Ground-based and remotely sensed nutrient availability across a tropical landscape.

Tropical soils often are assumed to be highly weathered and thus nutrient-depleted, but this prediction applies primarily to geomorphically stable surfaces. Topography complicates the assumption of nutrient depletion, because erosion can enhance the supply of nutrients to tropical ecosystems. Consequently, understanding nutrient availability across landscapes requires a spatially explicit assessment of the relative strength of depletion and enhancement. We document the relationship between foliar nutrients and topographic position across a 20-km(2), 4- to 5-million-year-old eroded landscape in Kaua'i, Hawai'i, and use this relationship to build a bottom-up map of predicted nutrient availability across this landscape. Only approximately 17% of the landscape is nutrient-poor, mostly on stable uplands; nutrient availability on slopes and valley bottoms is much higher, in some cases similar to the most fertile montane forests in the Hawaiian Islands. This pattern was corroborated by top-down remote sensing of area-integrated canopy phosphorus concentrations.