A few days ago we put some of my thesis work on the impacts of tropical forest logging on the preprint server PeerJ. The work is currently in review elsewhere but I thought I should put up a blog post about our findings, even if they change a bit after the review. I am really pro the idea of making results available as soon as possible so that they can be read and cited. I have lost count of the number of times a piece of work I have seen presented at a conference and wanted to cite has taken 1-2 years to come out as a paper. In short I think preprints are the future, so feel free to read, comment on and critique ours over at Peerj (and you can even cite it if you like).
A fifth of tropical forests have been logged in the recent past. Though logging is an important source of timber and jobs it also faces questions about its long-term sustainability particularly in its impacts on biodiversity and carbon. However, as I have written before, the results of studies on the impact of logging are very variable making generalisation difficult. Previous meta-analyses of the impacts of logging have indicated that biomass losses can be as high as 66% or as low as 4%, while tree species richness may be reduced by as much as 53% or show increases of up to 27%. However, none of these meta-analyses of the impacts of logging on tree biodiversity or biomass have explored the potential reasons for these differences. A recent study showed that differences in the impact of logging on animal species richness was explained by the variation in intensity of logging at sites, measured as the volume of wood removed per hectare. Interestingly this study showed that while species richness generally declined for amphibians, mammals and invertebrates with increasing intensity, bird species richness actually increased slightly.
Another recent piece of work by Jake Bicknell and colleagues has shown that a method called reduced impact logging (RIL), a technique which aims to reduce logging damage by altering extraction methods, reduces the negative effects of conventional logging techniques on animal population sizes. This is an important result as RIL has long been championed as a potential solution to the problems of logging sustainability and Jake’s work is the first to really show that it has positive effects across a number of sites.
So while two recent meta-analyses have indicated that logging intensity and method may have a profound influence on forest biodiversity no similar work has been done for trees, despite the fact that programmes that focus on conservation of carbon, such as REDD+, need this evidence to implement policy. So to fill this knowledge gap we performed a meta-analysis to determine what factors relating to logging intensity and method may cause variation in the impacts of logging on residual tree damage, aboveground biomass and tree species richness. In total we collected data from 62 studies across the tropics giving us 38, 43 and 9 data points for investigation of damage, biomass change and species richness change respectively. Promisingly we found that RIL seemed to reduce residual tree damage compared to conventional logging, with greatest differences to conventional logging found at low intensities. However, at higher intensities residual damage became more similar to that of conventional logging as suggested by previous work from Indonesia. However, the same wasn’t true of biomass. Though there was an apparent statistical difference in the slopes for RIL and conventionally logged forests the relatively low overlap in the intensities at which conventional and RIL are carried out means that slope estimation is not fantastic. As such it is not entirely clear whether, at the stand scale, RIL reduces biomass losses because of lower intensities or differences in practice as the two are confounded. Unfortunately there wasn’t enough information on species richness from forests logged using RIL to allow a comparison. However, the results were still interesting. There was a general decline in species richness with increasing logging intensity, but the plot hints that richness might increase at low intensities. Whether or not this is as a result of intermediate disturbance hypothesis type relationships is not a fight I want to get into, but this work does confirm that tree species richness is relatively insensitive to logging even at high intensities.
So our work suggests that the evidence for the positive effects of RIL is mixed, once we account for differences in logging intensity. I am well aware that this piece of work might annoy a few people who think I have something against RIL. Those I have spoken to at conferences where I have presented often think that I am saying that RIL doesn’t work. I’m not. It just isn’t entirely clear what it’s effects are. Frankly it would be remarkable if RIL and conventional logging had similar impacts at the stand scale given the differences in the two practices. What I think we lack is enough evidence to say what is going on.
So what would be a constructive way to determine the differences in impact of RIL and conventional logging? One thing we think would improve evidence is the quantification of logging intensity at the plot scale. Currently studies often report logging intensities for the entire landscape where plots are located, meaning that the variation between plots is not accounted for. There is likely to be a big difference amongst plots and so the impacts are likely to differ as well. As far as I can tell only a few studies have done this. One good example is the work of Lucas Mazzei and colleagues who showed that plots that had been more intensively logged showed a slower recovery in biomass. Metrics such as the basal area of trees removed per hectare might be useful and relatively easy to collect at the plot scale.
The results of our study and those of Zuzana Burivalova suggest that logging intensity drives carbon and species loss while Jake Becknell’s work suggest that RIL is less damaging for animal populations. As such, current evidence suggests that RIL at relatively low intensities is likely to be the best way to reduce carbon and biodiversity loss in tropical logged forests. However, given the massive area of tropical forest already designated for logging reductions in local intensity, and thus yield, may encourage expansion into previously unlogged areas. Recent work indicates that high intensity logging over a smaller area (‘land sparing’) may have better outcomes for tropical forest species than low-intensity extensive timber extraction (‘land sharing’) in Borneo, though there is a need for similar studies in other areas of the tropics. Although reductions in logging intensity may reduce impact, the high demand for timber requires novel solutions that do not drastically reduce current yields but reduce impacts on forest ecosystems. Methods such as silvicultural thinning techniques to remove pioneer species may aid recovery of floral community composition, carbon and timber stocks but further work is needed to assess their effectiveness. Although RIL may also provide a solution, further evidence is required to verify this for carbon storage in the form of above-ground biomass. Analyses that take into account plot level variation in logging intensities using collaborative networks such as The Tropical managed Forests Observatory offer a potential solution to this.