Estimating the number of trees and forest area necessary to supply internationally traded volumes of big-leaf mahogany (Swietenia macrophylla) in Amazonia

Abstract

Big-leaf mahogany's listing on CITES Appendix II requires producer nations to certify that exported supplies were obtained in a manner non-detrimental to the species' survival in its role in the ecosystem. Non-detriment findings based on annual export quotas should verify that current harvest rates are sustainable with respect to total commercial stocks. In order to assess this impact, a method for converting export-quality sawnwood volumes to numbers of standing trees is used to estimate the number of mahogany trees and forest area required to produce the original and revised 2007 export quotas set by Peru, correcting for systematic measurement error caused by buttresses and for stem defects caused by heart-rot (hollow bole). Based on large-scale inventory data from three forest sites in nearby south-west Brazil, the average commercial-sized (> 75 cm diameter in Peru) mahogany tree in this region would yield 6.4-8.5 m 3 of roundwood (standing volume), which in turn would be processed or milled into 1.7-2.2 m 3 of export-grade sawnwood. From this estimate, 6120-8070 commercial-sized trees would have been harvested to supply the original 2007 export quota of 13477m 3 , from a forest area of 407 300-536 750 ha at landscape-scale densities reported by the Peruvian CITES Scientific Authority. To supply the revised export quota of 4983m 3 , an estimated 2260-2980 commercial-sized trees will be harvested from a forest area of 150 600-198 450 ha. Both estimates exceed the number of trees the Peruvian Scientific Authority estimates can be sustainably harvested annually (961±144 trees > 75 cm diameter) based on preliminary inventory data. The method estimates that, since 1996, 154 000-203 000 mahogany trees have been logged in Peru from a forest area of 10.2-13.5 million ha to supply the total reported export volume during this period (including the revised 2007 export quota) of 339114 m 3 . This area corresponds to 18-25% of mahogany's total natural range in Peru, or 37-49% of mahogany's estimated remaining range in 2001. Without empirical knowledge of density patterns, surviving commercial stocks, and biological and technical issues linking processed lumber (sawnwood) to standing trees (roundwood), it will remain difficult to evaluate the sustainability of export quotas issued at the national level for mahogany or other tropical timber species.