By Manuel R. Guariguata
Principal Scientist, Center for International Forestry Research
As tropical forests give way to cities, roads and soybean fields, what’s left behind is a collage of forest remnants and ‘secondary’ forests that regrow after agricultural lands are abandoned. While protecting primary forests will always be essential for tropical conservation, these mosaic landscapes do retain a substantial proportion of forest species, even where forest products are extracted. Researching the impacts of timber harvesting on tropical forest plants and animals has kept ecologists busy over the past three decades. The question of just how much selectively logged forests contribute to global biodiversity conservation remains poorly analysed, and essentially, unanswered. But two recent meta-analyses of previously published research provide fresh evidence that selective logging, if carefully done, has relatively benign impacts.
In an article appearing in the journal Conservation Letters, Putz et al. (2012) found that, across a sample of 109 studies, the impacts of selective logging on the number of bird, mammal, insect and plant species were very modest overall. And good harvesting practices were not employed in most of the analysed cases, suggesting that their results are conservative. On the basis of 35 studies, the article by Gibson et al. (2011) in the journal Nature similarly reports that the impacts of selective logging on primary forest biodiversity are relatively small. Gibson and colleagues reinforced these conclusions by eliminating the ‘drawer effect’, i.e. the tendency to publish only when significant results are obtained.
There are some obvious flaws in the analyses: although meta-analyses are well-known statistical tools used to elucidate trends among a disparate set of studies with different experimental approaches and methods, the results are inevitably a caricature of reality. Studies from African forests are largely underrepresented in both articles. The number of trees harvested (the logging intensity) also varied considerably amongst published studies, as did logging techniques. And most of the published studies were of short duration, making it impossible for these meta‑analyses to comment on long-term consequences.
Despite these shortcomings, both articles provide compelling evidence that selectively logged tropical forests across the globe are critical for conserving the full spectrum of biodiversity — from beetles to orangutans. The question is whether or not the long-term persistence of forests can be guaranteed solely on the basis of profits from timber, so that sustainable forest management is competitive compared with other market forces like agro-industrial expansion. The results of the article by Putz and colleagues (2012) seem to indicate ‘not’; timber yields are expected to decrease over time because the 20–40 year logging intervals currently applied across the tropics cannot guarantee a constant supply of wood with attractive financial returns.
To help counter this trend, Putz et al. (2012) suggest a mixed strategy. One essential step is further promotion of sustainable timber harvesting practices, as these are directly linked to more forest carbon being retained, for which financial compensation might be available through REDD+ incentives. They also recommend increased recognition of locally based approaches to multiple-use forest management under clear land tenure arrangements. Other viable options include enhancing the financial benefits from forest certification, and making assurance of legality a prerequisite for international market access. The key to success is achieving the right balance across these approaches, so that climate, biodiversity and people all benefit, and the multiple demands for tropical forest resources are satisfied. With nearly 400 million hectares of tropical forest officially designated for production purposes worldwide, one could say that there is plenty of room to play.
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Putz, F.E., Zuidema, P.A., Synnott, T., Peña-Claros, M., Pinard, M.A., Sheil, D., Vanclay, J.K., Sist, P., Gourlet-Fleury, S., Griscom, B., Palmer, J. and Zagt, R. 2012 Sustaining conservation values in selectively logged tropical forests: the attained and the attainable. Conservation Letters. DOI: 10.1111/j.1755-263X.2012.00242.x
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Gibson, L., Ming Lee, T., Pin Koh, L., Brook, B.W., Gardner, T.A., Barlow, J., Peres, C.A.,
Bradshaw, C.J.A., Laurance, W.F., Lovejoy, T.E. and Sodhi, N.S. 2011 Primary forests are irreplaceable for sustaining tropical biodiversity. Nature 478: 378–383. DOI:10.1038/nature10425