Processes influencing community assembly during grassland restoration

Background/Question/Methods

Community assembly theory attempts to explain processes driving local community structure, which can extend to affect ecosystem functioning. If community assembly is deterministic, then community composition is predictable under certain physical (environmental) and biotic conditions. One approach to revealing the relative strength of each process on community assembly is to establish replicated communities under similar and different conditions. The same species were sown at the same rate into four plots delineated a former agriculture field in 2010 (Sequence I) and 2012 (Sequence II). We relied on interannual variability in climate to provide different environmental conditions at the start of each restoration sequence. The percent cover of all plant species was visually estimated in permanent 10 m²sampling areas. Aboveground net primary productivity (ANPP) was estimated by clipping plants at peak biomass. All plant responses were measured for three consecutive years in each sequence. Community composition and ANPP were compared between restoration initiation years (sequences) and over time using PERMANOVA and mixed model analyses, respectively. 

Results/Conclusions

Stochastic events are needed to reveal community assembly processes. Growing season precipitation in the establishment year was representative of average conditions in 2010 for Sequence I (402.3 mm) and drought conditions in 2012 for Sequence II (268.3 mm). Total, sown, and volunteer species composition differed between the sequences and interacted with age (PERMANOVA: P = 0.001). Community composition differed by year 3 between sequences due to a greater increase in sown species cover in Sequence I relative to Sequence II, which was dominated by a drought tolerant weed in year 2. Total, sown and volunteer ANPP also exhibited different patterns over time in each sequence (all mixed models: P < 0.05). Total ANPP in both sequences increased from year 1 to year 2. Total ANPP continued to increase into year 3 in Sequence I, but decreased from year 2 to year 3 in Sequence II due to the decrease in volunteer ANPP. Sown ANPP increased over time in both sequences, but to a greater extent in Sequence I coinciding with a decrease in the ANPP of volunteer species. These data suggest that interannual climate variability is a stochastic factor influencing community assembly and ecosystem function in restored grassland.