Difficulties in tracking the long-term global trend in tropical forest area

Abstract

The long-term trend in tropical forest area receives less scrutiny than the tropical deforestation rate. We show that constructing a reliable trend is difficult and evidence for decline is unclear, within the limits of errors involved in making global estimates. A time series for all tropical forest area, using data from Forest Resources Assessments (FRAs) of the United Nations Food and Agriculture Organization, is dominated by three successively corrected declining trends. Inconsistencies between these trends raise questions about their reliability, especially because differences seem to result as much from errors as from changes in statistical design and use of new data. A second time series for tropical moist forest area shows no apparent decline. The latter may be masked by the errors involved, but a "forest return" effect may also be operating, in which forest regeneration in some areas offsets deforestation (but not biodiversity loss) elsewhere. A better monitoring program is needed to give a more reliable trend. Scientists who use FRA data should check how the accuracy of their findings depends on errors in the data.

Fig. 1.

Trends in Natural Forest area 1980–2005 in 90 tropical countries (10⁶ ha) from data in Forest Resources Assessments (FRAs) 1990, 2000, and 2005, with estimates for 76 countries from FRA 1980 and its 1982 revision. (Sources: refs. –.)

Fig. 2.

Estimates of tropical moist forest area (10⁶ ha) for 63 countries 1973–2000. For clarity, the Grainger (1980) estimate derived from FRA 1980 and the FAOCB 1990 estimate are both shown as FAO estimates, and the GLC estimate is shown as a TREES estimate. (Sources: refs. , , , –, and –.)