| Forest harvest is associated with large negative effects on small streams. Typical changes observed in streams after upland forest harvest include elevated sediment inputs, altered hydrological regimes, leaching of nutrients, warming of stream temperature and, in boreal Sweden, leaching of toxic methyl mercury. All these physiochemical changes are also associated with impoverished aquatic communities across all trophic levels. Importantly, these negative effects, which are observed locally, may be carried downstream due to the cumulative nature of stream networks. Thus, we may not be able to conserve water for all end users and attain good ecological status of downstream reaches, if water arrives impaired from upstream impacted tributaries.
Riparian buffers, i.e., strips of forest along watercourses, are usually left during upland forest harvest to protect aquatic ecosystems. In Sweden, riparian buffers are commonly established along larger streams and rivers, whereas small streams (headwaters) receive narrow buffers or no protection at all. In my ongoing research project, I found that 70% of the 111 headwater streams studied had riparian buffers <5 m wide on each side, and many streams were lacking buffers all together. The results of this work further indicated that the thin buffers are likely inefficient in protecting aquatic ecosystems from the harmful effects associated with harvest locally, within the clearcuts. We do not know, however, how these local impairments propagate downstream.
In this project, I aim to address how local impairments caused by harvest propagate to downstream reaches and if upstream riparian buffers can mitigate downstream propagation. I will conduct a field study along longitudinal profiles of recently harvested streams to answer these questions. Twenty streams in recent clearcuts with different buffer configurations will be investigated, five in each buffer category of: a) no buffer, b) thin buffer (<5 m), and c) moderate buffer (>15 m wide). I will utilize sites from my ongoing project as well as include new sites to achieve five replicates per category. By comparing the streams with buffers to reference conditions (streams in intact forest) I will be able to address how effective the current practices are in protecting streams locally and downstream.
At three sites along each stream, I will measure a number of physical (water temperature, flow), biogeochemical (carbon, nutrients, oxygen) and ecological (organic matter decomposition, algae and macroinvertebrate communities) parameters which are typically linked to water quality. First site will be situated in the clearcut to provide an assessment of local effects and a baseline for downstream comparisons. Second site will be established along the stream reach right after it entered a downstream forest patch. Third site will be situated about 200-300 m downstream in the forest. Site 2 and 3 will provide me with information about propagation of local effects of harvesting to downstream reaches.
This project will significantly contribute to the ultimate goal of my research, which aims to develop new best-management-practices (BMPs) of riparian buffers along small streams in order to mitigate the negative effects of forestry on water on a catchment scale.
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