Adapting streams for climate change using riparian broadleaf trees and its consequences for stream salmonids

Summary The societal value, ecological importance and thermal sensitivity of stream‐dwelling salmonids have prompted interest in adaptive management strategies to limit the effects of climate change on their habitats. Additionally, in northern temperate regions, the management and restoration of riparian broadleaf forest is advocated increasingly to dampen variations in stream water temperature and discharge, but might have collateral effects on salmonids by changing allochthonous subsidies. Here, in a cross‐sectional analysis of 18 temperate headwaters with different riparian and catchment land use, we use classical fisheries data alongside stable isotope ratios in salmonids and their macroinvertebrate prey to examine whether increasing catchment cover of broadleaf trees could (i) increase the density, biomass and size of salmonids, (ii) increase brown trout (Salmo trutta) dietary reliance on production of terrestrial origin and (iii) mediate allochthonous energy flux between aquatic macroinvertebrates and brown trout. Contrary to expectation, catchment broadleaf cover had no systematic effect on salmonid density or individual size, although salmonid biomass was lowest in streams draining non‐native conifers. Moreover, there was no major effect of land use on the dependence of S. trutta on terrestrial production: averaged across all sites, trout used more production from in‐stream (62 ± 3%: mean ± 1 SE) than terrestrial (38 ± 3%) sources. Dependence on terrestrial production varied more substantially among individual streams than with riparian land use, mirroring site‐specific patterns observed in macroinvertebrates. Although increased broadleaf cover could benefit salmonids by offsetting the impacts of warming related to climate change, these results imply that broadleaf restoration along temperate, upland headwaters is neutral with respect to salmonid biomass, density and terrestrial subsidies. In contrast, the use of non‐native conifers for stream shading could have negative effects on salmonid production. Knowledge of the ecological implications of climate change adaptation remains rudimentary, and we advocate further evaluations like ours not only for fresh waters, but for ecosystems more generally.