1. Introduction
Annual tree rings record interactions between trees and environment [
1]. In conifers, the anatomical changes of tracheids through the growing season are characterized by the formation of narrower lumen areas, thicker walls and more dense wood along the gradual transition from earlywood to latewood [
2]. Variation in wood characteristics (e.g., tracheid lumen area, wood density, or latewood percentage) within and between trees is affected by a number of factors including stand structure, tree age, environmental conditions and genetics [
3,
4].
Physiological drivers of wood-density changes between and within tree rings are related to cambial activity and depend on factors modulating the activity of this meristem, including tree size and age, and seasonality in climate conditions [
5]. Moreover, wood density is sensitive to environmental changes, such as water availability, drought stress and temperature fluctuations [
6,
7]. However, the relationships between stand attributes, tree growth rate and wood density in conifers are unclear [
8].
Tree radial growth within forest stands mainly depends on the interactions between tree-to-tree competition and environmental conditions [
9]. Some of these stand factors, such as density, can be manipulated by forest managers to achieve desired wood quality outcomes [
10]. The effects of silviculture on wood properties, particularly timber density, are often assumed to be a consequence of their impacts on radial-growth rates. Wood density is also a reliable indicator of stem’s hydraulic properties, as it is related to both growth rate and hydraulic efficiency. A better understanding of the relationships between wood density and growth rates is relevant to comprehend the role played by forests in carbon sequestration and biomass production [
11]. Wood density has long been considered the most important wood quality attribute, and a high-density wood is usually associated with high lumber strength and stiffness [
12]. In general, wood density is negatively influenced by the tree growth rate in softwoods.
Tree growth and wood density can be directly managed by different silvicultural treatments to manipulate stand conditions [
13]. When properly used, thinning reduces long-term competition stress, and may increase resilience and resistance of trees to extreme drought events [
14,
15,
16]. On the other hand, radial growth responses to climate vary considerably according to stand tree density in water-limited areas such as the Mediterranean basin [
17]. A reduction of growth sensitivity to water shortage usually results from thinning practices in drought-prone areas [
18].
Forest management may therefore affect wood properties and tree growth performance through the intensity and type of thinning, changing tree species composition, altering trees’ competitive status and affecting whether juvenile or mature wood is being formed [
19]. Thinning interventions also positively affect the amount of incident light, available nutrients [
20] and temperatures inside the stand [
21]. All these factors can potentially impact tracheid growth and development, and in turn affect wood density. In fact, the effects of silvicultural management on wood properties have been studied over decades in productive tree species such as Scots pine (
Pinus sylvestris L.) [
22]. However, information on the effects of thinning on individual ring density and ring growth is meager and fragmentary [
23], and often shows conflicting results [
19,
24].
Thinning usually decreases competition among the remaining trees by improving the availability of water and nutrients in the soil, as well as solar radiation, thus increasing tree growth [
25]. However, different authors [
5,
26,
27] pointed out that there may be positive, negative or weak relationships between growth rate and wood density. Therefore, wood density has been recently described as a poor predictor of competitive ability among individuals of the same species [
28].
In this sense, small reductions in wood density after thinning have been reported for several conifer species, including
P. sylvestris [
19,
29],
Pinus banksiana [
30] and
Picea abies [
31]. Indeed, increased growth rate often decreases the density of earlywood and latewood [
31]. On the other hand, an increase in wood density after thinning has been reported in
Pinus taeda [
32] and
Pinus resinosa [
33]. Generally, overall ring density may remain unaffected although thinning may influence the distribution and density of earlywood and latewood components within annual rings [
29]. Such conflicting results arise because the effects of thinning on wood properties encompass many responses that may result in an increase, a decrease, or no effect on wood properties [
5]. Such responses are a function of multiple factors such as growing conditions, site, initial stocking or age at time of treatment [
34].
