The role of evolutionary history in a changing climate: Combined effects of phylogenetic dissimilarity, carbon dioxide, and nitrogen

Background/Question/Methods

Biodiversity is fundamental to ecosystem services, and ecosystem function generally improves with increasing species richness. The most common explanations for the relationship between biodiversity and ecosystem function are driven by plant traits; this suggests that evolutionary history, which generates trait diversity, may influence ecosystem function. Unfortunately, biodiversity is declining across the globe, and climate change is a major driver of this decline. Climate change will alter CO₂ and nitrogen (N) concentrations and also shift the composition of plant communities. However, the ecological importance of biotic environmental shifts relative to CO₂ and N is not well known. We established an experiment that used 26 Tasmanian eucalypt species (from subgenera Eucalyptus and Symphyomyrtus), planted in species monocultures and mixtures. The mixtures manipulated both species richness (3 or 6 species) and phylogenetic diversity (1 or 2 subgenera). We also manipulated CO₂ and N concentrations. We attempted to answer three broad questions: (1) Do different phylogenetic groups respond differently to increases in biodiversity (measured either at the species or subgenera level), (2) Do different phylogenetic groups respond differently to increases in CO₂ and N, and (3) How important is phylogenetic dissimilarity relative to concentrations of CO₂and N?

Results/Conclusions

We found that plants in pots that experienced elevated CO₂ produced more belowground and aboveground biomass. The effect sizes of CO₂ on aboveground biomass and belowground biomass were similar, suggesting that CO₂ was not having a disproportionately large impact on belowground plant structures. CO₂ and N appeared to be co-limiting, as manipulations of only CO₂ or only N had no effect on productivity, but manipulating both CO₂ and N simultaneously resulted in significant increases in total productivity, as well as aboveground and belowground productivity. We also found that the outcomes of species interactions were affected by the evolutionary relationships (i.e., phylogenetic similarity) between interacting plants, but the effects of evolutionary relationships were not dependent on manipulations of CO₂ and N. Within a pot, more genetic similarity generally increased total mixture-level productivity. The effect size of phylogenetic similarity was roughly 40% of CO₂ manipulation, and 35% of N manipulation. Determining the importance of phylogenetic similarity in light of these results is subjective and different researchers will likely arrive at different opinions. However, our results provide an initial example of how phylogenetic similarity can influence plant productivity to a degree that is comparable to manipulations of CO₂ and N.