Tania L. Maxwell, Nicolas Fanin, William C. Parker, Mark R. Bakker, Ariane Belleau, Céline Meredieu, Laurent Augusto, Alison D. Munson
This is a plain language summary of a Functional Ecology research article that can be found here.
Increasing the number of tree species within a forest has been shown to improve nutrient cycling due to factors such as complementarity between species that vary in structure and function. For example, mixing tree species may improve stand-level nutrient use efficiency (NutUE), which is the capacity to produce biomass per amount of nutrients used. However, the effect of mixing tree species should be further studied in relation to how climate change (e.g. changing water supply) may impact this effect. Our study tested the interactive effect of mixing tree species and increasing water availability on NutUE, using young temperate forest experimental plantations (ORPHEE, southwestern France and IDENT-SSM, northern Ontario, Canada). These sites manipulate both tree diversity (monocultures vs. mixtures) and water availability (via irrigation).
We found significant tree species richness and water availability effects on NutUE, but they were weakly and inconsistently expressed, detected only for specific nutrients, and differed between the two experimental sites. Species identity effects proved to be more explanatory in birch-pine mixtures at both sites. At ORPHEE, both nitrogen use efficiency (NUE) and phosphorus use efficiency (PUE) were significantly greater in the pine-birch mixture than in either monoculture. This was due to an increase in aboveground net primary production together with a decrease in litterfall nutrients, driven by the dominance of pine in this mixture. In the younger and denser plots of IDENT-SSM, birch dominated the canopy space and led to a positive effect of mixing on both biomass production and litterfall nutrient concentrations, leading to a neutral effect of mixing on NUE and PUE. Overall, the effects of mixing were similar in low and high water treatments, suggesting that species composition is more important than water availability for NutUE in young forest stands.
In conclusion, tree species composition (rather than species richness) is a stronger driver for nutrient use efficiency (NutUE) than water availability, and mixing certain tree species can be a viable way to improve stand-level NutUE in forests, provided that the complementarity of species is correctly assessed.
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