Scale, diversity, and community convergence in a grassland restoration experiment

Background/Question/Methods           Both the level and heterogeneity of resources can affect patterns of species richness.  Increases in resource heterogeneity may be expected to have a scale-dependent effect on richness, as within-patch alpha diversity is unaffected, but between-patch beta diversity is increased.  This idea was tested in a restoration experiment in coastal Californian grasslands in which resources were manipulated.  Since fall 2005, we have applied two resource manipulation treatments in each of two heavily invaded coastal grassland sites.  The first is a reduction of soil fertility, accomplished by adding sugar to the soil twice annually.  The second is a reduction of aboveground standing crop, accomplished by spring mowing. We sampled the plant communities in the summers of 2006 and 2007, using a nested plot design with plots ranging in size from 0.015 m² to 4 m². 

Results/Conclusions           Consistent with expectations, treatments had a scale-dependent effect on richness.  Mowing increased species richness markedly at small scales, but the observed strength of this effect diminished as plot size increased.  Carbon addition had no effect on species richness at small scales, but increased it at larger scales.  These changes are explained by increased within-plot subplot compositional similarity in mowed plots, and a corresponding decrease in carbon addition plots.  This pattern holds at the between-plot scale as well, as carbon addition plots diverged from one another in their species compositions, while mowed plots experienced compositional convergence.  This is consistent with theoretical predictions concerning the effect of productivity and disturbance on community convergence.