Adapting boreal streams to climate change: effects of riparian vegetation on water temperature and biological assemblages

Riparian vegetation directly or indirectly affects many important structural and functional processes in stream ecosystems. We used a contiguous paired-reach approach in 4 km of open and shaded reaches in 8 boreal streams to study the effects of canopy cover on in-stream physicochemical (temperature, nutrients), substratum, and biotic assemblages (benthic diatom, macrophyte, invertebrate). Canopy cover, woody debris, and summer temperatures differed between paired reaches. Variation partitioning (partial redundancy analysis) showed that benthic diatom traits, but not species composition, were related strongly to canopy cover (33% of the variance was explained by canopy cover alone). In contrast, using variation partitioning, we found no support for the hypothesis that canopy cover was a strong predictor of macrophyte or benthic invertebrate assemblages. However, multivariable regression (redundancy analysis) showed that canopy cover was a significant predictor of invertebrate taxonomic and trait composition (explaining similar to 26% of the variability), after accounting for covariance with other environmental variables. In agreement with predictions, univariate analyses revealed that macrophyte traits related to light and growth forms were correlated negatively with canopy cover. Our findings that canopy cover resulted in lower summer temperatures, higher amounts of woody debris, and differences in community structure and traits support the importance of riparian buffer strips as a management tool for maintaining biodiversity and for managing stream ecosystems for the warmer future.