Given the need to explore the responses of ring width and density to thinning and their relationships with environmental factors, we selected a long-term thinning trial in Scots pine stands located close to its southwestern distribution edge (northern Spain), where drought stress has been found to affect radial growth [
35]. Scots pine is one of the most important commercial conifer species in Europe and one of the tree species with the widest geographical distribution in the world [
8]. Several studies have documented the influence of silvicultural treatments on radial growth and wood properties of Scots pine. For example, some authors suggested that it is possible to affect the uniformity of wood properties by thinning [
36]. Other authors reported an important increase of mean ring width but not significant changes in density after thinning, while a negative correlation between ring width and density has also been found [
29]. In the western Spanish Pyrenees, it was showed that light and moderate thinning weakly affected radial growth, which was enhanced three years after tree removal [
35].
To better understand the relationships between forest management, environmental factors and wood properties, this study aimed to explore and compare the individual ring characteristics of trees in two Scots pine forests in northern Spain subjected to different climatic conditions for 14 years after different thinning treatments, using scanning X-ray densitometry. Both sites exhibit different water supply regimes (limiting vs. not limiting). Several tree-ring parameters (width and density among them) were retrospectively quantified at annual resolution using X-ray densitometry [
5,
37]. Ring characteristics are useful indicators in forest management and product manufacturing because they are strongly correlated with other tree traits, such as diameter growth and mechanical wood properties [
5]. Some of these properties are related to intra-annual wood features (earlywood and latewood width and density, latewood percentage), which can be quantified using X-ray densitometric profiles [
38]. Therefore, the effects of thinning intensity on different factors affecting wood density, such as latewood percentage, average ring density, and earlywood and latewood densities, were examined. Our specific objectives were: (1) to characterize ring characteristics (ring width and density variation) after different thinning intensity treatments; and (2) to analyze the effects of thinning and climate on wood growth and density in response to drought stress.
With those objectives in mind, we hypothesized that: (1) thinning will increase tree ring width and reduce wood density by releasing growth limiting factors for the remaining trees; (2) post-thinning growth response will be influenced by local soil and climatic conditions, expecting wider tree ring widths and lower wood density after thinning in the site with more intense combined growth limitations (water, nutrients, light and growing space); and (3) the thinning effect will not be beneficial under extreme drought conditions due to the cancellation of the positive effects on growth of increasing nutrient, light and growing space availability after thinning because of the extreme water limitation during drought events.
4. Discussion
One of the aims of this study was to analyze the effects of thinning intensity on wood growth and density of Scots pine over a period of 14 years after the first thinning in naturally regenerated stands. Scots pine is a useful species in dendroecological studies since its radial growth clearly responds to environmental factors including drought stress [
50,
51]. It is widely known that thinning affects growth and wood properties, while the positive effect on tree diameter depends on the thinning regime [
52]. Moreover, the thinning response and its dependence on changes in growing conditions provide highly relevant information for use in practical forest management, potentially allowing optimizing the timing and intensity of thinning [
36].
Our results support our first hypothesis that thinning enhanced radial growth (higher BAI, RW, EW, and LW values) as compared to unthinned plots. However, the effect of thinning intensity at the Mediterranean site depended on the frequency management interventions. Thus, the first thinning had no effect on radial-growth attributes but the second thinning did. In fact, effects of the second thinning on radial-growth attributes in the Mediterranean stand (Aspurz) were similar to the corresponding effects of the first thinning at the continental site (Garde). The absence of significant thinning effects on growth at the Mediterranean site might be due in part to the fact that the stand structure was initially highly variable among plots [
35]. Additionally, such differences in effects by the first thinning between sites could indicate a stronger nutrient limitation at the continental site (as previously reported for the same stands, [
53,
54]), despite a higher water limitation at the Mediterranean site [
43]. Although the Mediterranean site has a sandy texture that retains slightly less water than the continental soil, its soil is also deeper, and as a consequence both study sites show similar water retention capabilities. This indicates that difference in water limitation are due to water supply, rather than soil water retention capacity.
