Late-successional riparian forest structure results in heterogeneous periphyton distributions in low-order streams

Abstract

Late-successional riparian forests often regulate autotrophic microhabitats in low-order streams through shading provided by canopies. However, few studies have linked forest structure with periphyton microhabitat in adjoining streams. Our hypotheses were that (1) the heterogeneous horizontal structure in old-growth forests creates more spatially variable below-canopy light environments compared with mature forests and (2) site-specific light availability over streams correlates with spatial distributions of periphyton microhabitat. We surveyed 15 low-order stream reaches in late-successional northern hardwood–hemlock forests in the Adirondack Mountains of New York, USA. We measured forest structure and the below-canopy light environment at all reaches and the periphyton chlorophyll a concentration on artificial substrates in eight reaches. While stand age was not statistically significant, multivariate models of horizontal forest structure (e.g., gap distribution) and topography showed strong relationships (R2 > 0.70) with the below-canopy light environment across all late-successional forests. Furthermore, metrics of below-canopy light availability explained a small but statistically significant proportion of the variation in chlorophyll a concentration. This variation in chlorophyll a indicates that complex late-successional riparian forests, both mature and old-growth, create a mosaic of heterotrophic (shaded) and autotrophic (lighted) microhabitats along low-order streams. These results reveal important and previously unrecognized links between stream habitat heterogeneity and the horizontal heterogeneous late-successional forest structure.