ROOT GAP DYNAMICS IN LODGEPOLE PINE FOREST - NITROGEN TRANSFORMATIONSIN GAPS OF DIFFERENT SIZE

Belowground responses to aboveground disturbance were studied in exper imental gaps created in a 95-yr-old stand of Pinus contorta in southea stern Wyoming. One-, 5-, 15-, and 30-tree clusters were felled to crea te a series of gaps in the root mat, and solution-phase N was monitore d over two consecutive snow-melt periods via tension-tube water collec tors. We hypothesized that dissolved and extractable nitrogen concentr ations would not exceed predisturbance levels until a threshold canopy gap size had been achieved. As predicted, NO(x)-N attained significan tly higher solution N concentrations (2-5 mg/L) only with the death of 15 trees or more. However, dissolved organic nitrogen decreased gradu ally with increasing gap size. Net mineralization and nitrification we re studied using 30-d in situ incubation assays in each gap. Extractab le nitrate routinely was negligible until the 30-tree gaps had been at tained. Predicting the effects of disturbance on nutrient cycling, inc luding timber-harvesting practices, requires information on belowgroun d responses to gap formation. Our experiments suggest that gap size is important; removal of 15-30 tree clusters represented a threshold abo ve which significant losses of available N to the groundwater may be i ncurred, at least in Rocky Mountain coniferous forests.