Initial species pattern affects invasion resistance in experimental grassland plots

Question: Does the spatial patterning of plant species affect plant community dynamics in ways that are independent of effects attributable to species richness or abundances? Does the initial species pattern affect subsequent diversity and invasion in a perennial grassland system? Location: Field experiment, Iowa State University Horticultural Research Station, Ames, Iowa, USA. Methods: Experimental plots ( 4 m 2) were planted with seedlings of four grassland species arranged into increasingly larger groups ( patches) of conspecific individuals while controlling plot- scale richness and evenness. Available light ( photosynthetically active radiation; PAR) at the soil surface was measured each month for three growing seasons after planting. Species' relative abundances were quantified via point- intercept sampling within each plot at the end of each growing season. Results: In the three growing seasons after planting, planted species richness and evenness ( plot scale) did not vary among plots planted with different species patterns. However, early in the second growing season more light reached the soil surface in plots with initially larger conspecific patches. Invader abundance was also consistently higher in plots planted with initially larger conspecific patches. Conclusions: Our findings support the hypothesis that invasion resistance increases as communities becomemore heterogeneous at fine scales and suggest that within- plot heterogeneity should be considered as an additional factor when assessing invasion resistance in perennial plant communities.