Selective logging misses the forest for the trees

Some four million square kilometers of Earth’s tropical forests have been designated for logging. That’s an area larger than the size of India. And that’s a problem for the wildlife that calls those forests home. Compromises in conservation have always been a tricky sell, because environmental harms have a way of rippling out beyond what’s predicted. Still, assuming that some logging is going to occur regardless of what’s optimal, some researchers argue that there are better and worse ways to go about it.

Earlier this year, I reported on a paper published in the journal Current Biology by graduate student Zuzana Burivalova from ETH Zurich in Switzerland. In it, she argued that selective logging, which is the targeting removal of only certain trees, is preferable to the less careful sort of logging that wipes out entire swaths of forest. She further argued that a logging intensity of 10 cubic meters per hectare would be the upper limit to allow all taxonomic groups – she looked at birds, mammals, amphibians, and invertebrates – to retain their biodiversity in the face of habitat loss.

But University of Kent researcher Jake E. Bicknell argues that selective logging, for all its improvements over conventional logging, is not all its cracked up to be. Earlier this week in the journal Current Biology he and his colleagues argue that we can do better through a process they call “reduced impact logging” or RIL.

The problem with selective logging is that it doesn’t account for collateral damage. In practice, it’s no different from conventional logging; it simply targets fewer trees. But the practice of logging is a lot more expansive than the simple removal of trees. “Harvest intensities are not always indicative of actual disturbance levels resulting from logging,” writes Bicknell.

What are the sources of that collateral damage? Not pre-planning the path that roads should take, for example. In RIL, road networks are carefully and explicitly designed to minimize additional damage to the forest. Another is letting the tree fall in any random direction. It’s good to identify particular trees to chop down, but when they fall, they can take out additional trees as well. Being more explicit about the direction a tree is brought down, to avoid taking down additional trees that ought to remain standing, is another feature of RIL. Something else that selective logging ignores is the role of vines. Vines can move from tree to tree and if one tree is brought down, the others can come down like a row of dominos. In RIL, vines are pre-cut to avoid that.

On the surface, RIL seems far superior to selective logging, even if it does require a bit more logistical effort to execute properly. But what does the data say? Bicknell rounded up all investigations of logging effects that compared primary tropical forests with either conventional logging, selective logging, or reduced-impact logging. In all, he had more than 3400 comparisons from 41 different studies.

What they discovered was the forests that used RIL were less damaging to populations of arthropods, birds, and mammals compared with those that used the more traditional practices. And among mammals, bats in particular benefited.

Why not just advocate the lower logging intensities of selective logging? Bicknell argues that a push for lower intensity alone could result in larger expanses of primary forest being logged to meet the demand for timber. In other words, it could lead to a situation where everything is lightly logged, rather than a mosaic of heavily logged areas mixed with untouched primary forest. Perhaps surprisingly, “more biodiversity is retained where high harvest intensities are combined with the sparing of primary forest reserves, rather than universally harvesting at lower intensities,” write the researchers. – Jason G. Goldman | 05 December 2014

Source: Bicknell J.E., Matthew J. Struebig, David P. Edwards & Zoe G. Davies (2014). Improved timber harvest techniques maintain biodiversity in tropical forests, Current Biology, 24 (23) R1119-R1120. DOI: http://dx.doi.org/10.1016/j.cub.2014.10.067

Header image: shutterstock.com

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