The relationship between diversity and invasion resistance in wet-mesic tallgrass prairie restorations

Background/Question/Methods Grassland restoration is an important activity on many public lands, and ecologists are interested in seed mixes and seeding densities that will make grassland plant communities more resistant to invasion by aggressive species.  An understanding of the factors that increase grassland resilience will help improve the success of grassland restoration.  This study analyzed the effects of diversity and seeding rate on invasion resistance in experimental wet-mesic tallgrass prairie plots located in central Nebraska.  In the spring of 2006, twenty-four 0.30-hectare plots were planted to six replicates in each of four treatments: high diversity sites of approximately 100 species typically planted by The Nature Conservancy planted at the normal seeding rate, high diversity sites at twice the normal seeding rate, and low diversity sites using the Natural Resources Conservation Service’s CP25 seed mix (15 species) at normal and half-normal seeding rates.  The composition of the plant community was assessed in mid- to late June 2007 and 2008.  Basal stem cover was measured in six one-meter intervals (subtransects) along three 55-m long transects within each plot, a middle transect and two outer transects.  The stem cover of invasives was analyzed among the treatments with four linear mixed models using maximum likelihood.

Results/Conclusions In both 2007 and 2008, the highest mean stem cover of cropfield weeds was found in the low diversity, half-normal seeding rate plots.  The results of the linear mixed model analysis suggests the density of seeds sown was more important in predicting the stem cover of weeds than the diversity of plant species that make up that density, particularly in 2008.  When compared against the intercept in the best model, the mean stem cover of weeds in the two treatments with the higher seeding densities decreased significantly (P = 0.0411 and P = 0.0160).  There was little edge effect as sampling location within a plot was comparatively less important in explaining the occurrence of invasive plants.  These results seem to support the fluctuating resource ability theory, in which a plant community becomes more susceptible to invasion whenever there is an increase in the amount of unused resources.  As succession proceeds, more specialized native species from the seeding mixes may become more effective in exploiting resources than generalist invasive species and diversity may become more important in determining the invasion resistance of the plots.