The Export of Nutrients and Recovery of Stable Conditions Following Deforestation at Hubbard Brook

Abstract

We report the effects of deforestation on the export of particulate matter, erodibility of the ecosystem, and the relative importance of dissolved substances and particulate matter in exported materials. The mature forested ecosystem is little affected by erosion, with an average annual particulate matter export of only 2.5 MT (metric tons) km—2 yr—1. Deforestation and repression of growth for 3 yr increased export to a maximum of 38 MT km—2 yr—1, but the increase in export was exponential with rather minor increases in the first 2 yr after cutting and a sharp increase in the 3rd yr. This resulted because the ecosystem continued to exercise considerable control over erodibility for 2 yr after cutting without annual renewal of biotic regulation by primary productivity. Increases in particulate matter export are primarily due to increases in erodibility rather than increased flow rates. In the mature ecosystem, the average ratio of annual net export of dissolved substance to particulate matter is 2.3. Deforestation shifts this ratio to >8.0 during the first 2 yr after cutting. This shift results because the export trends, for dissolved substances and particulate matter following deforestation, are not synchronous. The first response to deforestation is mobilization of nutrients from the available nutrient and organic matter compartments and leakage in stream water. After 2 yr, particulate matter output rises sharply as biotic control of erodibility weakens, while dissolved substance export declines, probably because of diminution of readily available nutrients stored within the system. This nonsynchrony, coupled with the action of natural species adapted to take advantage of the abundance of nutrients and water that results immediately after disturbance, may be considered as part of a homeostatic mechanism that allows rapid recovery of a forest ecosystem while minimizing the effects of erosion. Thus, following destruction of the vegetation, the ecosystem maintains a residual resistance to erosion and has a high potential for repair through successional productivity. Nutrient flux and erosion losses return to previous levels as the intrasystem aspects of the hydrologic—nutrient cycle interaction and erodibility are increasingly regulated by biotic factors. These results relative to stable conditions and their reestablishment following disturbance may have applicability to a wide range of terrestrial ecosystems.