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How Does Changing Tropical Forests to Farms Affect the World Underground?

Karen Carney
Geological and Environmental Sciences Department
Stanford University
June 2001

My research seeks to understand whether the loss of plant diversity associated with the conversion of tropical forest into agricultural land affects soil microbial communities and the critical processes that they govern.

Anyone who has ever seen a tropical forest has surely been struck by the sheer number of species of all different life forms harbored there. For example, over 200 species of plants can be found in a single hectare of tropical forest in Panama. Over 400 species of birds inhabit the forests of Belize. Yet even these numbers are dwarfed when you consider the millions of species of beetles that reside in the wet tropical forests of the world. While hiking through one of these species-rich forests, your path might meander into a nearby pasture, and once again you may be struck, this time to find that the immense diversity you just witnessed in the natural ecosystem of the forest has given way to a relative wasteland of biological diversity. In the pasture, you may find a few species of grass, perhaps a few remnant forest trees, and a sparse collection of birds (and of course some cows!). In fact, you would find a similar pattern if you compared native tropical forest to croplands or tree plantations. Human management homogenizes tropical ecosystems, resulting in a huge loss of species. Such drastic changes in the ecosystems of tropical forests immediately beg the question How does simplifying ecosystems affect the way they work and the very processes that keep them going? Many ecologists around the world, myself included, are trying to find the answer.

Many recent studies have found that increasing plant diversity does, to a point, positively influence certain ecosystem properties, or "functions." Such functions include productivity (the amount of biomass of all plants in an ecosystem), resistance to drought (the ability to sustain productivity in the face of drought), and the stability of the ecosystem in general (how steady the productivity or any other property of an ecosystem is over time). Most of these studies have only focused on the ecosystem functions for which plants are directly responsible, but plants can also indirectly affect ecosystem function by influencing things that live in the ground beneath them, such as microbes.

Microbes (e.g. bacteria, fungi, and actinomycetes) perform critical processes in soil, including the decomposition of organic matter, the transformation of nutrients from one form to another (more or less making them available for plants to use), and, perhaps most importantly, the production of greenhouse gases (the great majority of all the nitrogen plants need in order to grow does in fact come from organic material that microbes decompose). Simply put, without microbes plants would just stop growing. But despite their obvious importance, we know very little about the most basic ecology of most microorganisms. To date, less than one percent of all microbial species have been described. Biologists have, however, recently developed new methods that allow us to begin to describe microbial communities and learn about the factors that are most important in determining which microbes are where and what they are doing.

In my research, I hope to use these new techniques to begin to answer some of the fundamental questions that remain in microbial ecology. Foremost, I will address the rather straightforward question of whether plant community composition (the number and abundance of different plant species) influences microbial community composition (the number and types of different microbial species). I will examine three different aspects of this question after taking soil samples from sites located in the tropical lowlands of Costa Rica. First, since one of my main interests is determining how people affect the ways ecosystems work, I will focus on the following three different land use types (which also differ in their plant communities) native forest, pasture, and cacao plantation. Second, I will try to determine how plant diversity can affect microbial community composition using a gradient of diversity in tree plantations. I will use four plots that have one, three, five, and over 30 species of plants in each, respectively. These plots were initially set up to determine the influence of plant species composition on the productivity and sustainability of tree plantations, and I will take advantage of their design to study the effects of tree species on microbes. Finally, since other investigators have found that plant species composition alone (what species are there, not just how many) can also be important to ecosystem function, I will compare microbes in plots that have the same number of plants but differ in plant species composition. In each of the above sites, I will sample the soil using a 2.5 cm diameter soil core with a 10 cm depth. I will extract the soil for both microbial fatty acids (molecules that reside in the cellular membranes of microorganisms) and DNA. By analyzing the presence/absence and/or quantity of different fatty acids and fragments of DNA in the soil, I will be able to compare community "fingerprints" in each treatment to detect differences.

If there are indeed differences in microbial communities in any of the above treatments, I will go on to test whether a change in microbial community composition can lead to a change in two critical ecosystem processes the decomposition of organic matter and the rate of nitrification (the rate at which ammonium is transformed to nitrate). I will conduct decomposition and nitrification assays in soils such that all factors influencing these processes other than microbial community composition (e.g. soil nutrient availability, temperature, pH, and moisture) are held constant.

As more and more people populate the planet, and as those people become more wealthy and thus consume more resources over time, there will be an increasing need to alter natural systems for grain, meat, and wood production. In the tropics, these alterations result in drastic losses of biological diversity. Although these changes have been occurring over the past 100 years, we still know very little about the short- and long-term effects of such homogenization on the fundamental way these ecosystems work. My research will further our understanding of the influence of plant and microbial diversity on the functioning of ecosystems in tropical forests.