Climate Modulates the Effects of Tree Diversity on Forest Productivity. Jucker, T., Avăcăriței, D., Bărnoaiea, I., Duduman, G., Bouriaud, O., & Coomes, D. A.
Climate Modulates the Effects of Tree Diversity on Forest Productivity [link]Paper  doi  abstract   bibtex   
[Summary] [::] Despite growing evidence that, on average, diverse forests tend to be more productive than species-poor ones, individual studies often report strongly contrasting relationships between tree species richness and above-ground wood production (AWP). In the attempt to reconcile these apparently inconsistent results, we explored whether the strength and shape of AWP-diversity relationships shifts along spatial and temporal environmental gradients in forests across Europe. [::] We used tree ring data from a network of permanent forest plots distributed at six sites across Europe to estimate annual AWP over a 15-year period (1997-2011). We then tested whether the relationship between tree species richness and AWP changes (i) across sites as a function of large-scale gradients in climatic productivity and tree packing density and (ii) among years within each sites as a result of fluctuating climatic conditions. [::] AWP-species richness relationships varied markedly among sites. As predicted by theory, the relationship shifted from strongly positive at sites where climate imposed a strong limitation on wood production and tree packing densities were low, to weakly negative at sites where climatic conditions for growth were most suitable. In contrast, we found no consistent effect of interannual fluctuations in climate on the strength of AWP-species richness relationships within sites. [::Synthesis]. Our results indicate that the shape and strength of the relationship between tree diversity and forest productivity depends critically on environmental context. Across Europe, tree diversity shows the greatest potential to positively influence forest productivity at either end of the latitudinal gradient, where adverse climatic conditions limit productivity and lead to the development of less densely packed stands. [Excerpt: Implications for Forest Conservation and Management] Identifying where and when tree diversity has the greatest potential to positively influence forest productivity has important implications for forest management and conservation practises, as well as efforts to mitigate climate change (Cardinale et al. 2012; Zhang, Chen & Reich 2012; Scherer-Lorenzen 2014). We found that in terms of maximizing rates of wood production, the benefits of maintaining diverse forests are most pronounced in systems where environmental conditions strongly limit productivity. While our study provides a useful framework for predicting under which conditions tree diversity is likely to matter most, there are, however, several reasons why practises aimed at maintaining diverse forests should not necessarily be limited to specific ecological contexts or geographic regions. For instance, in addition to promoting forest productivity, tree diversity has also been shown to help stabilize wood production over time across a range of forest types, highlighting the fact that mixed-species forests are able to remain productive under a wider range of environmental conditions than monocultures (Jucker et al. 2014a; Morin et al. 2014). Secondly, although we detected clear differences in the importance of tree diversity as a driver of productivity among forest types, only one site showed any indication of a negative association between diversity and productivity. Consequently, even though gains in productivity may be negligible for certain forest types, maintaining diverse forests is unlikely to adversely affect wood production and has the advantage of delivering a number of added ecological and economic co-benefits (e.g. reduced risk of pest and pathogen outbreaks, increased associated biodiversity, greater soil carbon storage; Scherer-Lorenzen 2014). [\n] In addition to highlighting under which circumstance tree diversity is currently most important for forest productivity, our study also provides a number of clues as to how climate change is likely to influence AWP-diversity relationships in future forests. By the end of this century, Mediterranean forests in Europe are expected to suffer more frequent and prolonged periods of drought, while boreal systems are predicted to warm considerably and experience longer growing seasons (Jacob et al. 2014). As a result, diversity effects may weaken in strength, particularly in the case of boreal forests where spring warming is expected to reduce the degree of phenological mismatch among coexisting tree species (Polgar & Primack 2011). More importantly, however, our results suggest that longer-term responses of forests to climate change - such as changes in species composition and forest structure - are what will ultimately determine how the relationship between tree diversity and forest productivity will look like in the future.
@article{juckerClimateModulatesEffects2015,
  title = {Climate Modulates the Effects of Tree Diversity on Forest Productivity},
  author = {Jucker, Tommaso and Avăcăriței, Daniel and Bărnoaiea, Ionuț and Duduman, Gabriel and Bouriaud, Olivier and Coomes, David A.},
  date = {2015-12},
  journaltitle = {Journal of Ecology},
  pages = {n/a},
  issn = {1365-2745},
  doi = {10.1111/1365-2745.12522},
  url = {https://doi.org/10.1111/1365-2745.12522},
  abstract = {[Summary]

