The Effect of Forest Clearing on the Chemistry and Productivity of Amazonian Streams
Although most of our knowledge about the effects of human activity on both terrestrial and aquatic ecosystems comes from studies in the temperate regions of North America, the most extensive and rapid conversion of natural ecosystems into crop and pastureland today is taking place in the tropics. Clearing of rain forest for cattle pasture is currently the most common form of land-use change on earth. This state of affairs is particularly true in the Amazon Basin, home to the world's largest remaining tropical forest ecosystem, where about 70% of the cleared land is used for cattle grazing.
Recently I have begun a collaboration with Ecosystems Center scientist Christopher Neill that focuses on how the clearing of forest for pasture changes the chemistry and ecology of small streams in the Brazilian Amazon. Small streams are of interest because they are the corridors that connect terrestrial ecosystems with larger rivers.
As the accompanying photographs show, some of the effects of forest clearing are direct and obvious. Elimination of the forest canopy increases light reaching the stream surface. Surface water runoff increases because rainfall interception and transpiration by trees are reduced, and the potential for increased sedimentation becomes greater. Cattle trample the banks of streams. Other consequences of forest clearing are more subtle. Clearing alters the availability and proportion of nutrients in formerly forested soils and thus the likelihood that these nutrients will make their way into streams.
We are conducting our study in Rondônia, a state in the southwestern part of the Amazon Basin, which has experienced an explosive growth in the conversion of forested land into cattle pasture since the early 1980s. Jerry Melillo and Paul Steudler of The Ecosystems Center as well as Neill have been working in this area since 1991 with Carlos Cerri and others at the Centro de Energia Nuclear na Agricultura (CENA) of the University of São Paulo. This team is studying the effects of forest clearing for pasture on soil organic matter, nitrogen cycling and exchanges of trace gases with the atmosphere.
The joint MBL-CENA group has identified several key changes in terrestrial ecosystems that are associated with the conversion of forest to pasture. Pasture soils have higher pH levels because of the ash deposited after forest vegetation is burned. They also have a higher availability of phosphorus, lower availability of inorganic nitrogen and lower production of nitrate than forest soils. With these results in mind, we developed a hypothesis about the ways in which changes in soil nitrogen and phosphorus dynamics are linked to stream chemistry and productivity (Figure 1).
According to our hypotheses, the relatively high availability of nitrogen and low availability of phosphorus in forest soils results in a high movement of nitrogen and low movement of phosphorus into streams. Because forest streams have high ratios of nitrogen to phosphorus, algal growth is limited by phosphorus where it is not already limited by low light levels in the shade of trees. After pastures are established, both soil nitrogen availability and the flow of nitrogen into streams decline.
Because phosphates associated with iron and aluminum, which dominate those of tropical soils, are more soluble at high pH levels, we suspect that the higher pH of pasture soils leads to greater availability of soluble phosphorus. Along with potentially greater erosion of particulate phosphorus from pastures, this availability increases the amount of phosphorus reaching streams. We hypothesized that this alteration in the relative availability of nitrogen and phosphorus would lower ratios of nitrogen to phosphorus in stream water and lead to nitrogen limitation of algal production in pasture streams.
We tested these ideas by examining the chemistry of streams draining small watersheds that were either entirely forested or were entirely covered by pastures that had been established five years before. We found that streams draining pasturelands had lower concentrations of inorganic nitrogen (nitrate and ammonium) but higher concentrations of phosphate than the streams draining the native forest (Figure 2 a-c). Ratios of nitrogen to phosphorus were 32:1 to 238:1 in the forest streams but 3:1 to 4:1 in the pasture streams (Figure 2d).
Ratios greater than 16:1 indicate phosphorus limitation of algal growth, whereas ratios less than 16:1 indicate nitrogen limitation. Therefore phosphorus should limit algal production in forest streams if the light limitation is removed, and nitrogen should limit algal growth in pasture streams. The higher nitrate concentrations in streams draining forested areas compared with those found in pasture streams differ from the pattern found in temperate zone ecosystems, where nitrate losses are generally higher from disturbed or altered sites.
In order to see how changes in nutrient inputs caused by pasture creation would affect the base of the stream food web, we also performed bioassays to determine the response of the algae to added nitrogen or phosphorus (Fig. 3). In these assays, nitrogen or phosphorus was added to agar contained in small bottles topped with porous ceramic disks. Algae grow on the disks and utilize the nutrients as they gradually diffuse out of the agar. One assay was located in a forest steam, the other in a pasture stream. In the forest stream assay, neither nitrogen nor phosphorus increased algal production, indicating that light rather than nutrients limited algal growth. In the pasture stream assay, algae, responded to both nitrogen and nitrogen plus phosphorus. These results supported the original hypothesis and demonstrated clearly how changes in soils of the upland, or terra firme, forests are linked to stream productivity. They also showed that watershed clearing caused a switch from one limiting nutrient to another.
While concepts gained from temperate North American streams can guide research on tropical ecosystems, there is no substitute for field studies. The chemistry of old, highly-weathered tropical soils and the intricate biological relationships in places like the Amazon are likely to yield surprises. We plan to expand our studies by examining rates of in-stream nutrient processing to determine how long the influences of forest or pasture environments are maintained downstream. This is important in a region where streams pass through a mosaic of forest and pasture. Will a stream segment of several kilometers through forest eliminate the effects of upstream pasture land use? These kinds of questions will be important for predicting how the cumulative changes in small watersheds will influence larger tributaries and ultimately the Amazon River itself.
The fate of algae produced in streams that now run through an increasingly deforested landscape will also be the subject of future studies. A diverse assemblage of aquatic animals, particularly freshwater fishes, uses the river networks of the Amazon Basin. Most have life histories and seasonal migration patterns that are closely tied to annual flooding and the location of areas with high algal production. We currently know very little about these complex relationships or how they will be affected by the changes brought by deforestation. But there is much knowledge about tropical aquatic ecosystems to be gained by focusing on small watersheds, where the ecological functioning of streams is tightly coupled with activities on the land.