki, 2004a ). In Fig. 2 the volume increment variation with the relative basal area for the extreme site qualities used in del Río et al . (2008) are presented, although for a longer study period. Despite the lack of significance of site in their study, it is noteworthy that at the better site, the loss in volume growth is greater and that at the poorer site, the maximum volume growth was found for light thinning. Therefore, one might recommend that the basal area after thinning should be higher on fertile sites compared to less fertile sites ( Nilsson et al ., 2010 ).

Regarding differences among regions, it seems that the volume losses with heavy thinning (more than 35% of total basal area removed) are higher in Northern and Central European (22-37%) than in SW-European Scots pine forests (19%), the losses being similar for moderate (10-15%) and light (5-10%) thinning regimes ( del Río et al ., 2008 ). These regional differences might be due to the lower mean site quality in the latter region; to the longer growing season in southern areas, which might allow the remaining trees to recover occupancy of the growing space more rapidly; or to the possible influence of genetic variation among regions, although more studies are needed to elucidate the regional differences.

When thinning treatments were combined with fertilization, the periodic annual volume growth and stand volume were greater in unthinned fertilized plots, which showed a greater response to fertilization than thinned stands ( Eriksson, 2006 ; Bergh et al ., 2014 ). The aforementioned study suggested that repeated thinning may reduce stand leaf area to below the limit required to maintain maximum growth rates and that this reduction is not sufficiently compensated by fertilization.

The effect of thinning on basal area growth and total basal area generally follow a similar pattern to that on volume, although the differences among thinning intensities are less pronounced ( del Río et al ., 2008 ). Despite this, findings for basal area growth should not be extrapolated to the corresponding volume growth ( Gizachew & Brunner, 2011 ). In some experiments, accelerated basal area growth was found at young stand ages ( Erteld, 1960 ; Kramer & Jünemann, 1984 ; Montero et al ., 2001a ), in accordance with findings for volume growth.

The tree selection criteria used in thinning has an important impact on the mean and dominant tree characteristics. In Scots pine thinning trials, thinning from below either has no effect on the dominant height of the stand ( Valinger et al ., 2000 ; Varmola et al ., 2004 ; del Río et al ., 2008 ) or only slightly reduces it in the case of heavy thinning intensities ( Mäkinen & Isomäki, 2004a ) since it is only the suppressed or intermediate trees that are removed. However, in a spacing-thinning experiment in Germany, where different initial stand densities were combined with thinning from above ( Pretzsch, 2010 , pp: 121-122), a higher top height was found at lower stand densities ( Fig. 1, left ). In plots with lowered density, the top height is one site class (up to 4 m) higher than in unthinned plots at an age of 40 years. Because of the shift in the calculation when eliminating the smaller trees, the superiority of the height of the tree with quadratic mean diameter of the thinned compared with the fully stocked plots is even higher (up to 6 m). The effects of the initial spacing on dominant height growth are still under debate, but significant height growth reductions as spacing decreases have been reported for a number of species ( Burkhart & Tomé, 2012 , pp: 166-167). Very high tree densities may drastically reduce space for tree growth, triggering a reduction in height, as found by Dippel (1982) and Spellmann & Nagel (1992) in the case of Scots pine. This kind of stagnation in top height development is typical in very dense plantations, where the homogeneity of age, space distribution and often genetics make social differentiation more difficult. However, in the abovementioned German experiment, height also differs between the control and the thinned plots for a given initial spacing. One hypothesis is that thinning significantly lowers drought stress and improves height growth at sites with low levels of precipitation, water storage capacity and mineral nutrient supply, as is the case at this site. The use of height as an indicator of stand productivity is therefore questionable, at least at dry sites; the effect of climate change might exacerbate this issue.

