Root Gap Dynamics in Lodgepole Pine Forest: Nitrogen Transformations in Gaps of Different Size

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Abstract
Belowground responses to aboveground disturbance were studied in experimental gaps created in a 95—yr—old stand of Pinus contorta in southeastern Wyoming. One—, 5—, 15—, and 30—tree clusters were felled to create a series of gaps in the root mat, and solution—phase N was monitored over two consecutive snow—melt periods via tension—tube water collectors. We hypothesized that dissolved and extractable nitrogen concentrations would not exceed predisturbance levels until a threshold canopy gap size had been achieved. As predicted, NOx—N attained significantly higher solution N concentrations (2—5 mg/L) only with the death of 15 trees or more. However, dissolved organic nitrogen decreased gradually with increasing gap size. Net mineralization and nitrification were studied using 30—d in situ incubation assays in each gap. Extractable nitrate routinely was negligible until the 30—tree gaps had been attained. Predicting the effects of disturbance on nutrient cycling, including timber—harvesting practices, requires information on belowground responses to gap formation. Our experiments suggest that gap size is important; removal of 15—30 tree clusters represented a threshold above which significant losses of available N to the groundwater may be incurred, at least in Rocky Mountain coniferous forests.

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