Why do some degraded tropical forests recover better than others?
By Dr. Leonardo Saenz, Technical Officer for Permian Global
The carbon sink properties of recovering secondary or degraded tropical forests varies across the Amazon, Borneo and Central Africa, but each holds less carbon than intact old growth forest. The priority must always be to protect intact forests, but it is also important to understand why some forests recover better than others.
A recent paper published in Nature, The carbon sink of secondary and degraded humid tropical forests by Heinrich, V.H.A., Vancutsem, C., Dalagnol, R. et al. (2023), looked to quantify the aboveground carbon sink of recovering forests across three main continuous tropical humid regions: the Amazon, Borneo and Central Africa.
The authors found that in the first 20 years of recovery, regrowth rates in Borneo were up to 45% and 58% higher than in Central Africa and the Amazon, respectively. Understanding these differences could provide important context for conservation efforts. We asked Permian Global’s Dr. Leonardo Sáenz for his impression of the research.
“According to the authors, the faster rates of accumulation in Borneo, when compared with the Amazon and the Congo, can be “linked to the distinct degradation drivers that are dominant in each region as well as environmental differences”. However, I also argue that the faster growth rates in Borneo could be associated with the fact that forests there have been most severely disturbed (up to 87% of the Above Ground Carbon – AGC- lost in degraded forests vs 50% only for the other two regions), so there should be more opportunities for rapid regrowth, as higher wood-density tree species likely face less competition.
I also believe that drivers such as population densities and land graving dynamics play a role, as well as the different topography of the regions, particularly slopes, which are less gentle in many parts of Borneo when compared to the vast flatlands in the Amazon.
That said, in my opinion the situation in Borneo and the Amazon are not directly comparable in terms of “recovering forests”, a term coined by the authors, as this does not necessarily apply to many degraded and degrading Amazonian forests. This is because many of these forests are subject to a continuous cycle of degradation, and they do not have time to recover. This cycle of degradation is likely mediated by their favourable topography for continuing logging and their subsequent transformation into livestock and/or mechanized soy production and agriculture. Those forests may also face an important tree mortality after last logging/disturbance, exacerbated by human induced fire. In Borneo, on the other hand, the land uses after last disturbance may be constrained due to for instance a much more undulated terrain, particularly in central and northern parts.
Nevertheless, what I appreciate about this paper is that they highlight the significant gaps that still exist to providing a clear estimate of the stocks of old-growth tropical humid forests. To this point, the authors state that their “median AGC estimate for old-growth forests was lower than field-study estimates in the three regions” and that “exploring these biases is a priority for the AGC fieldwork and remote-sensing communities, with scientific and policy implications”. This is a really important conclusion since we know that plot-based data may offer an underestimation of true stocks, since these have generally been implemented close to roads and rivers due to time or budget constraints, among other reasons, and therefore may be representative of forests at different levels of disturbance. These uncertainties may be greater and more complex in the Amazon, bearing in mind the very sparse and paused network of plots data and the vast areas of forest that these have to represent.”
Heinrich, V.H.A., Vancutsem, C., Dalagnol, R. et al. The carbon sink of secondary and degraded humid tropical forests. Nature 615, 436–442 (2023).