Degradation of forests through logging and fire in the eastern Brazilian Amazon

The status of the Amazon forest can no longer be expressed adequately by a simple concept of deforestation. Throughout the Amazon basin, logging and ground fires are degrading forest structure in ways that are not addressed by the dichotomous forest vs. non-forest classification schemes used to determine deforestation rates (e.g., Nepstad et al., 1999). For the purposes of national environmental accounting, these logged and burned areas remain classified as intact forests (Alves et al., 1998). However, to the extent that logging and fire degrade forest structure and species composition, it may be more accurate to describe these degraded forests as occupying an intermediate status between intact and cleared forest that demands a new classification scheme.

Forest degradation in the eastern Brazilian Amazon is often an incremental process where relatively low intensity impacts set the stage for the other, more profound, disturbances. The first step in this process is usually logging. In well-established logging frontiers an initial extraction commonly removes 3–9 trees ha⁻¹ of the nearly 100 tree species that are commercially valuable (e.g., Verı́ssimo et al., 1992). Although logging is selective, impacts to the residual forest stand are substantial including 20–40% reductions in canopy cover and tree density, in addition to threefold increases in the mass of woody debris that could serve as fuel for subsequent fire (Uhl and Kauffman, 1990; Verı́ssimo et al., 1992; Johns et al., 1996). As an established logging frontier begins to senesce, local log shortages combined with the high cost of transporting logs from distant forests can provoke a continuation of the “forest mining” sequence of repeat logging described by Uhl et al. (1997). In this case, a repeat extraction removes nearly all remaining individuals of marketable species including those deemed previously to be too small.

Fire, the other primary force driving forest degradation, is closely coupled to logging. Logging facilitates the spread of fire by disrupting forest canopy cover, thus allowing solar radiation to dry the otherwise humid forest floor and by providing large quantities of fine fuels in the form of residual logging debris (Uhl and Kauffman, 1990; Holdsworth and Uhl, 1997). Furthermore, logged forests often occur adjacent to pastures or agricultural plots where fire used in land clearing and pasture maintenance provides sources of ignition (Uhl and Bushbacher, 1985). In addition, once a forest has burned its ability to resist future burns is impaired (Cochrane and Schulze, 1999). Thus, the forest degradation that began with one or more logging episodes can enter into a positive feedback cycle where an initial burn increases the likelihood of additional, more intense burns (Cochrane et al., 1999). In some cases, however, this progression is shortened when fire from burning agricultural lands spreads into forest that has not been logged, but is suffering from drought and reduced canopy cover resulting from leaf shedding (Peres, 1999).

Given the multiple factors that drive the process of forest degradation, it is likely that concentrations of degraded forests will be found surrounding well-established centers of logging and agriculture. One such center is the town of Paragominas, Pará, where low intensity logging began during the early 1970s and expanded during the 1980s, as timber production boomed and the number of mills rose to over 200 (Verı́ssimo et al., 1992). In the wake of this timber boom remains a landscape largely made up of agricultural land and forest that is degraded to varying degrees. An initial analysis of a 1991 Landsat TM image of a 3600 km² region surrounding Paragominas classified 62% of the area as forest. However, more detailed analysis, based on the multi-temporal analysis of images from several years and interviews with landowners, revealed that only 10% of this forest had not been subjected to logging or burning (Nepstad et al., 1999).

Understanding both the local (e.g., reduced biodiversity and timber production capacity) and global (e.g., reduced carbon sequestration) implications of forest degradation requires detailed comparisons among forests in different states of degradation. The objective of this study was to assess the condition and future productive capacity of residual forests in an eastern Amazonian landscape that has undergone 30 years of logging and agricultural activity by comparing forest structure and composition among stands subjected to varying intensities of logging and fire.