Root gap dynamics in lodgepole pine forest: nitrogen transformations in gaps of different size

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, NO"x-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.