Plant species abundance mediates plant traits and ecosystem processes

Background/Question/Methods

Many biodiversity studies have demonstrated that plant species richness alters ecosystem processes. Few studies have focused on the impacts of changes in species abundance (i.e. evenness). However, species abundance is the measure of community composition that most often and rapidly responds to environmental conditions and anthropogenic stressors. Understanding how species abundance affects ecosystem processes is challenging because community effects are often “non-additive” (greater or smaller than what would be expected based on monoculture values, weighted by abundance). The goal of this study was to investigate how shifts in species relative abundance affect key ecosystem processes (nitrogen cycling rates, plant production, microclimate regulation) and plant traits (phenology, height). We planted experimental plots in a California grassland to create 8 replicates of two-species communities that varied in the relative abundance of component species (Aegilops triuncialis, noxious invasive grass and Bromus hordeaceus, forage grass). We designed seed mixtures to create communities with the following ratios of B. hordeaceus: A. triuncialis: 0:100, 5:95, 25:75, 50:50, 75:25, 95:5, 100:0. We measured ecosystem processes at three time points during the 9 month growing season (fall, winter, spring). Traits were measured at the end of the growing season when these species are flowering and senescing.     

Results/Conclusions

Species abundance had non- additive effects on plant production, but additive effects on soil microclimate regulation. In winter, monoculture biomass did not differ between species, but as the relative abundance of the species became more even, biomass increased (r² = 0.13, p = 0.028) and was higher than expected based on the monoculture biomass values. In contrast, in the spring, as the relative abundance of the species became more even, biomass decreased (r² = 0.14, p = 0.0036) and was lowest when the species proportions were approximately even. Soil temperature effects were additive. In monoculture B. hordeaceus had higher soil temperature than A. triuncialis (p = 0.0378), and in mixture, as B. hordeaceus increased in abundance there was a linear increase in soil temperature (r² = 0.22, p = 0.01). Species abundance had weak effects on phenology. For each species, as its abundance increased, its phenology tended to be earlier for initiation of seed production, senescence, and time of maximum height. Abundance had no effect on flowering phenology or end of season height. These results highlight the importance of considering species abundance when predicting the effects of a vegetation change on ecosystem processes.