Tree roots as a dominant agent of streambed habitat, profile pattern, and grade control

Abstract

Tree roots are a ubiquitous biotic agent in numerous fluvial settings, but their role in controlling streambed habitat and profile patterns is understudied. Although their importance in resisting bank erosion is more widely reported, tree roots also have the potential to span the channel bed, particularly in narrower first- to third-order streams. This enables tree roots to drive profile patterns and maintain important habitat elements such as steps that induce deep pools in both perennial and ephemeral systems. In a study across two humid-temperate ecoregions, tree roots were associated with more abundant and larger steps than large wood (LW), forming 91% of the steps in the high gravel load setting and 40% of the steps in the steeper, coarser, low sediment supply setting. When combined with LW, a majority of the steps in both settings were formed by biotic agents (97% and 55%, respectively). Furthermore, 70% of the deepest pools across all 40 study reaches were associated with biotic controls, with root steps being the most dominant driver: 45% of the deepest pools in all study reaches, including the maximum pool depths measured in both ecoregions (0.57-m and 0.38-m, respectively). Many questions need further exploration, for example, what tree sizes and species are most likely to form root steps (i.e. trunk diameters > 40 cm and American Sycamore (Platanus occidentalis) in this study); however, even this somewhat limited study has clear implications to land use planning and stormwater management. For example, on land developments draining to root-step dominated streams, maintaining the integrity of the riparian zone would seem to be as important as ensuring hydrologic mimicry if channel integrity is to be preserved. The study also has insights to the stream restoration industry where conventional practice typically involves large tree clearing operations to facilitate stream regrading.