Our results also indicated that diameter growth at both sites increased more substantially in the heavily and moderately thinned plots than in the unthinned plots, although differences between T20 and T30/40 were not significant. Most research supports that thinning treatments increase annual ring widths [
23,
29,
55], but the positive effect on tree diameter depends on the thinning regime [
52]. Some authors indicate that the increase in diameter growth immediately after thinning in Scots pine stands was most marked in the heavily thinned plots [
36]. Other authors found that radial growth increased more on heavily thinned plots than on less intensively thinned or unthinned plots in
Pinus taeda [
56,
57]. Similar observations were made for
P. abies [
58] and in
P. sylvestris [
59]. Moreover, thinning is proven to improve the recovery of radial growth following drought in Scots pine [
60]. In this study, a trend for larger LWP was only observed for T30/40 plots at the Mediterranean site. Similar to this result, a moderate thinning treatment produced higher LWP than other treatments in Mediterranean stands of
Pinus brutia [
55]. Furthermore, this author described significant positive relationships between RD and LWP from unthinned and thinned stands. Other researchers have also indicated that thinning could increase latewood growth, leading to increased wood density [
5]. Effects of the second thinning on the ring-width attributes at the Mediterranean site occurred much faster than after the first thinning, suggesting faster crown development after the second thinning.
A general decrease in values of density attributes relative to pre-thinning conditions was observed after the second thinning at the Mediterranean site (Aspurz). On the contrary, all these values increased after thinning at the continental site (Garde). Such a density reduction pattern in Aspurz might be related to a more pronounced decreasing trend over time as a result of water surplus at this site (see
Figure 1). Nevertheless, the thinning effect on ring density was stronger at the continental site, a fact that did not support our second hypothesis. Similar to our results, other studies have reported contradictory results on the effects of thinning on wood density. Different authors stated that thinning has little or no effect on wood density [
5,
29,
55]. On the other hand, other researchers concluded that thinning resulted in increased growth and a slight reduction in wood density [
31], or suggested that wood density is reduced by thinning treatments [
61,
62]. Moreover, some reports have shown that thinning treatments can cause an increase in wood density [
63,
64].
Indeed, heavy thinning caused significantly higher ring densities (RD) in various periods (i.e., 2002–2003, 2008–2012) and larger RWs were found after moderate thinning (i.e., 2007, 2010–2013) at the Mediterranean site, as previously reported for RW changes in the same study area [
35]. Conversely, the same thinning treatments did not produce any significant changes at the continental site. In this way, the second thinning treatment carried out in the Mediterranean site led to RD and RW increases. However, thinning treatments did not cause immediate increments of RW nor RD in the first year after thinning.
Tree responses to environmental conditions occur at seasonal temporal scales [
2]. For instance, drought events during the growing season can trigger remarkable changes of wood density [
65], and can even induce adult tree death as a consequence of long or intense events [
66]. Estimation of drought events impacts on Scots pine growth thus becomes important considering that previous studies in Spain have demonstrated that Scots pine displays the highest mortality rate among several pine species under severe droughts [
67,
68]. Particularly severe droughts in some specific years after the thinning treatments were observed during the studied period.
Although the results revealed differences between study sites in annual changes, important decreases in ring width were found for similar years (2005 and 2012–2013). Similarly, the driest and warmest years on record at both sites after the first thinning was applied were 2003, 2005, 2011 and 2012, with 2011 also being the driest year since 1920 at both sites (
Figure 1). At both study sites, a significant and common reduction of RD and RW occurred in 2003, 2005 and 2012. The 2003 heat wave is considered proof of the global warming effects on European productive systems including agricultural and forestry sectors [
69]. For instance, in northern Spain, 2003 was one of the warmest years at both study sites during the last 50 years. At the Mediterranean site, 2005 and 2012 showed a rainfall decrease of 24.8% and 21.2%, respectively, compared with the mean annual precipitation. However this effect was significantly more pronounced at the continental site, where a higher temperature variation was observed for that year. At the continental site, an increase of 32.4% in mean temperatures was detected in 2003, which was observed as the warmest year during the studied climatic series. In addition, a rainfall decrease of 27.5% compared with the mean annual precipitation was recorded in 2005, which was also considered one of the driest years during the study period (
Figure 1).
