During assembly, communities may develop: deterministically towards a single composition, stochastically, into unpredictable compositions contingent on biotic or abiotic factors, or into alternative stable states depending on arrival order. Assembly rules are found when the presence or abundance of early arriving species restrict the establishment of later arriving species (e.g. priority effects). Restorations allow for experimental tests of these alternative hypotheses. We examined the role of early arriving species on later community composition using cover crop species seed treatments and control plots. Cover crops are hypothesized to facilitate establishment by reducing weed abundances without competing with the target mix. Treatments included seeding one annual or biennial cover crop species (tillage radish, oats, or gray primrose), a perennial species Canada wild rye, two grass-forb species combinations, or nothing as a control in fall 2015. All plots received a diverse prairie seed mix in March 2016. Community composition and species diversity were sampled in the following three years and were compared among treatments with perMANOVAs and ANOVA.
Results/Conclusions
Species composition and species diversity differed among cover crop treatments in all four sampling dates (perMANOVA, P values all < 0.001), suggesting the existence of assembly rules. The first year, wild rye reduced Amaranthus sp. and side oats gramma from the seed mix, and this expanded to other species from the seed mix in years two and three. The strength of the effect on cover crops increased over time (i.e., F increased from 3.9 to 12.2). Cover crop species varied in their effectiveness at reducing weed abundances, with wild rye being especially effective at reducing weeds. However, wild rye also greatly reduced establishment and species diversity of the prairie seed mix. Compared to oats, species richness was 41% lower in wild rye plots (p<.0001). Other cover crops such as oats and primrose were largely neutral in their effects on prairie establishment. Cover crops can have beneficial effects in early years, but they may also reduce prairie establishment, especially when the cover crop species is perennial. We found strong evidence that priority effects did influence the outcome of community assembly in a perennial grassland restoration.