Linking stream ecosystem integrity to catchment and reach conditions in an intensively-managed forest landscape

Abstract

Forests are vital to maintain headwater stream integrity in forested biomes, which in turn ensures the delivery of aquatic ecosystem services downstream. Forest harvesting, however, can alter land-water linkages and compromise stream integrity. Although the main effects of forestry on streams have been documented, most studies have focused on short-term, post-harvest effects and used only a few (mainly abiotic) indicators. To address this, I investigated the effects of varying forest management intensities (FMIs) on different components of stream ecosystems in a holistic two-year study. In 12 catchments ranging in FMI, I measured a suite of abiotic and biotic indicators of stream ecosystem integrity to determine which explanatory catchment and reach variables were driving the observed differences. Most indicators detected the gradient in FMI, with abiotic indicators responding most strongly: streams in catchments with highest FMI tended to have higher fine inorganic sediment deposition and entrainment, water cations and carbon, DOM aromaticity and humification, and water temperature. These abiotic differences were associated with higher biofilm biomass and shredder densities, but lower leaf decomposition and contribution of algae to stream macroinvertebrate diets. Therefore, higher FMIs promoted the delivery of terrestrial water-borne materials to streams, which were incorporated into food webs. However, there were hardly signs of impaired biological communities in these 12 streams compared to data from 3 reference streams. Fixed-width riparian buffers are used to protect stream ecosystems from forestry, but this fails to acknowledge areas with strong hydrological connectivity (variable source areas – VSAs) that may warrant special protection due to being vegetation and biogeochemical hotspots. To assess whether these hotspots have an effect on the receiving waters, I compared stream ecosystem integrity between VSA and non-VSA stream reaches in a paired study. Although I detected some differences (higher understory vegetation density, deposited organic matter, % gatherers, lower % riffles, DOC, algal biomass), I found little evidence that inputs from VSAs had significant effects on stream communities and functions. Overall, this study demonstrates the advantages of assessing several indicators for a more holistic understanding of the linkages between forestry and streams, it shows that current management practices do not fully protect against an increased delivery of terrestrial materials derived from high FMI to streams, and calls for more studies assessing the ecological implications of VSAs.