Graduate Thesis Or Dissertation
 

Arthropod communities on understory plants in thinned and unthinned Douglas-fir forests in the Oregon Coast Range

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  • This study assessed the effects of thinning on arthropod communities on understory plants in the Central Oregon Coast Range. Arthropods were sampled from five understory plants in five pairs of thinned and unthinned, young (50-80 yrs), managed Douglas-fir stands, from late May to mid-July of 1998. Vine maple (Acer circinatum), salal (Gaultheria shallon), understory hemlock (Tsuga heterophylla), bracken fern (Pteridium aquilinum), and sword fern (Polystichum munitum) were sampled for arthropods using beating sheet and aspirators. Arthropod taxa were sorted and identified to family, species where possible. Intensities and diversity indices were calculated both for all arthropods (including spider taxa) and for spiders separately. Arthropods were placed into functional groups based on trophic level/feeding habits. Spiders were placed in guilds based on prey capture strategies. Collembolans, psocopterans, aphids and linyphiid spiders together made up 50% of total arthropod abundance. Spiders made up 23% of the total arthropod abundance. Functional groups overall were not found in different intensities between thinned and unthinned stands. Relative representation of spider guilds was different between thinned and unthinned stands. Cobweb weavers and nocturnal hunters had higher intensities in thinned stands. Sheetweb weavers, orbweb weavers and agile hunters had higher intensities in unthinned stands. Sap-suckers on vine maple were more abundant in unthinned stands. Parasitoids on vine maple were more abundant in thinned stands. Agile hunters on salal were more abundant in unthinned stands. Salal, especially in unthinned stands, provided important structure for spider communities. Salal supported high spider diversity. Hemlock had the highest species richness, for both arthropod and spider communities. Communities on vine maple were diverse, despite low overall arthropod abundance. The arthropod communities showed significant segregation by plant species and treatment condition. Shrub cover, shrub diversity, patch size, light levels, tree density and stand age explained arthropod community differences. Each one of the understory plants I studied supported a unique portion of the overall understory arthropod community and should be maintained in managed forests to support this important element of diversity. Differences in species distribution and structure of understory vegetation between treatments, resulting in arthropod community differences, suggests that maintenance of both treatment conditions across a landscape is important for maintaining diversity of understory arthropod communities.
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