Investigation of Catchment-Scale Wildfire and Harvesting Disturbances on Riparian-Stream Linkages in Boreal Headwater Streams

Abstract

Forested streams are highly interconnected to adjacent terrestrial landscapes and are therefore highly sensitive to natural (e.g., wildfire) and anthropogenic (e.g., forestry) disturbance. In forest management, emulation of natural disturbance (END) has been proposed to promote long-term sustainability of riparian forests and aquatic ecosystems; however, further understanding of ecological responses to natural disturbances is required to implement END strategies effectively. The objective of this research was to investigate long-term effects of catchment-scale wildfire and harvesting (with riparian buffers) on riparian-stream linkages in boreal headwater streams. I assessed a suite of physicochemical, hydrological, and biotic indicators representing riparian-stream linkages including riparian forest condition (e.g., vegetation composition), leaf and terrestrial invertebrate subsidies, decomposition dynamics using leaf packs, and aquatic macroinvertebrate communities associated with leaf packs and in drift across reaches within catchments with wildfire, harvesting, and minimally-impacted disturbance histories. Most indicators of riparian forest condition differed significantly between wildfire and reference and harvested sites, but rarely between harvested and reference sites. Wildfire-disturbed forests contributed greater leaf subsidies to streams compared to reference and harvested forests. Macroinvertebrate communities at wildfire sites had greater taxa richness and unique shredder taxa. I then explored how disturbance-driven compositional differences in leaf subsidies influenced aquatic macroinvertebrate communities and decomposition using mixed-species leaf packs modelled after wildfire-disturbed and intact riparian forests. Wildfire-modelled leaf packs decomposed quicker than reference-modelled leaf packs in wildfire-disturbed streams and contained higher shredder abundance. In drift samples, wildfire-disturbed streams contained greater aquatic macroinvertebrate abundance, biomass and different community composition compared to reference and harvested streams. Finally, I assessed the relative importance of hydrology, shredders, water chemistry and temperature on leaf decomposition over three years in streams. Shredder-related models consistently explained the greatest variation in yearly decomposition rates compared to water chemistry, temperature, and hydrology-related models. The effects of wildfire persist beyond a decade in boreal headwater streams. Long-term differences in riparian composition can influence macroinvertebrate communities. Differences in riparian forest condition and instream ecological structure and function suggest that management under END could support aquatic biodiversity by inducing riparian forest succession, enhancing leaf subsidies, organic matter processing, and riparian habitat complexity.