Geologic controls on nutrient availability in terrestrial ecosystems

The southern alps of New Zealand have some of the fastest uplift and erosion rates in the world.  Soil residence times are on the order of a few hundred years, landslides restart the system after that.  Understanding the role of uplift and erosion in controlling soil age requires integrating ecosystem ecology, biogeochemistry, and geomorphology.  It also requires going to a lot of beautiful places to collect samples!

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 funded by the Andrew Mellon Foundation, and is a multidisciplinary collaboration with George Hilley, Peter Vitousek, and Oliver Chadwick.  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!  Joaquin Chaves, a post-doc in the lab, has begun to test our model results in some very interesting places.  He has established sites in eastern Venezuela, where erosion rates are very low and soils are as old as they get in the tropics.  We are using cosmogenic isotope analyses to date how long soils stay in this landscape before being eroded away, and we will couple this information with measurements of nutrient availability.   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!  Because we expect these sites to be some of the oldest soils on earth, we can use this site as an end member for nutrient availability in very old tropical rainforests. We are in the process of establishing another series of sites in southern Costa Rica, where erosion rates are very rapid and soils are quite young, with the idea that this two sites will span the range of soil ages that are common in the lowland tropics.  We will then fill in the erosion rate continuum with sites in Puerto Rico and Mexico.

Publications from this project

• 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.

Nutrient availability across Hawaiian landscapes

 
An airphoto of the island of Kaua’i, where weathering, erosion, and deposition combine to produce a complex patchwork of fertile and infertile forests.  Photo by Greg Asner.

This project, a collaboration with Peter Vitousek, Greg Asner, and  Adina Paytan 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 across Hawai’i, parent material is constant (basalt), wet forests are dominated by the same tree (ohi’a), and undisturbed sites with very similar climate exist on each island.  By controlling these four variables, we are able to address the role of ecosystem age and landscape-forming processes (erosion, deposition) on the biogeochemistry of Hawaiian forests.  Some of the most interesting results to date include strong evidence that in old ecosystems, erosion can rejuvenate the supply of nutrients to forests, and reverse the decline of fertility that occurs in non-eroding ecosystems over thousands of years.  Furthermore, this variability can be detected by airborne instruments, allowing us to map biogeochemical variation at large spatial scales.

Publications on this topic:

• Porder, S., G.P. Asner., and P.M. Vitousek (2005).  Ground-based and remotely-sensed nutrient availability across a tropical landscape  Proceedings of the National Academy of Sciences 102 (31), 10909 – 10912.
• Porder, S., A. Paytan, and P.M. Vitousek (2005).  Erosion and landscape development affect plant nutrient status in the Hawaiian Islands.  Oecologia 142, 440 – 449. 
• Vitousek, P; Chadwick, O; Matson, P; Allison, S; Derry, L; Kettley, L; Luers, A; Mecking, E; Monastra, V; Porder, S.  (2003)  Erosion and the rejuvenation of weathering-derived nutrient supply in an old tropical landscape.  Ecosystems 6(8) 762-772.

 Landscape biogeochemistry of Costa Rica

 
The lowland tropical forest of La Selva, Costa Rica, as seen from an above
canopy tower used to measure CO2 flux and rainwater composition.  Photo by Matt Clark.

This project, in collaboration with Deborah Clark, Peter Vitousek, Alan Townsend and Carleton Bern, compares the results from the Hawaiian study described above with a similar set of measurements taken in Costa Rica (at La Selva Biological Station and on the Osa Peninsula).  In many ways, the Hawaiian and Costa Rican sites are similar.  Both are tropical forests on volcanic parent material, both a quite wet (over 2.5m rain/year, and up to 11m/year in Kaua’i).  La Selva is dominated by a single species of tree (though not the same species in Costa Rica as in Hawai’i).  However, La Selva is much warmer than the Hawaiian sites, the forests are at least ten times as diverse, and the system was thought to be as old as the oldest Hawaiian Islands.  This study turned up some surprising results. First, the forests of La Selva, which we expected to be similar to the oldest, most nutrient poor Hawaiian systems, were actually quite a bit more like intermediate-aged, fertile Hawaiian forests.  In fact, La Selva soils showed very little indication of old age.  We were forced to conclude that La Selvas soils are fairly young, and more nutrient rich than previously believed.  While this was an interesting result for research in La Selva, this project also did a lot to shape my thinking about nutrient availability in the tropics.  It has led me to question how nutrient poor the tropics actually are (a common assumption in the literature), and to try to incorporate a better understanding of landscape dynamics into studies of nutrient availability.  Recently I’ve collaborated with Carl Bern and Alan Townsend, who have been doing really interesting work on landscape dynamics on the Osa Peninsula, and we’ve been trying to understand what drives the differences between Hawaii and Osa. 

Publications from this project:

• Bern, C.R., Porder, S. and A.R. Townsend. (2007).  Erosion and landscape development decouple strontium and sulfur in the transition to dominance by atmospheric inputs.  Geoderma 142: 274-284.
• Porder, S., D.B. Clark, and P.M. Vitousek (2005).  Persistence of rock-derived nutrients in the wet tropical forests of La Selva, Costa Rica. Ecology 87(3), 594-602.

Carbon storage in the rangelands of the Rocky Mountains

Just south of Yellowstone, and one of my favorite places in the world, lie the Teton Mountains. 
A great place to ask interesting scientific questions and the best skiing in the lower 48. 

Not all of my projects are in the tropics.  I have worked on several different projects in the Rocky Mountains, predominantly in the Greater Yellowstone Ecosystem.  My current research is focused on understanding how different grazing practices affects native plant diversity and carbon storage in rangeland ecosystems.  This project has both a scientific and an applied side.  In terms of the science, we need a better understanding of the effects of grazing on below ground carbon storage if we are to accurately assess the future trajectory of climate change.  Forest conversion to pasture has the potential to greatly alter carbon storage in ecosystems.  Different grazing practices, and stocking rates, may affect carbon storage as well.  Thus we need to ascertain which grazing practices lead to the least carbon loss (or the most carbon gain) in soils.  On the more applied side, ranches in the Rocky Mountain West are under heavy development pressure, and with development comes subdivision, loss of open space, and loss of wildlife habitat.  We are working in collaboration with the Chicago Climate Exchange, Beartooth Capital Management and the Sun Ranch in the Madison Valley of Montana, to ascertain whether changes in grazing practices will result in carbon sequestration, and whether ranchers can sell carbon credits on the climate exchange.   This work is just underway, but I’m very excited about the questions we’re asking, I think it will lead in some really interesting, and highly relevant, directions.

Publications from my work in the Greater Yellowstone Ecosystem (though not on carbon storage)

• Porder, S., A. Paytan and E.A. Hadly (2003).  Mapping the origins of faunal assemblages using strontium isotopes.  Paleobiology 29(2) 197-204.

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