Short‐term forest harvesting effects on emergence rates of aquatic insects from small streams treated with different riparian management

Abstract Riparian zone management significantly influences adjacent aquatic habitats, particularly in the context of commercial forest harvesting. Tree removal can significantly alter light exposure, organic matter supplies and nutrient inputs, among other impacts. Riparian buffers are often used to mitigate harvesting's impact on freshwater ecosystems, but their effectiveness is limited. We experimentally tested the effects of riparian forest harvesting, using treatments of different buffer widths, on adult stream insect emergence from temperate, coniferous rainforest streams of coastal British Columbia. We measured the emergence of Plecoptera and Trichoptera taxa before and after harvesting activities. Using a replicated, Before‐After‐Control‐Impact (BACI) design, we examined emergence from 13 streams each treated with one of three riparian buffer widths (30, 10 and 0 m) versus unharvested reference streams. Insects were captured at regular intervals over the three‐and‐a‐half‐year study period using 0.5 m 2 emergence traps. At the level of Order, Plecoptera emergence was negatively impacted by harvesting activity, whereas Trichoptera emergence was not. Mean Plecoptera abundance at reference streams doubled (an inter‐annual effect unrelated to harvesting) after the harvesting period, whereas the observed increases in 10 and 0 m streams were, respectively, 92% and 114% less than the expected increase based on reference streams. At the genus level, several Plecoptera taxa were significantly lower in harvested sites (two Nemouridae and one Leuctridae), while one leuctrid species, Despaxia augusta , increased significantly following harvest in streams with 30‐m buffers relative to reference streams. Synthesis and applications . These results highlight the detrimental impact of harvesting along stream ecosystems where riparian buffer width is insufficient and demonstrate that a 30‐m buffer may offer substantial protection, strengthening the scientific foundation for sustainable riparian forest management.