Graduate Thesis Or Dissertation
 

Relationships between forest-floor invertebrate distribution, movement, and microclimate under alternative riparian management practices

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  • Headwater streams and their riparian zones are a common, yet poorly understood, component of Pacific Northwest landscapes. I sought to describe the ecological significance of headwater stream riparian zones as habitat for forest-floor invertebrate communities, and to assess how alternative management strategies for riparian zones may impact these communities. I compared community composition of forest-floor invertebrates at five distances along 70 m trans-riparian (stream edge to upsiope) gradients in three treatments: mature forests; clearcuts; and across riparian buffers of ~30 m width. In the buffer treatments, I looked for evidence of microclimatic edge effects, and also biological edge effects, as characterized by species distribution and movement patterns across the forest-clearcut boundary. Invertebrates were collected in pitfall traps, in five replicate blocks of three treatments each, in the Willamette National Forest, OR. Air and soil temperature, and relative humidity were measured at a subset of pitfall locations at each site. A pitfall grid was installed at one riparian buffer site for a mark-release- recapture study to record carabid beetle and lycosid spider movements across the buffer edge. Ordination revealed a distinct "riparian" invertebrate community within 1 m of the stream edge in mature forest treatments, which was strongly related to a cool, humid microclimate. The stream appeared to influence microclimate at least 20 m up slope in the mature forest treatments. Invertebrate community composition in buffer treatments was far more similar to that of mature forests than was that of clearcuts, a pattern mirrored by microclimate. Microclimatic edge effects were not evident in the buffer, suggesting that the stream's cool, humid influence on microclimate may be modifying any warm, drying effects coming in from the forest-clearcut edge. While biological edge effects were not clear for invertebrate communities, individual species showed various responses to the buffer edge, depending on their habitat affinities and mobility. These results suggest that invertebrate distributions are strongly associated with microclimate, and that riparian buffers of ~30 m width provide suitable habitat for many forest species. However, buffer edges may serve as barriers to dispersal for some forest interior species, or be permeable to invasion by open-habitat species, with possible consequences for long-term population and community dynamics within the buffer.
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