Effects of neighborhood-scale diversity on plant establishment

Plant distributions may be very important in determining invasibility (colonization), competitive exclusion (local extinction), and therefore species diversity.  Here, plant distributions are defined by the degree of conspecific clumping (species aggregation) that occurs in a community.  We directly manipulated plant distributions in 24 plots (2 x 2 m) planted in the spring of 2006 to test potential mechanisms of diversity maintenance.  Four common grassland species were transplanted into three spatial treatments of low, medium, and high species aggregation (high to low neighborhood- scale diversity), while keeping species richness and abundance constant at the plot scale.  All plots were weeded during the first season to allow transplant establishment.  At the end of the first growing season, species relative abundance data were collected through point-intercept sampling in a 0.5m² subplot of each plot to determine if the treatments established as planted.  As expected, high aggregation treatments had lower neighborhood-scale diversity than low aggregation treatments and there were no differences between planted diversity and diversity at the end of the growing season.  In addition, transplant mortality rates were consistent across treatments suggesting that there was no differential competitive exclusion (local extinction) during establishment among treatments.  In 2007 colonizers will be allowed to persist to determine if invasion differs among the treatments.  These first year results suggest that the processes maintaining species diversity are not affected by neighborhood scale plant distributions.  Findings from this study will have important implications for grassland reconstructions where the method of initial seed distribution may alter long-term community dynamics.