Nutrient availability across tropical landscapes: Examining old paradigms at new scales

My dissertation explores the influence of landscape development on nutrient availability and provenance in tropical forests. Geomorphic processes such as erosion and deposition can redistribute nutrients across the landscape, and alter the rate of nutrient influx to a system by introducing new rock-derived nutrients to soils and plants. I quantified the magnitude of these effects, and tried to understand how they vary in different landscapes. Using analyses of plants and soils, remote sensing, and geographic information systems (GIS) I asked: (1) how does erosion affect biogeochemical patterns in Hawaiian landscapes of different ages? (2) how does the interplay between nutrient depletion and rejuvenation affect the distribution of nutrients across a tropical landscape in Kaua'i, Hawai'i? and (3) how well do patterns of landscape-level variation in nutrient availability in Hawai'i help predict patterns across a similar tropical forest in La Selva, Costa Rica?; My results indicate that erosion can rejuvenate soils by significantly increasing the rate at which bedrock-derived nutrients are supplied to ecosystems. In older systems, where stable surfaces are severely depleted in rock-derived nutrients, this input can fertilize forests on slopes, in some cases doubling foliar nutrient concentrations. I found that ∼75% of a montane forest landscape in Kaua'i was relatively nutrient rich---a result which challenges the paradigm that old tropical forests are uniformly nutrient depleted, and highlights the need to incorporate rejuvenation via erosion into models of landscape and ecosystem development.; My results from La Selva, Costa Rica also offer a surprising contrast to my results from Hawai'i. In La Selva, a wet tropical forest sitting atop 6.8 million year old bedrock has high inputs of rock-derived elements even on geomorphically stable surfaces. Erosion does increase these inputs, but has little effect on plant nutrient status. Thus the forests at La Selva are, in terms of nutrient inputs and the effects of erosion, much more similar to the young forests of Hawai'i than the old, nutrient depleted forests of Kaua'i---further highlighting the need for cross-ecosystem comparison to shape our understanding of ecosystem development across tropical landscapes.