Graduate Thesis Or Dissertation
 

Growth and mycorrhiza formation of Douglas-fir seedlings grown in soils collected at different distances from hardwoods pioneering southwest-Oregon clearcuts

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  • A greenhouse bioassay was used to compare the effects of soils collected at different distances from hardwood species on the growth, mycorrhiza formation, and foliar nutrient concentrations of Douglas-fir seedlings. Soil nutrient concentrations and bulk densities were also determined. Soils were collected from two southwestern Oregon sites that had been clearcut and broadcast burned 5 years previously. The sites, poorly stocked with conifer reproduction, were occupied primarily by grasses, forbs, and scattered individuals of tanoak (Lithocarpus densiflora (Hook. and Arn.) Rehd.), Pacific madrone (Arbutus menziesii Pursh), and canyon live oak (Quercus chrysolepis Liebm.). Five-month-old seedlings grown in media containing mineral soil collected beneath hardwood crowns had an average 70% greater height, 2.2 times greater weight (roots plus shoots) and almost 2 times more total and mycorrhizal root-tips than seedlings grown in media containing soil collected farther than 4 m from a hardwood. Rhizopogon sp. and Cenococcum qeohilum dominated on seedlings grown in hardood soils and an unidentified brown mycorrhiza on seedlings grown in open-area soils. The "hardwood effect" did not vary among the three hardwood species or between the two sites. A study of soils collected at various distances from hardwoods indicated that the effect extended between 2 and 3 m. Average foliar nitrogen was higher for seedlings grown in hardwood soils, but differences were not statistically significant. Differences in other foliar nutrients of seedlings grown on soils from beneath the three hardwood species were inconsistent. There were no consistent differences in soil nutrient concentrations; however, rates of mineralizable nitrogen (anaerobic) were from 2 to nearly 6 times higher in hardwood than in open-area soils, and soil pH was higher. Results suggest that the pioneering hardwoods strongly influence soil biological activity in these clearcuts and impose one or more soil patterns that favor establishment and growth of conifer seedlings.
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