PS 42-48 - Is water storage potential of a restored grassland influenced by beaver dam structure and density?

Background/Question/Methods

Within forested landscapes, American beaver (Castor canadensis) behavioral impacts on river morphology lead to increased ecosystem function; however, the ecological impacts of beaver dams within grasslands are virtually unstudied. Grassland groundwater recharge rates are less than within forests, while streamflow rates are greater in grasslands compared to forests. The hydrological dynamics of larger water movement and storage impacted by beavers in grasslands could have significant implications for mitigating climate change impacts and restoring clean water resources. If the central place foraging theory applies to beavers within grasslands, the distribution and abundance of beaver dams will be limited by availability of woody habitat. Our goal was to compare dam density of grasslands to forests and investigate hard edge (preserve boundary) as a limiting factor to relative water storage of dams.

We located beaver dams by wading through all streams within Nachusa Grassland, a restored tallgrass prairie fragmented within an agricultural landscape in northwestern Illinois during the summer of 2021. We ranked water retention above a selection of beaver dams according to level of stream flow and assessed the relationship of distance to the hard edge and the extent of water retention per dam using ordered logistic regression.

Results/Conclusions

We located 148 dams distributed across the three major streams that run laterally through the property. Dams were largely absent in most of the branching streams within isolated property allotments. Within the 1200 ha of Nachusa, dams occupy 1.4% of the 10.4-km accumulated length of stream reaches, a dam density approximately one-tenth as dense as beaver dams in forests. Our analysis revealed that distance to hard edge was not a strong indicator of dam water retention and contradicts central place foraging patterns. Though dams in this grassland are less dense than those in forests, the frequency and spatial scale of dams within Nachusa is surprising due to the limited woody resources in the grassland.

Grassland beavers may occupy a unique foraging niche, potentially transplanting woody species across the grassland and maximizing dam water storage. Increases in water storage potential could create drought resilience and boost carbon sink loads within grasslands. In future work, we will examine how hard edge and forest patches shape beaver dam density and water storage in a restored tallgrass prairie. This work will be a unique contribution to our understanding of grassland beaver behavior and a resource to restoration managers navigating beaver facilitated restoration in grasslands.