Novel communities, defined as collections of species that do not share an evolutionary history, are becoming increasingly common. Understanding how they impact community dynamics is important for preserving historical communities as well as predicting community structure in the future. Exotic species spread as well as global climate change leads to new species assemblages and interactions. Here, I examine in perennial grasslands, the impacts of novel plant communities on (1) community assembly and priority effects and how differences between native and novel species interact with water variability, (2) community diversity maintenance and how species diversity decline is associated with niche overlap or equalizing and stabilizing mechanisms, and (3) soil legacy effects of native and novel communities.

In Chapter 2, I present results from a greenhouse experiment that used native and novel species pairs to look how priority effects and water variability impacted community establishment. Native species had weaker priority effects than their exotic counterparts regardless of water variability. Additionally, the timing of water variability treatments altered priority effects, which were stronger when the growing season began dry.

In Chapter 3, I test mechanisms behind diversity maintenance in native and exotic communities. I show that high temporal overlap in exotics drive diversity decline, as well as low canopy height overlap suggesting exotics increase asymmetrical competition and highlighting the importance of equalizing mechanisms for diversity maintenance in native communities.

In Chapter 4, I use a bioassay to determine differential soil legacy effects of native and novel experimental communities. I examined previous biomass (correlated with nutrient depletion), microbial community differences and functional groups to determine possible mechanisms behind legacy effects. Exotic communities showed larger soil legacy effects that lowered germination the following season, and legacy effects were most associated with the previous dominant species functional group. Exotic legumes caused the largest negative effects on growth the following growing season, suggesting possible allelopathy.

In conclusion, I find that novel communities can alter community assembly, diversity maintenance and legacy effects in central U.S grasslands. Early arriving exotic plant species suppress native species from establishing from seed, lower species diversity and leave behind legacy effects that make it less likely for native species to reestablish.