Modelling biophysical controls on stream organic matter standing stocks under a range of forest harvesting impacts

Abstract

Forest harvesting could induce diverse responses of terrestrial-derived coarse particulate organic matter (CPOM) quantity in small streams. Understanding the basis of such variation requires the assessment of the independent and interactive effects of the controlling processes of stream CPOM quantity. Here we simulated post-harvest responses of leaf litter-derived CPOM quantity in a coastal rainforest stream in British Columbia, Canada, using a published process-based model. We compared the relative importance of major biophysical controls of CPOM quantity, including riparian litterfall, discharge, and stream temperature, across a range of severity of forest harvesting disturbance, using a sensitivity analysis. This range represented published post-harvest responses of these model drivers in temperate North America. We then varied the values of model drivers to examine possible changes in CPOM quantity (within ˜4 years post-harvest) under different harvesting scenarios, and to characterise the interactions among pairs of drivers. The effects of litterfall reductions due to forest harvesting on depleting CPOM quantity were at least an order of magnitude greater than those of elevated peak flows. Summer stream warming of 4 °C or more could lead to a smaller magnitude of CPOM reductions, possibly due to decreases in CPOM consumption and shredder biomass that lasted until fall. Warming-induced CPOM increases could counteract the effects of reduced litterfall and elevated peak flows on lowering CPOM quantity, depending on disturbance severity. CPOM depletions were highly likely when litterfall was below 50% of that in undisturbed conditions. Our heuristic modelling revealed that non-additive, antagonistic interactions between paired model drivers could emerge at higher severity levels of disturbance. We suggest that establishing riparian buffer zones would more likely mitigate post-harvest changes in CPOM quantity through minimising alterations in litter inputs and stream summer temperature. Our study illustrates the utility of process-based simulations and scenario analysis in evaluating the ecological impacts of biophysical processes operating at reach to catchment scales. A wider adoption of these modelling approaches can improve the predictions of stream ecological responses to watershed disturbances.