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

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Abstract
A greenhouse bioassay was used to compare the growth, ectomycorrhiza formation, and foliar nutrient concentrations of Douglas-fir seedlings grown in soils collected at different distances from hardwood species. Soil nutrient concentrations and bulk densities were also determined. The soils were collected from two southwestern Oregon sites that had been clear-cut and broadcast burned 5 years previously. The sites, poorly stocked with conifer reproduction, were occupied primarily by grasses, forbs, and scattered individuals of tanoak (Lithocarpusdensiflora (Hook. & Arn.) Rehd.), Pacific madrone (Arbutusmenziesii Pursh), and canyon line oak (Quercuschrysolepis Liebm.). Five-month-old seedlings grown in media containing mineral soil collected beneath hardwood crowns had on average 60% greater height, 2.2 times greater weight (roots plus shoots), and almost 2 times more total and ectomycorrhizal short roots than seedlings grown in media containing soil collected more than 4 m from a hardwood. Rhizopogon sp. and Cenococcumgeophilum dominated on seedlings grown in hardwood soils, and an unidentified brown ectomycorrhiza dominated on seedlings grown in open area soils. The effect of hardwoods 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 area than in open area 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. Differences in soil nutrient concentrations were also inconsistent; however, rates of mineralizable nitrogen (anaerobic) were from 2 to nearly 6 times higher in hardwood area than open area soils, and soil pH was higher. Results suggest that the pioneering hardwoods strongly influence soil biological activity in these clear-cuts and impose one or more soil patterns that favor establishment and growth of conifer seedlings.