[::] Despite growing evidence that, on average, diverse forests tend to be more productive than species-poor ones, individual studies often report strongly contrasting relationships between tree species richness and above-ground wood production (AWP). In the attempt to reconcile these apparently inconsistent results, we explored whether the strength and shape of AWP-diversity relationships shifts along spatial and temporal environmental gradients in forests across Europe.

[::] We used tree ring data from a network of permanent forest plots distributed at six sites across Europe to estimate annual AWP over a 15-year period (1997-2011). We then tested whether the relationship between tree species richness and AWP changes (i) across sites as a function of large-scale gradients in climatic productivity and tree packing density and (ii) among years within each sites as a result of fluctuating climatic conditions.

[::] AWP-species richness relationships varied markedly among sites. As predicted by theory, the relationship shifted from strongly positive at sites where climate imposed a strong limitation on wood production and tree packing densities were low, to weakly negative at sites where climatic conditions for growth were most suitable. In contrast, we found no consistent effect of interannual fluctuations in climate on the strength of AWP-species richness relationships within sites.

[::Synthesis]. Our results indicate that the shape and strength of the relationship between tree diversity and forest productivity depends critically on environmental context. Across Europe, tree diversity shows the greatest potential to positively influence forest productivity at either end of the latitudinal gradient, where adverse climatic conditions limit productivity and lead to the development of less densely packed stands.

[Excerpt: Implications for Forest Conservation and Management]

Identifying where and when tree diversity has the greatest potential to positively influence forest productivity has important implications for forest management and conservation practises, as well as efforts to mitigate climate change (Cardinale et al. 2012; Zhang, Chen \& Reich 2012; Scherer-Lorenzen 2014). We found that in terms of maximizing rates of wood production, the benefits of maintaining diverse forests are most pronounced in systems where environmental conditions strongly limit productivity. While our study provides a useful framework for predicting under which conditions tree diversity is likely to matter most, there are, however, several reasons why practises aimed at maintaining diverse forests should not necessarily be limited to specific ecological contexts or geographic regions. For instance, in addition to promoting forest productivity, tree diversity has also been shown to help stabilize wood production over time across a range of forest types, highlighting the fact that mixed-species forests are able to remain productive under a wider range of environmental conditions than monocultures (Jucker et al. 2014a; Morin et al. 2014). Secondly, although we detected clear differences in the importance of tree diversity as a driver of productivity among forest types, only one site showed any indication of a negative association between diversity and productivity. Consequently, even though gains in productivity may be negligible for certain forest types, maintaining diverse forests is unlikely to adversely affect wood production and has the advantage of delivering a number of added ecological and economic co-benefits (e.g. reduced risk of pest and pathogen outbreaks, increased associated biodiversity, greater soil carbon storage; Scherer-Lorenzen 2014).

[\textbackslash n] In addition to highlighting under which circumstance tree diversity is currently most important for forest productivity, our study also provides a number of clues as to how climate change is likely to influence AWP-diversity relationships in future forests. By the end of this century, Mediterranean forests in Europe are expected to suffer more frequent and prolonged periods of drought, while boreal systems are predicted to warm considerably and experience longer growing seasons (Jacob et al. 2014). As a result, diversity effects may weaken in strength, particularly in the case of boreal forests where spring warming is expected to reduce the degree of phenological mismatch among coexisting tree species (Polgar \& Primack 2011). More importantly, however, our results suggest that longer-term responses of forests to climate change - such as changes in species composition and forest structure - are what will ultimately determine how the relationship between tree diversity and forest productivity will look like in the future.},
  keywords = {*imported-from-citeulike-INRMM,~INRMM-MiD:c-13917201,~to-add-doi-URL,biodiversity,climate,diversity,habitat-suitability,mixed-forests,wood-production}
}
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