Tree diameter growth response may depend mainly on the thinning schedule and the social status of the tree. For a given thinning intensity, the effect on the quadratic mean diameter and dominant diameter can vary significantly with the thinning method. Heavy thinning from below significantly increases the quadratic mean diameter of the stand, although part of this increase is due to the shift caused by removing smaller trees. The observed effect on the dominant diameter differs among studies for Scots pine; some reporting positive ( Mäkinen & Isomäki, 2004a and references therein) and others, neutral effects ( Chroust, 1979 ; Varmola et al ., 2004 ; del Río et al . 2008 ). These differences might be due in part to differences in the timing of thinning treatments, the thinning intensities, and site conditions. Thinning through selection from above and based on the selection of future crop trees result in greater diameter increments for the selected trees ( Kramer & Röös, 1989 ; Ditmar, 1991; Nickel et al ., 2007 ), improving both stand stability and wood quality. However, the use of these thinning methods may not always be advantageous since they frequently involve heavy thinning intensities, with the associated yield loss ( Franz, 1983 ; Küster et al ., 2004). Moreover, in the case of the future crop tree approach, it is initially necessary to select a larger number than that expected to remain at the end of the rotation due to the occurrence of damage to trees and changes in tree social classes (Ditmar, 1991). The German combined spacing-thinning experiment provides insight into trade-offs between tree size and stand productivity using the thinning from above method ( Fig. 1 ). The mean diameter can increase by up to 8 cm in the case of the 100 tallest trees ( Fig. 1, centre ) and 12 cm in the case of quadratic mean diameter. Fig. 1 , (right) shows that the cost of obtaining these increases in mean tree size can be up to 80 m ³ /ha in maximum total yield. This finding agrees with those reported by Franz (1983) .

Other factors such as initial spacing, site characteristics and stand age may also affect the thinning response in terms of tree diameter growth. For Scots pine, a better diameter growth response to thinning has been reported at younger ages and at better sites ( Franz, 1983 ; Mäkinen & Isomäki, 2004a ). In the German combined spacing-thinning experiment, the benefit in terms of tree size along with the loss of total yield at the age of 40 years is caused, to a similar extent, by both the low initial density and the reduction in density through thinning ( Küsters et al ., 2004 ), highlighting the importance of initial spacing along with age at first thinning intervention for future stand development in this species. Regarding the combination of thinning and pruning, Moreno-Fernández et al . (2014) found no significant difference in tree diameter growth between thinned and thinned-pruned stands.

As regards the response to thinning depending on tree social status, some studies which have analyzed growth response to thinning according to tree size have found less difference between thinning intensities for larger trees and future crop trees in comparison to smaller trees ( Kramer & Röös, 1989 ; Mäkinen & Isomäki, 2004b ). However, Mehtätalo et al . (2014) found that tree size mainly affected response time, rather than maximum tree growth response, with much shorter response times in larger trees. Mäkinen & Isomäki (2004b) reported that in absolute terms, the effect of thinning decreased with decreasing relative size, although in relative terms, relative basal area growth in smaller trees was greater than in larger tress. Pukkala et al . (1998) reported that the best response to thinning occurs in trees of medium diameters.

Whereas silviculture only has a minor effect on the quality of wood to be used for paper and fuel, the characteristics of building and construction wood, like strength, stiffness, knottiness, density, distortion, wood heterogeneity and compression wood are highly dependent on thinning ( Low, 1964 ; Pretzsch & Rais, 2016 ). However, the formation of heartwood seems not to be dependent on the thinning regime ( Mörling & Valinger, 1999 ). The species-specific morphological plasticity and the spatial constellation of the tree within the stand have a specific effect on tree morphology and timber quality. Scots pine belongs to the morphologically highly plastic species with low apical dominance and strong plagiotropic crown extension ( Pretzsch & Rais, 2016 ). Its morphological development and wood quality depends strongly on the spatial constellation within the stands ( Agestam et al ., 1998 ). Nevertheless, the proportion of large trees with high quality wood at the rotation age also increases with the intensity of the silvicultural schedule ( Montero et al ., 2001b ).

When Scots pine is densely surrounded by competitors of similar size, the strong lateral restriction causes slender stems, low knottiness, high wood density, and low distortion of the timber. This pattern of response is widespread in densely established and conservatively managed even-aged pure and mixed stands. As this was the standard silvicultural paradigm in the past, high proportions of timber harvested today display these attributes of quality and allocation pattern. Strong dominance achieved either by vertically overtopping neighbors or by heavy thinning allocates resources to lateral crown extension to improve light interception and to stem growth for mechanical stabilization rather than to height growth ( Dippel, 1982 ; Hynynen, 1995 ). This results in tapered stem shapes, large-sized knots along the stem axis, as well as lower wood density, eccentric stem cross sections, and crowns with uneven lateral growth. Normally, there is a correlation between stem growth and amount of thick branches but the variation is large ( Klang, 2000 ). In order to counter the tradeoff between high density (improving the quality but slowing down size growth) and low density (reducing the wood quality but accelerating size growth) early tending, crop tree selection, and pruning are common management methods employed ( Cameron, 2002 ; Mäkinen & Isomäki, 2004a ; Peltola et al ., 2002 , 2007 ). Due to the frequent presence of dominant trees with many large branches or ‘wolf trees’, even at young ages, it is important to remove them through early thinning ( Kramer & Jünemann, 1984 ). Early thinning or pre-commercial thinning can be used for improving quality attributes in young Scots pine stands, and this measure is more effective for increasing stem quality in a stand at later rather than at younger stages ( Ruha & Varmola, 1997 , Fahlvik et al ., 2005 ).

