As countries ponder incentives to slow the degradation of their tropical forests, a huge, unanswered question looms: What exactly is a degraded forest?
Programs that provide such incentives, such as REDD+ (Reducing Emissions from Deforestation and forest Degradation), a U.N.-backed initiative, face the challenge of accurate measurements of deforestation and degradation.
New criteria can help address that problem.
“The difficulty is that what some people consider a degraded forest may not look degraded to others,” said Manuel Guariguata, a principal scientist with the Center for International Forestry Research (CIFOR).
“There are hundreds of definitions of forest degradation, but they don’t clarify where the threshold lies for defining what is degraded and what is not.”
Guariguata and colleagues aim to remedy the problem with a set of five guideline criteria that forest managers and land-use planners can use to evaluate the state of a forest and determine whether use of its resources is sustainable.
Those criteria: long-term production of forest goods and services; biodiversity; unusual disturbances such as fire or invasive species; carbon storage; and the forest’s ability to protect soil. The criteria can be given a different weighting depending on the forest-management goals.
The researchers describe the criteria and how they can be measured in a paper, “An Operational Framework for Defining and Monitoring Forest Degradation”, published in the journal Ecology and Society.
“We did not create a specific definition of degradation, but our work provides guidance about how land planners and managers can apply different dimensions of degradation to their own work,” Guariguata said.
Forest managers can decide which criteria are most important in their own situations, he said. In many cases, they can then use remote-sensing technology, such as satellite images, to continue to monitor the state of the forests.
Toward a definition
The Collaborative Partnership on Forests provided a definition that relates degradation to the loss of ecosystem goods and services. However, that definition still required a way to make it operational for land managers to use. The five new guidelines seek to provide this.
Because forests store carbon and are a source of timber and products such as fuel, fruit and nuts, the first criterion for measuring degradation is how well they provide those products and services, the researchers said.
A forest’s ability to produce timber and fuel wood is judged by its “growing stock” — the volume of all trees of a particular height and diameter. Signs of degradation could include a decrease in that volume over time, the number of certain types of tree, or in the harvest of such non-timber forest products as fruits or nuts, the research shows.
The second factor is biodiversity — vital because a wide range of plants, insects, animals, fungi and other living things perform crucial functions in tropical forests, such as seed dispersal, pollination, disease control and decomposition, the authors said. These functions are often directly related to the provision of ecosystems goods and services.
Land managers can measure biodiversity by monitoring changes in vegetation and certain important species, including insects and birds. They can also track forest fragmentation, a type of forest degradation that can result in the loss of habitat and of species — animals, birds, insects or other creatures — that were dependent on it.
Sometimes degradation is more obvious — a forest may be scarred by excessive fires or overrun by an invasive exotic plant or insect that threatens native species. Such “unusual disturbances,” which can be aggravated by climate change, are the third criterion.
Forests are not only a source of products, but they also protect soil and maintain moisture by regulating the flow of water in an ecosystem, releasing water into the atmosphere through their leaves, in a process known as evapotranspiration, and controlling the way in which water filters into the ground.
The researchers designated water retention as the fourth criterion and recommended monitoring this type of degradation by measuring soil erosion and the quantity of water.
The fifth criterion in defining forest degradation reflects the key role that tropical forests play in carbon storage, as forests hold about half the world’s carbon stocks in living and dead trees and the soil.
Degradation from forest fragmentation, a reduction of tree size or in the number of species in a forest can release carbon and limit its future accumulation in the forest. The researchers recommended monitoring both stored carbon and the presence of high-density tree species, which store the most above-ground carbon, in the forest.
For all five criteria, the key to monitoring lies in having a reliable baseline, or reference level, against which to measure degradation, Guariguata said. Although the “gold standard” is an intact, old-growth forest, he cautioned that trees alone do not make a functional forest.
“You can have a very beautiful, old-growth forest, but no animals, because of overhunting,” he said. “From the standpoint of forest structure, the forest is not degraded, but there are no seed dispersers, game species or herbivores, and that will eventually have an impact on the structure of the forest.”
One danger — and a reason why the researchers drew up criteria for measuring degradation — is that policymakers may be tempted to write off any forest where there has been logging or other activity, saying that because it is “degraded,” it no longer serves a purpose.
That could pave the way for clearing for road building, agriculture or other activities that further threaten the forest’s survival, Guariguata said.
“There is a perception that if you say a forest is degraded, it’s not a good forest,” said Ian Thompson, research scientist in forest ecology with the Canadian Forest Service who co-authored the report. “If you use good logging practices to harvest timber, it won’t be the same as an old-growth forest, but it will still be a productive forest. We wanted to clear up these misconceptions and show that there are many dimensions to degradation.”
In other words, a well-managed forest, considering all the criteria, could equally serve as a baseline, Thompson added.
