Stream Temperatures Influenced by Riparian Vegetation as Mediated by Terrestrial Wildlife in Yellowstone National Park

ABSTRACT

Temperature is a fundamental control on many ecological processes, and factors that alter thermal regimes can shift species interactions and ecosystem function. Across eight streams, we assessed five aspects of summer temperature (daily mean, daily maximum, daily minimum, daily range and coefficient of variation) in relation to riparian vegetation status, geomorphology, hydrology and elevation. Using these metrics, we compared thermal regimes of streams in Yellowstone National Park with contrasting degrees of ungulate herbivory. Sites with primarily herbaceous vegetation experience heavy riparian browse by bison ( Bison bison ), which are less vulnerable to predation than other native ungulates. In contrast, sites dominated by woody vegetation, especially willows ( Salix spp.), are associated with lower ungulate browse, attributed in part to restoration of large carnivore populations. Mean daily temperatures varied during summer and were not consistently correlated with riparian vegetation state. However, streams with herbaceous vegetation had higher summer maximum temperatures than streams dominated by woody vegetation, with greater differences in later summer. Throughout summer, daily stream temperature ranges and coefficients of variation were up to five times greater in herbaceous sites. We conclude that terrestrial factors, including trophic cascades, can contribute to heterogeneous riparian vegetation states and alter stream temperature regimes. Furthermore, the link between these terrestrial dynamics and stream temperature was strongest in the late summer, when stream temperatures often exceed thermal thresholds for cold‐water biota. As this region's hydrology shifts from snowmelt to rain‐dominated, exacerbating late‐summer temperature spikes, this link between terrestrial wildlife dynamics and stream temperature may increasingly influence aquatic ecosystem processes and food webs.