Graduate Thesis Or Dissertation
 

Physical processes affecting soil biotic and abiotic responses to disturbance in forest ecosystems of southwestern Oregon

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  • The effect of physical factors such as soil structure, bulk density, parent material, and topographic variables on soil C and N dynamics and ectomycorrhizal inocululu potential in forests of southwest Oregon were investigated In the high-elevation white fir (Abies concolor (Gord. & Glend.) Lindl.) zone, two old poorly-vegetated clearcuts with different soil textures (sandy loam and silt loam) were compared with adjacent uncut areas. Significant differences in soil C and N were not detected at either site, but the site with silt loam soil had 20-25% lower C and N concentrations in several particle size fractions. Compared to adjacent forests, anaerobically mineralizable N (Nmin) was lower only in the clearcut with sandy loam soil. A larger pool of physically-stabilized but chemically labile N probably sustained Nmin levels on the clearcut with finer-textured soil. In another investigation on these sites, Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings were used to bioassay six soil particlesize fractions for ectoniycorrhiza (EM) formation. Six major EM types were found to be heterogeneously distributed among size fractions. Increased colonization by one EM type following mechanical disaggregation of soil from a clearcut indicated propagules had a "clumpy" distribution in soil or had been suppressed by actinomycete activity. A separate investigation in a lower elevation vegetation zone (the mixed evergreen zone) of southwest Oregon compared soil and forest floor C and N in forests and 5-year-old clearcuts that had been broadcast-burned (BB) or hand piled-and-burned (PB). Total C and N, and Nmin were determined variously in litter, F-layer, 0-5 cm soil, and 5-15 cm soil. In clearcuts, decreases in C and N stored in these layers exceeded amounts typically removed by harvest. BB clearcuts had significantly lower Nmin levels in the 0-5 cm layer compared to adjacent forests. The disappearance of F layers from BB and PB clearcuts represented the largest loss of N from the layers we sampled. The proportion of N lost from these layers was related by regression of C:N ratios of the two soil layers. This proportion and both C N ratios were in turn correlated with slope, aspect, and soil bulk density. We incorporated the relationships into a conceptual model depicting a complex topographic influence on N losses following forest disturbance.
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