It’s commonly thought that edge effects are the main reason that tropical forest fragmentation leads to loss of biomass and carbon stock. However, a new review, recently published in Global Change Biology, points out that there’s limited empirical evidence to support this. In response, the authors propose a new framework and sampling protocol that link biomass loss to landscape-level traits such as matrix quality and patch configuration.
Tropical forests store disproportionately large amounts of carbon, and are at the center of many climate change and carbon trading policies. Understanding the consequences of tropical forest fragmentation helps improve predictions about both the forests themselves and their impacts on climate. Because fragmentation increases the amount of edge in the forest, edge effects are generally seen as the driver behind loss of large, shade-tolerant trees and overall biomass.
While the systematic review of studies related to edge effects on tropical forest biomass turned up almost 11,000 publications, only 12 of these empirically tested how edge affects aboveground biomass. Those studies that did report edge effects provided contrasting results, with some showing biomass decreasing and some showing it unaffected. This gap in knowledge means that estimates of carbon emissions might not be very accurate.
The new proposed framework is based on the idea that landscape composition and configuration affect the severity of edge effects. The framework hypothesizes a fragmentation threshold of habitat amount, below which biomass would shrink rapidly and patch isolation would have a greater impact. Edge effects would also be mediated by the degree of contrast in the surrounding matrix, and the time since fragmentation.
Based on this framework, in order to empirically test the link between landscape composition and configuration to biomass you should consider (both collectively and separately): edge effects, habitat amount, habitat isolation, fragment size, and matrix type. To help meet that goal, the authors present a unique sampling design meant to account for these different components that explicitly lays out a usable spatial design. It can be applied to a wide range of tropical forests, both within and outside the Amazon.
By getting better estimates of how tropical forest fragmentation affects biomass, we can get a better understanding of how the fragmentation process affects carbon loss. This in turn can lead to better climate change models, and more effective policies on carbon trading and offsetting (e.g. REDD+).
Melito, M., J. P. Metzger, and A. A. de Oliveira. 2018. Landscape-level effects on aboveground biomass of tropical forests: a conceptual framework. Global Change Biology 24: 597-607.