Persson et al . (1995) and Blumenrother et al . (2001) described the wide variation in wood quality of Scots pine according to provenance, particularly between the apical oriented and straight-growing mountain provenances with high wood quality and the rather more laterally expanding lowland provenances which present a lower quality. However, in the analysis by Person et al . (1995), spacing was found to be significant for wood density and fibre length, as well as the interaction provenance-spacing for fiber length and fiber ratio, indicating the importance of controlling stand density for wood quality.

Climate change may be also be a consideration when dealing with wood density, since drought stress can significantly modify stem and wood properties, causing narrower tree rings and higher slenderness values ( Mette et al ., 2015 ). Furthermore, the increasing incidence of Viscum album ssp. austriacum is widely assigned to climate change ( Dobbertin, 2005 ; Walentowski et al ., 2007 ; Sangüesa-Barreda et al ., 2013 ) and means an increasing susceptibility to drought and pathogens as well as a reduction in wood quality ( Richter, 2015 ).

The most common non-wood forest products (NWFP) considered in the management of Scots pine forests are berries and mushrooms, although their abundance and socio-economic relevance varies considerably among regions. Most of the research focusing on the influence of silviculture on berry yield has been developed in Nordic countries, while studies relating silviculture and mushroom yield are more widespread geographically. However, our knowledge is still somewhat limited due to the difficulties involved in studying these NWFP, particularly as the conclusions of many local or regional studies cannot be extrapolated to larger scales. Among other factors, berry and mushroom yields show great spatial and temporal variability, hence the necessity for yield data recorded over long periods and from different sites.

Bilberry ( Vaccinium myrtillus L.) abundance is closely linked to site conditions ( Miina et al ., 2009 ), this species being frequently found in many Scots pinewoods. Cowberry ( Vaccinium vitis-idaea L.) is common in pine forests in northern Europe, particularly at poor sites ( Tomé & Faias, 2014 ). Thinning can affect both the coverage of berry plants as well as berry yield, although there are few studies corroborating this effect in empirical experiments ( Hedwall et al ., 2013 ). In a Finnish study based on yield measurements over a period of 14 years, it was found that bilberry coverage in Scots pine stands increased with age and basal area until a maximum was reached, while the berry yield depended only on bilberry coverage ( Miina et al ., 2009 ; Turtiainen et al ., 2016 ). As regards basal area, this maximum was around 25 m ² /ha, decreasing with larger basal areas. In a similar study for cowberry, based on 12 year berry yield data, the coverage increased with stand basal area while berry yield decreased with basal area, although obviously the yield also depends on coverage ( Turtiainen et al ., 2013 ). Using this model, a temporary positive effect of thinning might be expected, probably due to a higher level of light under the tree canopy.

The kind of information available regarding the effect of thinning on mushroom yield is similar to that for berries, with some models relating mushroom yield to stand basal area or density but very few studies focusing on the effect of thinning. Several studies developed in Scots pine forests in Spain have revealed the importance of basal area for mushroom production. Bonet et al . (2008) reported a maximum mushroom production in the Central Pyrenees for a basal area around 20 m ² /ha, and found similar results when focusing exclusively on Lactarius spp. yield. A similar optimum basal area was found in an updated version of the model by de-Miguel et al . (2014) . However a larger basal area maximum (around 40 m ² /ha) was reported for Boletus edulis in Scots pine stands in Central Spain ( Martínez-Peña et al ., 2012 ).

Concerning the effect of thinning, Bonet et al . (2012) found a positive short term effect on Lactarius group deliciosus yield , although this study was carried out in Pinus pinaster Ait. stands. Salerni & Perini (2004) reported a larger production of B. edulis after light thinning than with heavier thinning (20% and 40% of the trees removed) under canopies of different species, including Scots pine stands. Nevertheless, thinning might affect mushroom production since it can produce microclimatic changes in the soil layer which can influence mushroom composition and sporocarp production ( Bonet et al ., 2012 ).

