Combined effects of an anthropogenic (forest harvesting) and natural (extreme rainfall event) disturbance on headwater streams in New Zealand

Abstract

Abstract Although extreme hydrological events are a natural component of river ecosystem disturbance regimes, their frequency is predicted to increase with climate change. Anthropogenic activities have the potential to exacerbate the impact of such disturbances but there are few studies on the combined effects of both anthropogenic and extreme hydrological disturbances on stream ecosystems. We investigated the recovery of stream ecosystems over a 5‐year period following the impact of an anthropogenic (forest clear‐cut harvesting) and an extreme rainfall disturbance (estimated one‐in‐100 year average return interval) that generated debris flows in three headwater streams in New Zealand. Initially, most of the riparian vegetation was eliminated and showed little recovery 1 year later. Subsequent riparian recovery was led by wind‐borne, light‐demanding, pioneering exotic weed species, lengthening and altering the long‐term successional and recovery trajectories to a pre‐disturbance composition of indigenous shrubs. Stream shade, water temperature, and habitat had largely recovered after 5 years. However, the contribution of large wood to channel morphology and in‐stream habitat was compromised due to diminished wood supplies in the stream channel and a hiatus in up‐slope wood inputs until the riparian vegetation re‐establishes and the next crop of trees matures. After an initial decline, most indigenous fish taxa thrived in the post‐disturbance conditions, with significant increases in densities and biomass. The more sensitive fish taxa were scarce or absent, particularly those taxa that prefer pools with overhead and in‐stream cover provided by riparian vegetation and wood. Recovery of these taxa was outside the time frame of this study. Riffle dwelling fish communities were more resilient than pool dwelling fish communities. Invertebrate densities showed a similar response to fish. Post‐event invertebrate community composition differed from that typically found in post‐harvest headwater streams, comprising comparatively lower proportions of Chironomidae, Oligochaetes, and Mollusca taxa, and higher proportions of Trichoptera taxa. Progression toward pre‐event community composition was evident 5 years after the event. The compounding effect of forest removal from harvesting, along with riparian vegetation and wood removal by debris flows, lengthened the recovery of riparian vegetation and wood supplies with cascading effects on in‐stream habitat and biological communities.