Land-sparing/sharing in tropical logged forests

The dichotomy between land-sharing and land-sparing has been used a lot in studies on the impacts of agriculture on biodiversity to compare between relatively intense, highly productive agriculture that spares natural ecosystems from conversion and extensive, wildlife friendly agriculture with lower yields. The comparison between these two extreme ends of the land-use spectrum could potentially be applied to a whole host of problems relating to how we use land, such as urban planning, electricity production and timber production. While making changes to a manuscript I have been pondering the last of these problems a bit, in the context of tropical selective logging.

Our recent preprint, as well as in 2 other papers in the last year (here and here), showed how the impact of logging biodiversity and carbon storage vary over a gradient of logging intensity. Where large volumes of wood are extracted species richness of trees and animals are negatively impacted, animal populations are reduced as is carbon storage in tree biomass. This gradient of logging extraction represents potential different intensities at which tropical forests could be logged, extensive and low intensity, or high intensity and spatially concentrated. Though it is a topical subject (indeed there has been an NCEAS working group set up to deal with it and who have a flashy website here) there has been little empirical study of land-sparing/sharing in the context of tropical forests, with the only study published so far suggesting that land-sparing presents a better option for birds, dung beetles and ants in Borneo. Typically the gradient of timber extraction is calculated as the volume of trees felled per hectare. However, there are a number of problems that make this metric far from ideal.

Firstly, it tends to be calculated at very large scales, often covering an entire forest concession of hundreds of hectares. To get a better idea of the impact of logging across a gradient the scale of the measurement needs to be reduced so that variation between plots can be examined. Also, though the volume of trees felled obviously tells us quite a lot about the gradient of disturbance, it doesn’t actually tell us what we want to know – the yield. Just as crop yields are what is most important for a farmer the yield of timber from a logging concession is the primary concern of logging companies. Importantly the volume of trees harvested is not always very good at measuring this, since some logging operations are more efficient than others. For example, some trees that are felled but never actually make it to the sawmill. Such wastage is more likely in unplanned logging when lack of co-ordination can result in logs being left behind after being cut, and as a result the yields per hectare can be lower than measures of logging intensity might otherwise suggest.

Though there is currently some discussion of whether sharing or sparing are likely to result in better outcomes in tropical logged forests, the truth is that we currently don’t know much since we lack the sufficient evidence. In order to get this much needed evidence we need to make sure that when studies are designed to answer the land sparing/sharing question in tropical forests they use timber yield, not logging intensity as their gradient and species density as their response variable. Doing this will require closer collaboration with logging companies in order to get detailed information. Some people have cited the fact that logging appears to have relatively little effect on species richness at low intensities, however as I have discussed these relatively modest reductions in species richness may mask large changes in what species are present. As such species richness has no place in the debate about the configuration of landscapes in the context of tropical logging.

In addition to the populations of priority conservation species any future assessment of land-sharing/sparing must recognise that recovery times for carbon and timber tree populations are likely to be longer when logging intensities are high. Given this it seems likely that in order to reach as many goals as possible logging intensity should be high enough to reduce the area impacted but low enough to allow recovery within cutting periods – often around 30 years. Finding this balance will be difficult in the current data vacuum.

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