It is well known that resource supply plays an important part in structuring plant communities, however, it is still unclear whether increases in resource supply will quickly restructure communities, and then have consistent and persistent effects, or instead gradually and increasingly restructure communities over time. Understanding these dynamics is important because ecosystems are facing numerous long-term pressures which are changing resource supply in a directional and cumulative manner. Therefore, there is a need for a comprehensive understanding of how plant communities respond to chronic resource changes through time in order to make robust predictions about ecosystem structure and functioning in the future. Here, we examine how chronic nitrogen and water addition (both projected to increase at our study site) impact grassland plant community composition trajectories through time. We utilized a 10 year, full factorial water and nitrogen addition experiment at Cedar Creek (East Bethel, MN). Specifically, we considered responses of functional trait mean and variance shifts (functional dispersion, functional richness, and functional distance), species richness, evenness, and individual trait and species abundance shifts through time. Trait data (height, leaf N, SLA, and photosynthetic pathway) were gathered from TRY and Cedar Creek.
Results/Conclusions
We found that most aspects of community composition continue to deviate away from ambient trajectories through time. All plots were becoming more similar through time when considering presence-absence of trait combinations (decrease in functional distance), however, when considering abundance-weighted presence of trait combinations, plots that differed in either their nitrogen or water treatment became more dissimilar than plots that had the same treatments through time. Specifically, we found that nitrogen addition had an increasingly negative effect on functional richness (a measure of amount of trait space occupied) through time. Further, the interaction between nitrogen and water addition reduced functional dispersion (a measure of the spread of traits) increasingly through time while the individual treatments increased functional dispersion increasingly through time. Both nitrogen and water addition decreased species richness increasingly through time. Nitrogen addition increased the presence of species with larger leaf area and decreased the presence of C3 species through time, and water addition decreased the presence of species with high leaf nitrogen. Overall, our results show that the effects chronic resource addition tend to grow through time, indicating that both the duration and type of pressure are important determinants of future community structure.