Wednesday, August 6, 2008: 8:20 AM
201 A, Midwest Airlines Center
Erica Smithwick, Department of Geography, The Pennsylvania State University, University Park, PA, Daniel M. Kashian, Department of Biological Sciences, Wayne State University, Detroit, MI, Michael G. Ryan, Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, CO and Monica G. Turner, Department of Zoology, University of Wisconsin- Madison, Madison, WI
Background/Question/Methods Nitrogen (N) is considered to limit lodgepole pine productivity in high elevation Rocky Mountain forests. Nitrogen availability is altered by stand-replacing fire, which affects > 4 million ha yr-1 nationally. Both immediate and longer-term effects of severe fire on N cycling have been elucidated. However, few general hypotheses have emerged regarding how post-fire patterns of N availability and stocks vary at broad spatio-temporal scales. Here, we present results from a replicated chronosequence study in Yellowstone National Park (Wyoming, U.S.A.) directed at measuring N availability (ion-exchange resin bags) and N pools among stands at varying time since fire and post-fire densities.
Results/Conclusions Among age-classes, N availability ranged from 0.63 (<25 yrs) to 1.67 µN bag-1 d-1 (170-230 yrs), largely due to increases in nitrate availability, and remained high thereafter (1.10 µN bag-1 d-1, >250-yrs). N availability was lower in the young, high-density compared to low-density stands (0.47 vs. 0.92 µN bag-1 d-1). N availability was similar among beetled-killed and non beetle-killed stands. Total N was lowest in the <25 yr age-class (130.6 gN m-2) and did not increase significantly beyond 40 yrs (average = 276.3 gN m-2). Live pool N ranged from 2% ( <25 yr) to 53% (80-130 yr); soil N was 58% of total N in the <25-yr age-class, but 33 to 36% >80 years. Total N was positively correlated to Bray-P availability (p<0.005) but relative nitrification was negatively correlated to P (p<0.0001). Both total N availability and total N were negatively correlated with pH (<0.0001). Models to predict N availability (R2= 0.41) included stand tree density, pH and P availability, whereas models for total N (R2=0.43) included pH, P availability, and cation exchange capacity. We conclude that although patterns in total N stocks largely follow patterns of biomass accumulation through succession, increased nitrate availability in older stands did not support the notion of progressive N limitation with stand age and may reflect variation in tree density, pH, and soil P.