Moreover, anomalous ecophysiological reactions have been reported in several tree species after those heat waves [
65]. For example, the 2005 and 2012 droughts have been also reported as some of the most important dry spells affecting Mediterranean Spanish forests [
70], particularly because they followed previous years that were also drier than usual. At the continental site, an important decrease of RD and RW occurred during 2012–2013. In this sense, Primicia et al. [
35] indicated that the 2005 drought might have cancelled out the effects of thinning on growth because an abrupt growth reduction was observed in trees from the Mediterranean site. Water stress promotes a strong reduction in stomatal conductance, together with an enhancement of respiration rates and a decrease in the stored C pool [
71], eventually leading to decreased growth. One of the main consequences of these drought and heat waves for the trees was not only a dramatic reduction in growth, but also lower ring density, as a response to a reduction in soil water availability [
66]. This is consistent with the production of less dense latewood tracheids, less lignified and with wider lumen areas and narrower cell walls leading to a more effective water-conducting (albeit more vulnerable to cavitation) hydraulic architecture [
72]. Thinning influenced the relationship between climate and ring width at the Mediterranean site, and also eliminated the pre-thinning climatic significant influence on growth at the continental site. In addition, we found a significant influence of drought on wood density only at the Mediterranean site. Scots pine, as a water stress avoider, can close stomata during the critical insolation hours of the hottest days [
73]. In addition, lowering LD during exceptionally dry years could have been an adaptation to autumns with scarce water availability following dry summers, as experienced at our sites during the last 20 years.
For the period before thinning was carried out, the driest years at the Mediterranean site were 1989 and 1993, and the warmest year was 1997. One year after each of these climate extremes, a drop in wood density was found (
Figure 5). Similarly, at the continental site, 1993 and 1998 were the driest years before thinning (
Figure 1), also coinciding with significant drops in ring density (
Figure 5). However, our results are in contrast with previous reports on dry spring conditions leading to denser earlywood, theoretically characterized by narrower lumen tracheids, as has been observed in Spanish juniper (
Juniperus thurifera, [
6]). Such contradictory results could be explained because dry spells and heat waves lead to abrupt changes in wood phenology and water use, causing the enlargement of water-conducting cells even if growth is reduced [
72]. Previous research indicated that trees surviving heat and drought waves have a significantly higher mean ring density, i.e., they form tracheids with more narrow lumens less susceptible to embolism [
2]. Our results could indicate that trees under water stress face a complicated balancing act: they could adapt to severe drought by reducing hydraulic resistance during that year (lowering RD), but the increased embolism risk may force trees to quickly increase RD the year after (
Figure 7). As these extreme drought events are becoming more frequent under the general scenario of climate change [
74], it should also be expected an increase in variation in temperatures and precipitation. In this context, previous and recent studies in northern Spain already suggested that the decrease in rainfall and the increase in mean temperature (and therefore evapotranspiration) must be considered as the most important factors that will affect growth of the Scots pine [
50,
75]. Therefore, greater fluctuations in Scots pine wood density are to be expected. Consequently, a great natural variability could appear in terms of wood density, strength, and appearance because of the wide range of growth conditions for the trees, affecting pine wood’s potential for different uses and its economic value [
12].
In this study, moderate or heavy thinning treatments generally enhanced Scots pine radial growth, as the results showed that average ring width values increased significantly in the thinned plots (T20 and T30/40) compared to the unthinned (T0), although two thinning events were needed at the Mediterranean site to reach such an effect, whereas at the continental site, one thinning event was enough to cause significant differences among treatments. However, a general decrease of density attributes was detected after the second thinning at the Mediterranean site, whereas an increase in these values was observed at the continental site. The dynamics over time of such effects depended on site and thinning treatment and were modified by droughts and heat waves, supporting our third hypothesis. These results demonstrate the complex interactions between site, management and climate. Moreover, our results indicate that there were significant, albeit weak, relationships between width and density attributes in Scots pine trees at the two study sites, as indicated by other authors [
23]. This fact agrees with the review which shows that growth rate is one of the many factors that can influence wood density [
5]. In general, thinning effects on ring density are smaller than effects on ring width [
29]. Several researchers have found different relationships between tree ring growth and density. Some authors reported that RD generally tends to decrease with increasing RW [
61]. Other authors also stated a negative relationship between annual growth rate and wood density in Scots pine [
8,
29]. It has also been reported that wood density is not significantly influenced by growth rate [
76], and that wood density is independent of the growth rate [
77]. As a consequence, it is increasingly challenging to predict the effect of future thinning on wood mechanical properties of these forests.