Assessing grassland restoration success: relative roles of seed additions and native ungulate grazing
Which aspects of community structure and ecosystem processes are restorable for most ecosystems, and how can seedling emergence in restorations be increased? I quantified success for 8 - 10 year old large tallgrass prairie restoration sites managed with native ungulates and fire by comparing it to three nearby remnants. I used the additive partitioning model of diversity, where [Alpha] is neighborhood (quadrat) scale diversity, [Beta] is accumulation of species diversity across neighborhoods, and [Gamma] is total diversity, and I decomposed [Alpha] into richness and evenness. Proportion of exotic biomass was similar between restorations and remnants, but proportion of exotic species and ANPP was two to four times higher in restorations. Alpha diversity and richness in restorations were half that of remnants, and alpha evenness was similar between restorations and remnants. Distance between quadrats per se was not related to diversity after accumulated quadrat area was taken into account. Proportion of [Beta] diversity was twice as high in restorations than remnants, possibly because patches of individual species were larger in restorations. Grassland restorations often lack rare forb and grass species found in intact grasslands potentially due to many reasons. I hypothesized that it could be due to seed limitation, dominance of C4 grasses (which can decrease number of microsites), or a combination of both. Realistic disturbances such as native ungulate activities may create microsites in vegetation. I experimentally tested these hypotheses in tallgrass prairie restoration by adding rare forb and grass seeds in two trials inside and outside native ungulate exclosures and measured seedling emergence. I measured light, water, and standing crop biomass to test whether they were related to emergence. Grazing increased light availability and increased ANPP in June-August 2004. Seedling emergence was consistently greater with seed additions and was conditionally greater with a combination of seeds and grazing. Grazing alone did not increase emergence. This suggests that low diversity in grassland restorations may be caused by combinations of seed and microsite availability. Grassland restoration may be improved if species coexistence can be increased. Restorations may be seed limited, and grazing alone may not increase emergence without seed additions.