Wfj. Parsons et al., ROOT GAP DYNAMICS IN LODGEPOLE PINE FOREST - NITROGEN TRANSFORMATIONSIN GAPS OF DIFFERENT SIZE, Ecological applications, 4(2), 1994, pp. 354-362
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.