Divergent impact of grazing on plant communities of created wetlands with varying hydrology and antecedent land use

Abstract

2Destruction of natural wetlands has warranted the creation of wetlands to mitigate the 3 reduction of valuable ecosystem functions and services. However, the complex 4 interactions between key drivers of wetland community structurehydrology, nutrient 5 availability and herbivorymakes creation of functional wetland replacements 6 challenging. We examined interactions among these drivers, and their impacts on plant 7 communities and soil characteristics in two created wetlands with different hydrology 8 and land use histories: a shallow emergent marsh on a previous gravel depository and a 9 seasonally flooded marsh on a former cattle pasture. In paired plots open to or 10 protected from large wetland grazers we observed seasonal variation in grazing impacts 11 on plant communities and an increase in effect size over time. At the permanently 12 flooded marsh with high grazing waterfowl densities and low nutrients, open plots had 13 significantly reduced plant growth and diversity, and an increase in submerged 14 vegetation. In contrast, grazer density was lower and nutrients were higher in the 15 seasonally flooded marsh, where grazer access enhanced plant diversity and reduced 16 invasive plant cover. These results suggest the possibility of long-term grazer-induced 17 shifts in community composition and delivery of key ecosystem services in young, 18 vulnerable created wetlands. To improve created wetland design and function, we 19 suggest that in addition to hydrologic conditions, the impact of prior land use on present 20 nutrient availability be considered. Further, enhanced heterogeneity of spatial and 21 bathymetric structure can provide conditions for diverse plant communities and 22 balanced habitat use by wetland grazers.23 24