Natural and historical variability in fluvial processes, beaver activity, and climate in the Greater Yellowstone Ecosystem

Abstract

Two centuries of human activities in the Greater Yellowstone Ecosystem (GYE) have strongly influenced beaver activity on small streams, raising questions about the suitability of the historical (Euro-American) period for establishing stream reference conditions. We used beaver-pond deposits as proxy records of beaver occupation to compare historical beaver activity to that throughout the Holocene. Forty-nine carbon-14 ( 14 C) ages on beaver-pond deposits from Grand Teton National Park indicate that beaver activity was episodic, where multi-century periods lacking dated beaver-pond deposits have similar timing to those previously documented in Yellowstone National Park. These gaps in the sequence of dated deposits coincide with episodes of severe, prolonged drought, e.g. within the Medieval Climatic Anomaly 1000-600 cal yr BP, when small streams likely became ephemeral. In contrast, many beaver-pond deposits date to 500-100 cal yr BP, corresponding to the colder, effectively wetter Little Ice Age. Abundant historical beaver activity in the early 1900s is coincident with a climate cooler and wetter than present and more abundant willow and aspen, but also regulation of beaver trapping and the removal of wolves (the beaver's main predator), all favorable for expanded beaver populations. Reduced beaver populations after the 1920s, particularly in the northern Yellowstone winter range, are in part a response to elk overbrowsing of willow and aspen that later stemmed from wolf extirpation. Beaver populations on small streams were also impacted by low streamflows during severe droughts in the 1930s and late 1980s to present. Thus, both abundant beaver in the 1920s and reduced beaver activity at present reflect the combined influence of management practices and climate, and underscore the limitations of the early historical period for defining reference conditions. The Holocene record of beaver activity prior to Euro-American activities provides a better indication of the natural range of variability in beaver-influenced small stream systems of the GYE.

Beavers affect carbon biogeochemistry: both short‐term and long‐term processes are involved

“…Breaching of the abandoned dam can result in at least partial erosion of the pond sediment, although the establishment of rooted aquatic macrophytes and other wetland vegetation commonly limits evacuation of pond sediment (Wohl & Scott ). Beaver pond deposits can remain for thousands of years, as has been shown from late Holocene deposits dated with radiocarbon (Persico & Meyer , , Polvi & Wohl ) or ground‐penetrating radar (Kramer et al. ) methods.…”

The impact of beaver dams on the morphology of a river in the eastern United States with implications for river restoration

“…Our measurements from Kimages Creek add quantitative support for a recent hypothesis emphasizing a limited impact of beaver dams on landscapes (Persico and Meyer, ; Burchsted and Daniels, ; Levine and Meyer, ). Previous studies of beaver dams have indicated the long‐term impact on channel morphology (Butler, ; Polvi and Wohl, ) included bed aggradation and a decrease in bankfull area, particularly in ponds upstream of beaver dams (Naiman et al ., ; Butler and Malanson, ).…”

Landscape‐scale carbon storage associated with beaver dams

“…Estimated total carbon storage during periods of active beaver meadows does not imply that all of the extensive meadows were continuously occupied during the Holocene. Although these sites represent optimal beaver habitat, they may all have been simultaneously occupied only during cooler, wetter intervals of the Holocene [ Persico and Meyer , , ]. The estimated carbon storage using 12% TOC and total sediment volume in extensive beaver meadows represents maximum potential carbon storage at the landscape level relative to contemporary, relatively low values.…”

“…This valley‐bottom metamorphosis likely has important implications for ecosystem resilience to climate change, drought, and wildfire. Beaver populations and their dam‐induced hydrologic effects have varied during the Holocene in response to climate fluctuations [ Persico and Meyer , , ] and will presumably vary in future. The persistence of at least some beaver, however, and the maintenance of higher riparian water tables probably created Holocene refugia for more mesic communities during drier intervals.…”