THE PERSISTENCE OF BEAVER-INDUCED GEOMORPHIC HETEROGENEITY AND ORGANIC CARBON STOCK IN MOUNTAIN RIVER CORRIDORS

Abstract

Beavers are engineers of their ecosystems that manipulate their environment by building dams, flooding ponds, digging small canals, and altering riparian vegetation to suit their needs. Through these activities, beavers create beaver meadows, which are segments of river corridor characterized by high geomorphic heterogeneity, attenuation of downstream fluxes, and high biodiversity. We examined the effects of beaver engineering on the surface and subsurface geomorphic heterogeneity of 7 beaver meadows on the eastern side of Rocky Mountain National Park, Colorado, USA. The 7 meadows are divided into four categories of beaver activity: active, partially active, recently abandoned (< 20 years), and long abandoned (> 30 years). We survey and characterize distinct geomorphic units across the river corridor in each meadow and calculate metrics of surface heterogeneity relative to the category of beaver activity. We also examine the subsurface geomorphic heterogeneity through measures of soil moisture, clay content, soil depth, and organic carbon concentration across the beaver activity categories. Finally, we calculate the organic carbon stock in the upper 1.5 m of each meadow in order to make comparisons of organic carbon storage between the different categories of beaver activity. We find that surface geomorphic heterogeneity, and mean soil moisture differ significantly only between active and long abandoned meadows, suggesting a nonlinear decrease in these variables with time following beaver abandonment of a meadow. We find that there is no consistent trend in the soil depth and organic carbon stock among the different beaver activity categories, suggesting that larger-scale geologic factors that affect the creation of deep floodplain soils can continue to store considerable stocks of organic carbon well after beaver have abandoned a meadow. We find that the effects of beaver engineering can persist for nearly three decades following the disappearance of beaver from a river corridor.