Researchers find the world’s forests are fragmenting at a quick pace

4 February 2016, Mongabay news - Recent research by the U.S. Forest Service finds that the world lost interior forest at three times the rate of forest loss as a whole. They write that this fragmentation could severely jeopardize the ability of remaining forests to provide critical wildlife habitat and other ecological functions.

In total, the team found there was a global net loss of 1.71 million square kilometers of forest cover from 2000 to 2012 — 3.2 percent of total global forest area.

But the Forest Service researchers argue that focusing on forest area loss alone risks ignoring the ecological threats of forest fragmentation. In addition to the direct loss of forest, they found a "widespread shift" in the world's remaining forests to a more fragmented condition.

They calculated a net loss of 3.76 million square kilometers of interior forest area amounting to 9.9 percent of global interior forest cover, according to their study in the journal Landscape Ecology.

It may seem counter-intuitive that there can be more interior forest loss than total forest loss, but depending on the spatial distribution of deforestation, it's entirely possible to have more forest area converted to "edge conditions" — say, when a road is built through a forest that penetrates its interior, converting the former heart of the forest into a new forest edge — than the total amount of forest that was destroyed.

The researchers' findings indicate that the world's interior forest cover was lost at a rate three times higher than global forest area as a whole was lost — and this could have important consequences for ecological processes.

Kurt Riitters, a research ecologist at the U.S. Forest Service Eastern Forest Environmental Threat Assessment Center and the lead author of the study, told Mongabay, "Some natural ecological goods and services are provided only by interior conditions, and some are adversely impacted by ecological 'edge effects' that occur in non-interior conditions."

In other words, as forests become fragmented by the building of roads and other human activities as well as natural drivers of deforestation like wildfires, the proportion of interior forest generally decreases and the proportion of edge area increases. And Riitters said that leads to two kinds of risks: the loss of ecosystem functions that require interior conditions, and the degradation of functions that are sensitive to edge conditions.

Riitters and his team studied data on changes to interior forest area between 2000 and 2012 and compared them to changes in total forest area in 768 ecological regions. They found that the difference in loss rates was consistent across most of those regions — more interior forest was lost than numbers on total forest loss would indicate.

"The pattern of change is important and that is why one must conduct a spatial analysis if one cares about spatial attributes like 'interior' or 'edge' – analysis of area totals is only part of the story of fragmentation," Riitters told Mongabay.

Pinpointing any specific numbers on total forest loss versus interior loss depends greatly on how interior is defined and measured, Riitters notes. He and his team have interpreted the data using several different definitions, but used just one in the paper in order to focus on the geography of the difference between total loss and interior loss as opposed to absolute values.

"We believe that if we had measured interior in a different way, the area numbers would be different but there would still be more interior loss than total loss, and we would see the same geography of the differences," Riitters said.

Riitter also says he and his team's analysis of land cover data assumed that any new or replacement tree cover was equivalent to tree cover that was lost, regardless of whether it was natural or planted "That's a constraint of using the data we have available," he said.

"We analyze 'interior' from a purely structural point of view, without any guarantee that structural interior is in fact functional interior," he said. In other words, the researchers did not differentiate between natural (structural and functional) and planted (just structural) forest. "At the same time, the area of structural interior is at least an upper limit to the available functional interior, so we are still concerned if analysis of structural indicates a decline in the potential area of functional."

The Forest Service researchers write that sustaining forest interior is arguably as important as sustaining forests themselves.

"Monitoring sudden changes in forest interior area may provide an early warning of impending tipping points in dependent ecological functions."