The extent of young postfire conifer forests is growing throughout western North America as high-severity fires increase, making it important to understand structure and function in early seral forests. Chronosequence studies in lodgepole pine (Pinus contorta var. latifolia) indicate recovery of nitrogen (N) stocks 40-70 years postfire, but N-cycling studies during early decades of succession are few. We re-sampled lodgepole pine stands in Yellowstone National Park (Wyoming, USA) that regenerated naturally after the 1988 fires to ask: (1) How have lodgepole pine foliage, litter, and soil N changed from 15 to 25 years postfire? (2) How do N pools and fluxes vary with lodgepole pine density and productivity? (3) Does evidence suggest N limitation in 25-yr old postfire lodgepole pine? In 14 plots (0.25-ha) that varied in postfire tree density (1,500-344,000 stems ha-1), we measured N concentrations and pools in lodgepole pine foliage, annual litterfall, and forest-floor litter; soil total N and annual inorganic N availability; and δ15N of lodgepole pine foliage, litter, and soil. Over time, we expected foliar N concentrations to decrease, foliar N pools to increase, and soil N pools to decrease. Among stands, we expected positive relationships between lodgepole pine productivity and soil N.
Results/Conclusions
Lodgepole pine foliar N concentrations (1.33 and 1.11%N in current-year and composite needles, respectively) did not change between postfire years 15 and 25, but forest floor litter N concentration increased by 25% to 0.82%N. All measured ecosystem N pools increased substantially. Total foliar N increased to 89 kg N ha-1 (+85%), forest floor litter increased to 39.4 kg N ha-1 (+38%), and soil N increased to 0.08% (+33%). Inorganic N availability also increased to 0.69 µg N g-resin-1 day-1 (+165%). Thus, soil N pools did not decline as live biomass N pools increased over time. Among stands, foliar and litterfall N concentrations declined with stem density and productivity, whereas the foliar N pool increased. Lodgepole pine productivity was negatively correlated with annual resin-sorbed N, and there was no evidence of widespread N limitation. Soil δ15N values were positive and declined as soil total N increased; foliar δ15N values were negative and increased with foliar N concentration. These results cannot be explained by atmospheric deposition or presence of known N fixers. Rather, N dynamics in these young postfire stands are consistent with recent reports of N fixation in young lodgepole pine and facultative N fixation in early seral forests following disturbance.