Brown University Dept. of Ecology & Evolutionary Biology

A major focus of my lab is exploring the effects of tectonic uplift and erosion on landscape and ecosystem-scale nutrient availability – a topic that I think is ripe for integration of biology and geology, and one that may lead us to a fundamentally different understanding of the importance of geologic processes in structuring ecosystems. This question is of fundamental importance because nutrient availability is a prime determinant of how systems respond to changes in climate and land use. Particularly in the tropics, where some of the last relatively intact forests are rapidly being converted to pasture and agriculture, we need to develop a better understanding of how soil fertility varies spatially, and how this influences ecosystem properties and the sustainability of human land uses.

This project is currently funded by the Andrew Mellon Foundation and the National Science Foundation, and is in collaboration with George Hilley. We are taking two different approaches: 1) a modeling effort to understand how different landscapes evolve through time and how this affects ecosystem dynamics and 2) a series of field measurements to help us understand what is right, and more importantly what is wrong, in the way we are thinking about important processes. To date, one of the most exciting results from the modeling tier of the project is the suggestion that erosion may be one of the dominant factors in controlling nutrient availability worldwide. As we scale up this result, it calls into question one of the most commonly made assertions in the literature, that tropical soils are nutrient poor. Furthermore, we are making progress on a first-principles based model that will predict nutrient availability across tropical landscapes.

Of course the fun is not in the modeling, the fun is in the field! We have established field sites in Costa Rica, Venezuela, and Puerto Rico (soon to add Brazil and others) where we are using cosmogenic isotope analyses to date how long soils stay in this landscape before being eroded away. Cosmogenics are a very new tool in biogeochemistry, and these will be the first dates of soils from a geologically dynamic tropical rainforest anywhere in the world! We will will couple this information with measurements of nutrient availability. Ultimately, the goal of this project is to come up with a predictive framework that describes spatial variation in nutrient availability across the tropical biome.

Chaves, J. and S. Porder (submitted). Inorganic and organic nitrogen losses from a montane tropical forest in Las Alturas, Costa Rica. Biogeochemistry.

Porder, S. and G.E. Hilley (submitted). Linking chronosequences with the rest of the world - predicting soil phosphorus content in eroding landscapes. Biogeochemistry.

Hilley, G.E. and S. Porder (2008). A framework for predicting global silicate weathering and CO₂ drawdown rates over geologic time. Proceedings of the National Academy of Sciences 105(44) 16855-16859 (doi/10.1073/pnas.0801462105)

Porder, S., P.M. Vitousek, O.A. Chadwick, C.P. Chamberlain and G.E. Hilley. (2007). Uplift, erosion, and phosphorus limitation in terrestrial ecosystems. Ecosystems 10: 158-170.

This project, a collaboration with Oliver Chadwick, is focused on understanding variation in nutrient availability across several Hawaiian landscapes. Hawaii is an ideal model system for ecological and biogeochemical research, because it provides an unparalleled opportunity to control important variables across large spatial scales. There are six factors thought to control ecosystems properties: climate, parent material, age, topography, organisms, and human activity. This study takes advantage of the fact that in Hawaii it is possible to have extraordinary variation in one of these factors at a time. For example, walking up a single lava flow on the Big Island can take you from a desert getting 200 mm/yr rainfall to a rainforest getting 3,000 mm/yr, all within a short distance on the same age parent material. Stepping off that lava flow may put one on soil that is hundreds of thousands of years older, but experiencing the same climate! onto the adjacent one may alter the substrate age by hundreds of thousands of years. We are currently working on how climate and vegetation control nutrient losses and soil development.

Porder, S. and O.A. Chadwick (2009). Climate and soil-age constraints on nutrient uplift and retention by plants. Ecology 90 (3): 623-636.

Porder, S., O.A. Chadwick and G.E. Hilley (2007). Chemical weathering, mass loss and dust inputs across a climate by time matrix in the Hawaiian Islands. Earth and Planetary Science Letters 258: 414-427.

This project, in collaboration with Chris Neill, explores the biogeochemical and hydrologic effects of this dramatic landuse change in the world's largest intact tropical forest ecosystem. Myriad questions remain to be answered. Will agricultural intensification spare other, intact, land from development? What will the effects of this intensification be on surrounding forest ecosystems? What are hydrologic and climatologic effects of this radical shift in land use? This project is the basis for Shelby Hayhoe's Ph.D. thesis in the Brown/MBL joint program. Deforestation in the Amazon is an emerging focus at Brown, from the ecological, sociological, and economic perspectives.

Hayhoe S., Neill C., McHorney, R., Porder, S. and Lefebvre, P. (in prep). The effects of soy agriculture on stream discharge and flowpaths in the southeastern Amazon.