Development in a six year tall grass restoration experiment: Effects of planting season and density on diversity

Background/Question/Methods

Ecological drivers that alter restored plant communities diversity are not well understood. Factors that control diversity during succession are often not isolated from each other. Instead, they interact and may operate at different spatial and temporal scales. Here we test the effect of planting season, planting density, and mammalian herbivory on diversity and composition during six years of a tall grass restoration experiment at the Morton Arboretum, in Chicago, Illinois from 1998 to 2003. Experimental restoration plots were originally designed to test the effect of small herbivores on planted tall grass community of 18 species. Here we ask how poorly understood factors, such as planting season, and planting density affected diversity and how they interacted with each other and herbivory.

Results/Conclusions

Repeated measures ANOVA reveal that Simpson’s diversity is higher in December planting than June planting plots, and this difference persists across all six years (F_5,64 = 15.45 , Wilks’s lambda P≤0.001). Low planting density treatments had lower diversity than high planting density treatments for most of the experiment. However, by the end, diversity was the same in high and low treatments plots (F_5,64 = 3.11 , Wilks’s lambda P≤0.05). Interactions between planting season and mammal treatments were important for diversity (F_5,64 = 2.73, Wilks’s lambda P≤0.05). Planting season and planting density interactions were important for diversity across years too (F_5,64 = 3.28, Wilks’s lambda P≤0.05). This analysis revealed three important outcomes. First, planting season is very important for diversity. Early winter planting favors diversity relative to early Summer planting and this effect persisted for six years. Second, planting density and mammals effects on diversity were dynamic, changing from year to year. This is important because herbivores are known to respond to planting densities, facilitating the growth of some species over others under such conditions. This may be a reason for no evidence of interactions between mammal and density treatments. Finally, the differences in diversity due to planting season were consistent with differences in plant composition.