PARENT SESSION
Oral Session #68: Nutrient Cycling. Presiding: D. Johnson.
Thursday, August 9, 2001. 1:00 PM to 5:00 PM. Hall of Ideas G.

Stoichiometric equilibration of N and P losses over four million years of tropical forest development.

HEDIN, LARS¹, VITOUSEK, PETER², MATSON, PAMELA², BUTTEL, LINDA¹, LEVIN, SIMON³, PACALA, STEPHEN³, DURRETT, RICHARD¹, ^{1 2 3}

ABSTRACT- Theoretical and simulation models predict that N and P cycles ought to equilibrate over the course of forest ecosystem development. Inputs of N from biotic fixation and abiotic deposition should accumulate only as long as N remains limiting relative to P at the ecosystem scale. We studied hydrologic and gaseous losses of N and P from montane tropical forests across a four million-year developmental age gradient across the Hawaiian Islands. Patterns of N and P loss were consistent with N-limitation in developmentally young forests, and P-limitation in forests 20,000 years and older. N to P loss ratios equilibrated at molar ratios (~ 1,400:1) that greatly exceeded plant N:P demand ratios (~ 37:1), indicating persistently high supplies of N relative to P in developmentally old forests. We used simulation and analytical models to explore different explanations for such sustained high N to P loss ratios, including differences in internal recycling efficiencies, differences in external N to P loading ratios, and the layered structure of terrestrial ecosystems.

KEY WORDS: Forested ecosystems, Tropical forests, Nutrient losses, Nitrogen, Phosphorus