Nutrient cycling encompasses processes above- and below-ground. In a simplified soil-plant system the allocation of nutrients to foliage, resorption and litterfall production determine the return of nutrients to the soil, whereas litter accumulation, decomposition, rhizodeposition and subsequent immobilization-release patterns control the rate of nutrient inputs. Some of these nutrients, after transformation to plant-usable forms, will be uptaken by plants whereas others will be lost from the forest via leaching or consumption by animals. The balance between nutrient inputs and outputs in the cycle is mediated by abiotic and biotic drivers. The importance of a balanced nutrient budget is that a correct supply of nutrients supports all other ecosystem services ( Lavelle et al ., 2005 ).

The effects of thinning on nutrient cycling include decreasing litter mass in the forest floor due to reduction of litter production. This effect might be cancelled in the short term due to canopy closure some years after thinning ( Roig et al ., 2005 ), however long-lasting effects on forest floor have also been observed and if it is accompanied by a reduction in nutrient concentration in green foliage, the nutrient budget may be unbalanced ( Jonard et al ., 2006 ). Notwithstanding, the type of harvest must also be considered as it will greatly affect the final nutrient stock in the forest floor ( Nave et al ., 2010 ). Thinning can also modify the abiotic environment needed for correct nutrient cycling in forest ecosystems. In Temperate and Boreal forest ecosystems, the gap opened by thinning allows more radiation to reach the soil, thus increasing the temperature and accelerating the decomposition rate and nutrient concentration in foliage ( Thibodeau et al ., 2000 ; Vesala et al ., 2005 ). This higher soil temperature and subsequent faster decomposition in thinned stands might explain the higher carbon stock found in F and H (fragmented and humus) layers of the forest floor in unthinned stands ( Ruiz-Peinado et al ., 2013 ). However, in areas where soil moisture is the limiting factor, like in the Mediterranean, the increasing levels of radiation reaching the soil could impair decomposition ( Blanco et al ., 2011 ; Lado-Monserrat et al ., 2015 ; Bravo-Oviedo et al ., 2017 ).

Few well-designed studies have been conducted into the impact of thinning on nutrient cycling in Scots pine stands, although two thinning trials in the North of Spain under Mediterranean and Continental-type climate were compared for litterfall production, decomposition and nutrient flux after thinning ( Blanco et al ., 2005 ). The results indicated that litterfall production decreased with thinning intensity whereas site climatic descriptors such as soil temperature correlated with litterfall production ( Blanco et al ., 2006 ). Nutrient return was found to reduce in thinned stands but the effect was greater at the Mediterranean-type site ( Blanco et al ., 2008 ). Similarly, the reduction in the decomposition rate was also greater at the Mediterranean site ( Blanco et al ., 2011 ). In this suite of studies, the authors suggested that Scots pines in the Mediterranean region are more sensitive to thinning in the short term. However, another study addressing the long-term impact of thinning on soil condition in a natural Scots pine forest showed that thinned stands, as opposed to unthinned plots, had higher P, K, Mg concentrations in the forest floor and similar carbon and N levels ( Bravo-Oviedo et al ., 2015 ). The properties of the mineral soil remained unchanged in terms of bulk density and nutrient concentration, although stable stocks of K, Mg and Na throughout the profile in thinned plots would indicate leaching from upper layers. Ruiz-Peinado et al . (2016) in a study conducted in a Scots pine afforestation in northern Spain found that thinning had no effect on carbon stocks in either the forest floor or the mineral soil. This pattern has also been observed for different tree species after thinning ( e.g . Powers et al ., 2011 ; Ruiz-Peinado et al ., 2013 ).

In the long-term, Ruiz-Peinado et al . (2016) observed that with regard to carbon stocks on-site (biomass, deadwood, forest floor and mineral soil) and off-site (biomass removed) after thinning, there were statistical differences in total carbon stocks between unthinned and heavily thinned Scots pine stands. These differences were associated with the amount of total biomass carbon; a loss in biomass production was identified in heavily thinned stands in line with the general pattern observed for volume production. More information regarding the general effects of thinning on carbon sequestration for different tree species is presented in another paper in this issue by Ruiz-Peinado et al . (2017) .

In northern Europe, soil nutrient sustainability following whole tree harvesting has been the focus of research due to higher demand for fuel wood used in bioenergy. In general, whole tree harvesting during thinning tends to decrease growth more than stem harvesting only, although the effect is slight and soil nutrient and carbon pools do not seem to be notably affected ( Helmisaari et al ., 2011 ; Tamminen et al ., 2012 ).

