African Tropical Forests Are under Stress
Tropical deforestation is well known to have serious negative consequences for biodiversity, terrestrial carbon sinks and the balance of atmospheric greenhouse gases. By contrast, selective logging of tropical forests is often regarded as having a lesser impact on the ecosystem particularly in the long term, even though there have been few critical evaluations of the practice, particularly in Africa.
Recently, we published new research which shows how significant the impact of selective logging on African tropical forests can be. As this practice is the most widespread form of land use in world tropical forests, our results, together with a growing body of evidence, indicate that selective logging is not as benign as companies and researchers in commercial forestry usually suggest.
Our paper (Cazzolla Gatti et al. 2015) summarizes more than 3 years of field research in tropical Africa (funded by European Union through an ERC project) conducted in Ghana, Sierra Leone, Cameroon and Gabon by our team at the University of Tuscia, in Italy and with the support of international renowned researchers such as D. Coomes and J. Linsday.
We compared field data collected in 511 plots in the tropical forest of the four countries. These plots were subject to different forest management practices: no recent logging (primary forests), selective logging (up to 30 years old) and re-grown secondary forests post clear-cutting (at least 20 years ago). Our findings suggest that the vertical structure, stem density, the prevalence of vine and weed species and plant richness of the selectively logged and secondary forests differ greatly from those of primary forests.
We compared several variables across different management forests to evaluate the impacts of selective timber harvesting on forest ecology and we found that even low-intensity selective logging causes significant changes to ecosystems. This is one of the first studies which approaches the effects of "sustainable" selective logging not only on wildlife but directly on tree diversity and biomass.
In other words, we demonstrate that the negative effects, even at small logging volumes, is evident on selectively logged forests and it worsens with time (even after 50 years). We estimated a loss of 50 % of tree diversity (even where the removed trees are 1-2 per hectare!) and of 60 % of biomass (which means that it is only 40 % less than clearcutting!).
Moreover, we show that the effects of selective logging are greater than those expected simply from the removal of commercial species, and can persist for decades. Selective logging, unless it is practiced at very low harvest intensities, can significantly reduce the biomass of a tropical forest for many decades, seriously diminishing aboveground carbon storage capacity, and create opportunities for weeds and vines to spread and slow down the ecological succession.
This practice is causing long-term changes to tropical forest ecology in Africa by altering tree communities and, in this way, indirectly affecting animal diversity (most of the forest analysed are the habitats of great apes, rare birds, monkeys and cats, forest elephants, endangered reptiles and amphibians, unclassified insects, etc.). Our paper published in the journal Ecological Research clearly demonstrates that the effects of logging (even if selective), and carried out under "responsible forest management" schemes, are considerable and common in all the countries where this practice is allowed.
Given our results and other recent findings, we suggest that certification schemes - like the Forest Stewardship Council (FSC) and the Pan-European Forest Certification Council (PEFC) - may be completely unsustainable from an ecological point of view.
There appears to be a consensus that selective logging causes relatively minor disturbance and is thus acceptable within the requirements of some certification schemes such as the FSC. However, evidence of long-term effects of selective logging on carbon sequestration and biodiversity is poor and sometimes overlooked, particularly in Africa, despite the increasingly wide adoption of this practice.
We conclude our paper arguing that policymakers and conservationists should not consider selective logging a sustainable practice because our paper suggests exactly the opposite and it seems true for different countries and forest types. This practice has several important negative effects on forest structure, dynamics, biodiversity and ecosystem services and these effects can be truly evaluated only in the long term by analysing the evolving dynamics of repeated logging and, not the mean structural values, but the indices linked to the arboreal density.
The objective of this research is to alert not only the scientific community but also NGOs and governments to the necessity of reviewing licence and forest management. This is reinforced by another paper (Battipaglia et al. 2015) - led by G. Battipaglia of University of Caserta - we published recently on PlosONE, of whom I am one of the authors. Here we tried to evaluate to what extent African tree growth is influenced by global anthropogenic disturbances, such as rising concentrations of CO2 and climate change. Long-term tree-ring chronologies of three widespread African species were measured in Central Africa to analyze the growth of trees over the last two centuries. Growth trends were correlated to changes in global atmospheric CO2 concentration and local variations in the main climatic drivers, temperature and rainfall. Our results provided no evidence for a fertilization effect of CO2 on tree growth. On the contrary, an overall growth decline was observed for all three species in the last century, which seems to be significantly correlated to the increase in local temperature. Our work advances our current understanding of the growth responses of trees to atmospheric CO2 concentration, clarifying possible interaction with temperature. This is a key requirement for assessing the long-term responses and feedback between forest ecosystems and future climate.
Other evidence on the reduction of the growth of tropical forests, contrary to what was previously suggested, together with the fact that these ecosystems are being damaged by selective logging, underline that our attention should be paid not just to totally destructive practices such as deforestation (clear-cutting) for alternative land uses (crops or grazing, commonly in the Amazon, or the palm oil plantations that are typical of Southeast Asia), but also to the selective logging of the last virgin forests of Africa and of the regrown secondary forests that are already stressed by climatic changes. This may be a more serious cause of forest degradation than what has been thought to date. These first results suggest that it will be crucial to increase research about the key question for forest management and conservation: is selective logging really sustainable for tropical forests? Our answer is: no!
Roberto Cazzolla Gatti
Cazzolla Gatti, R. et al. (2015): The impact of selective logging and clearcutting on forest structure, tree diversity and above-ground biomass of African tropical forests, Ecological Research 30 (1), 119-132
Battipaglia, G. et al. (2015): Long Tree-Ring Chronologies Provide Evidence of Recent Tree Growth Decrease in a Central African Tropical Forest. PLoS ONE 10 (3), e0120962. doi:10.1371/journal.pone.0120962