Can forest fragments reset physical and water quality conditions in agricultural catchments and act as refugia for forest stream invertebrates?

Abstract

Forest fragments embedded within agricultural landscapes have the potential to provide a “forest reset effect” by mitigating agricultural effects on water quality, and acting as refugia and conservation reserves for aquatic species. We investigated the ability of forest fragments to reset agricultural effects using four catchments in the South Island, New Zealand. Two catchments were dominated by agricultural activities, but each had an isolated forest fragment in the lower valley, and two catchments had continuous riparian forest along the valley floor. Riffles sampled in continuous forest were generally deeper than those in agricultural and forest fragments, and not surprisingly streams in forest fragments and continuous forest received less light than those in agricultural land. All sites had circum-neutral pH, but both conductivity and temperature were significantly lower at continuous forest sites than agricultural and forest fragment sites. Taxonomic richness, Margalef’s index and numbers of Ephemeroptera, Plecoptera and Trichoptera (EPT) taxa were significantly higher in continuous forest than at forest fragment sites, but overall invertebrate densities did not differ between fragments and continuous forest. Several taxa were abundant at agricultural and forest fragment sites, but absent or at low densities in continuous forest. They included the blackfly Austrosimulium spp. and two caddisflies Pycnocentrodes sp. and Hydrobiosis parumbripennis. Conversely, the mayflies Austroclima sp. and Coloburiscus humeralis and the blepharicerid Neocurupira chiltoni were either restricted to continuous forest, or abundant in continuous forest but rare in agricultural and forest fragments. An ordination of communities separated those in agricultural and continuous forest sites, but communities at forest fragment sites were clustered among the agricultural sites. In this study forest fragments of 5–7 ha, located in the lower reaches of the catchment did not mitigate the negative upstream effects of agriculture on stream functioning. Fragment size (or riparian forest length), riparian forest width and vegetation type, and fragment location in the catchment may have critical roles in enabling forest fragments to reset the negative impacts of agriculture. Determining these characteristics of fragments has important consequences for stream remediation.