Enhancing biodiversity in forest ecosystems is currently one of the main objectives of sustainable forest management. Existing flora and fauna diversity, habitat, structural diversity, etc. should be taken into account in forest planning, and the effects of forestry on biodiversity should also be understood by forest practitioners ( Ferris & Pritchard, 2000 ). Thinning interventions modify stand structure, so they may affect different components of forest ecosystems. Among biodiversity components, the understory vegetation, deadwood, and stand structural diversity are often considered when studying the effects of forest management on biodiversity ( Lafond et al ., 2015 ).

Thinning results in greater availability of light, water and nutrients to the remaining trees and facilitates the development of an understory layer with a greater number of herbs, shrubs or tree species, mainly comprising early-successional species and aggressive native or non-native taxa. In contrast, repetitive thinning may lead to the appearance of one or just a few dominant understory species or there may be no evident effect when canopy cover pre-thinning is light ( Thomas et al ., 1999 ; Ares et al ., 2010 ). Hence, the impact will depend on thinning intensity and rotation time as well as on pre-thinning species composition. In the case of Scots pine stands in the Nordic countries, a peak in plant species richness is often found in young forest successions. Therefore young stand management ( i.e . pre-commercial thinning and initial thinning) may promote and sustain this diversity ( Widenfalk & Weslien, 2009 ). For example, some larger gaps could be opened up during the thinning operations to increase plant diversity. Over time there has been a trend towards denser forests in this region, which has led to a decrease in light demanding species and grassland species ( Hedwall & Brunet, 2016 ). Accordingly, in a study involving various forest species in North-Eastern Spain, including Scots pine, Torras & Saura (2008) observed that stand thinning had a positive effect on shrub abundance and on shrub and tree species richness.

Deadwood is a key component in the functioning of forest ecosystems and is both a habitat for fauna and host for decomposer biota. In general, managed stands that have been exploited for timber and firewood contain only a limited amount of deadwood in comparison to unmanaged forests. Thinning operations reduce tree mortality ( Mäkinen & Isomäki, 2004a ) and existing snags and logs are normally extracted from the forests. The harvesting process produces a small amount of small-diameter deadwood (logging residues) which, in order to reduce the risk of forest fire (mainly in the Mediterranean area) or pest attacks, is chipped and left on the floor or extracted for bioenergy purposes. Although studies focusing on the effects of thinning on amounts of deadwood and decay processes are lacking, various modelling studies have been carried out to estimate coarse woody debris in Scots pine forests ( e.g . Rouvinen et al ., 2002 ; Montes & Cañellas, 2006 ; Herrero et al ., 2010 ).

Structural diversity of stands is often used as an indicator of biodiversity ( McElhinny et al ., 2005 ; Alonso et al ., 2016), although more complex structures do not necessarily mean higher diversity ( Hunter, 1999 ). Thinning alters the structural diversity by reducing stand density along with the selective removal of trees from specific social classes, depending on the thinning method employed. Some studies concerned with thinning trials in Scots pine stands have reported that thinning from below reduced the coefficient of variation of the diameter distribution; this distribution being more skewed to the right and with higher kurtosis ( Montero et al ., 2000 ; Mäkinen & Isomäki, 2004b ; Crecente et al ., 2009 ). This greater homogeneity of tree size can also be expressed through the Gini coefficient ( de Camino, 1976 ), with lower values in thinned stands ( Fig. 4 ). Thinning from below may also reduce the range and standard deviation of tree heights ( Barbeito et al ., 2009 ).

Although public preferences for different types of forest may change over time (for educational reasons among others), many people consider unmanaged forests or old-growth forests to be unsuitable for recreation ( Lindhagen & Hörnsten 2000 ). Managed forests, which are generally more open and have more light, may be preferable for recreation ( Hale et al ., 2009 ). Thus, the forest management adopted must also consider recreational use, for example, by creating larger gaps and thinning the understory to create better views inside the forests ( Jankovska et al ., 2014 ). Thinning treatments can enhance preferences for recreation by facilitating accessibility and maintaining intermediate stand densities ( Jensen & Skovsgaard, 2009 ). Longer rotations are also preferable since harvesting and clear-cutting are often looked upon as disturbances that hinder recreation ( Holgen et al ., 2000 ). These studies provide a general picture of recreational preferences in all types of forest. However, specific studies focusing on thinned Scots pine stands are required. For example, in Scots pine stands it may be less important to open up gaps in the forest to improve views or provide amenities than in darker spruce or fir stands.