Trajectories of species and functional change during California serpentine grassland assembly

Background/Question/Methods

Understanding the forces that govern the structure and development of communities are among key challenges in ecology.  We used a series of experimental plant communities in California serpentine grassland test: a) how initial composition affects subsequent plant community development and b) whether species and functional compositions show similar responses to differences in initial composition. The communities were established in 1992 for a previous experiment investigating how functional richness and composition influence ecosystem processes. Four functional groups that differ in the timing of peak growth and other functional traits important for resource capture (e.g. rooting depth) initially comprised the experimental communities: Early Season Annuals (E), Late Season Annuals (L), Perennial Bunch Grasses (P), and Nitrogen Fixers (N).  The experimental treatments included: no plants (B-bare), a single functional group (E,L,P,or N), two functional groups (EL,EP, LP), three functional groups (ELP) or all four functional groups (ELPN).  Since 2002, these plots have been naturally colonized by species from the surrounding grassland.  During the first six years of colonization (2002-2007) and again in 2012, we measured species composition and relative abundance in these plots, allowing us to examine how community composition has changed over the past eleven years. 

Results/Conclusions

We tracked changes in community composition, both at the species and functional levels, using nonmetric multidimensional scaling (NMS), Analysis of Similarities (ANOSIM), and distance metric analysis.  Over the first five years of the study period, the communities converged in functional composition while remaining distinct in species composition. This initial pattern was reversed in 2012, where functional composition diverged and species composition converged relative to 2007 data.  Our results suggest that plant communities are dynamic and structured by a myriad of factors (e.g. climate, dispersal, resource availability, species interactions, etc.) that interact over time.