COS 147-4 - Functional and taxonomic diversity impacts on aboveground biomass in a model Populus system

Abstract: There is mounting empirical evidence from natural and managed ecosystems of a general positive relationship between biodiversity and productivity. However, there remain numerous counter examples where species richness is unrelated or negatively correlated with productivity in plant communities. A possible explanation for these exceptions is that functional diversity, rather than taxonomical diversity drives biodiversity/productivity relationships. The goals of this study were to test the relative impact of taxonomical diversity and functional diversity on the productivity of trees using short rotation Populus spp. and their hybrids as a model system. We established monocultures and mixed clonal plantings of six Populus clones from three different taxa (P. deltoides, hybrids of P. deltoides × P. maximowiczii, and hybrids of P. deltoides × P. trichocarpa). The mixed clone plantings were designed to be either more functionally diverse, with two clones of the same taxa but contrasting nitrogen use characteristics, or more taxonomically diverse with three clones, each of a different taxon, but with more similar nitrogen use characteristics. There was also a treatment that included all six clones. The different clonal treatments were replicated eight times at two different sites (n=16) as part of a larger study. We hypothesized that the more functionally diverse mixture was most likely to have increases in productivity. Following two years of growth, the diameter and height of all trees were measured, and a subset of trees were destructively sampled to generate allometric relationships to estimate the above ground biomass of all trees. Here we report above ground biomass, as a proxy for productivity, of mixed clone plantings compared to the weighted average of their constituent monocultures. We found that, on average, functionally diverse two-clone mixtures overyielded by producing more above ground biomass than either of their constituent monocultures, while the more taxonomically diverse three-clone mixtures underyielded, producing less above ground biomass than the average of their constituent monocultures. The mixture with all six clones was not significantly different from the average of all monocultures. The results of this study support the importance of funcitonal diversity when considering biodiversity productivity relationships, particularly when considering increasing biodiversity in managed ecosystems. Our results do suggest that, with careful implementation, increasing biodiversity in managed ecosystems, like Populus biomass plantations, may be a viable strategy for increasing production.