Land use history alters the relationship between native and exotic plants: The rich don't always get richer

Observational studies of diversity have consistently found positive correlations between native and exotic species, suggesting that the same environmental factors that drive native species richness also drive exotic species richness, i.e., "the rich get richer". We examined patterns of native and exotic plant species richness in temperate forests that have been undergoing reforestation since the turn of the twentieth century to test the influence of disturbance arising from land-use history on this relationship. Overall, we found no relationship between native and exotic plant species richness. Instead, we found a positive relationship between native and exotic richness in older but not younger-growth forests, suggesting that the same processes that drove exotic plant richness in older forests also facilitated native plants. In contrast, younger forests had similar numbers of native species relative to older forests, but 41% more exotic species and 24% more compacted soils. Moreover, exotic but not native species richness was positively correlated with increasing soil compaction across all sites. Overall, our results suggest that elevated exotic plant invasions in younger forests are a legacy of soil disturbance arising from agricultural practices at the turn of the century, and that native and exotic plants may respond differentially to disparate environmental drivers.