PARENT SESSION
Oral Session #94: Nutrient Cycling.
Presiding: R. Yanai
Thursday, August 8. 1:00 PM to 3:45 PM. Grand Ballroom East, Radisson.
Foliar nutrient status and element ratios in northeastern forests following debris avalanche disturbance.
Bryant, David*,1, Aber, John2, Innes, James3, Zarin, Daniel4, 1 Harvard University, Cambridge, MA2 Complex Systems Research Center, Durham, NH3 University of New Hampshire, Durham, NH4 University of Florida, Gainesville, FL
ABSTRACT- Erosion of mountain slopes during and following debris avalanches is extensive, often removing 50% to nearly 100% of organic matter, mineral soil and the nutrient reservoirs. Debris flow processes grind and fracture the regolith providing fresh surfaces for weathering of P, K. Ca, and Mg. Anthropogenic deposition of nitrogen may coincidentally provide N for plant colonization in lieu of biological N fixers, however these inputs represent only a minor fraction of annual soil N mineralization in mature forests of the northeast. We hypothesized that foliar and litterfall N content would increase with site age concurrent with the accumulation and cycling of N in the forest floor. Conversely, we expected rock derived nutrients to decline with site age as P, K, Ca and Mg are sequestered in developing biomass. These hypotheses were tested using foliar and litterfall macronutrients measured in 8 stands on a chronosequence of debris avalanche deposits in the White Mountain National Forest, NH USA. Additionally, mass loss and nutrient turnover rates were determined using litter bags of the four dominant species incubated for 2 years at each site. Neither foliar nor litterfall N, Ca, or Mg content showed a strong relationship with site age. Foliar K content showed a positive age correlation (r2 = 0.6837 p <0.01), while foliar and litterfall P was negatively correlated with site age (r2 = 0.7040 p <0.01 and r2 = 0.8144 p <0.01, respectively). Also, seed rain was positively correlated with foliar P content (r2 = 0.5444 p <0.01). Molar N:P, N:Ca and N:Mg ratios were much greater than published empirical ratios of these elements. Foliar N:P ratios were also strongly correlated with site age (r2 = 0.7541 p <0.01). These results suggest that P may limit production and reproductive effort in forests recovering from severe disturbance. However, regional data, compiled from the MAPBGC project, are consistent with these findings and indicate that low P and Mg foliar content extends to the WMNF region in general.
KEY WORDS: Ecosystem development, Phosphorus limitation, Magnesium limitation, Element ratios