Climate‐ and successional‐related changes in functional composition of European forests are strongly driven by tree mortality

Abstract

Intense droughts combined with increased temperatures are one of the major threats to forest persistence in the 21st century. Despite the direct impact of climate change on forest growth and shifts in species abundance, the effect of altered demography on changes in the composition of functional traits is not well known. We sought to (1) quantify the recent changes in functional composition of European forests; (2) identify the relative importance of climate change, mean climate and forest development for changes in functional composition; and (3) analyse the roles of tree mortality and growth underlying any functional changes in different forest types. We quantified changes in functional composition from the 1980s to the 2000s across Europe by two dimensions of functional trait variation: the first dimension was mainly related to changes in leaf mass per area and wood density (partially related to the trait differences between angiosperms and gymnosperms), and the second dimension was related to changes in maximum tree height. Our results indicate that climate change and mean climatic effects strongly interacted with forest development and it was not possible to completely disentangle their effects. Where recent climate change was not too extreme, the patterns of functional change generally followed the expected patterns under secondary succession (e.g. towards late‐successional short‐statured hardwoods in Mediterranean forests and taller gymnosperms in boreal forests) and latitudinal gradients (e.g. larger proportion of gymnosperm‐like strategies at low water availability in forests formerly dominated by broad‐leaved deciduous species). Recent climate change generally favoured the dominance of angiosperm‐like related traits under increased temperature and intense droughts. Our results show functional composition changes over relatively short time scales in European forests. These changes are largely determined by tree mortality, which should be further investigated and modelled to adequately predict the impacts of climate change on forest function.

Multiple abiotic and biotic pathways shape biomass demographic processes in temperate forests

“…We found that biomass loss resulting from the mortality of mature trees accounted for most of the fine-scale variation in net biomass change in our plots, thus supporting recent studies that have found mortality to be a key driver of aboveground biomass dynamics in natural forests (Delbart et al 2010, Ruiz-Benito et al 2017b. In contrast to previous empirical studies , van der Sande et al 2017, our results suggest that biomass mortality in our region can be adequately predicted using the combination of biotic and abiotic variables we selected (Fig.…”

“…For instance, tree mortality (both natural and as a result of logging) has played a key role in driving recent changes in the functional composition of European forests (Ruiz‐Benito et al. , b). These, in turn, would likely alter the relationship between functional diversity and ecosystem functioning in these ecosystems.…”

“…Based on a priori expectations (Ruiz‐Benito et al. , van der Sande et al. ), we constructed one structural equation model (SEM) for each of the three demographic processes as outlined in Fig.…”

“…Prior to analyses, the three demographic variables were log-transformed and all predictors were standardized to have a mean of 0 and a standard deviation of 1 in order to improve the interpretability of regression coefficients (Schielzeth 2010). Based on a priori expectations (Ruiz-Benito et al 2017b, van der Sande et al 2017), we constructed one structural equation model (SEM) for each of the three demographic processes as outlined in Fig. 1.…”

Spatial variation in climate modifies effects of functional diversity on biomass dynamics in natural forests across Canada

“…Functional identity was measured as the CWM of the continuous trait values (Ratcliffe et al, ). Similar to previous studies (Ruiz‐Benito et al, ), we performed a principal components analysis (PCA) with CWMs of the eight traits to obtain a comprehensive functional identity to represent them, because these values were highly correlated with each other [e.g., CWM Nmass with CWM Pmass ( r = .88), CWM SLA ( r = .88), CWM Habit ( r = .88), CWM Struct ( r = .88), CWM WD ( r = .53); CWM DT with CWM ST ( r = .62); Supporting Information Figure S2]. We used the first axis (CWM PC1 , which explained 53% of the variation) of the PCA as a variable of functional identity, because it collectively represents traits associated with resource acquisition (Chave et al, ; Niinemets & Valladares, ; Reich, ; Wright et al, ), being strongly related to higher CWM Nmass , CWM Pmass , CWM SLA , CWM Habit (i.e., deciduous), CWM Struct (i.e., broadleaves) and CWM WD and relatively associated with lower CWM ST (Supporting Information Figure S2).…”