According to new research led by Oregon State University that quantifies the effects of forest “degradation” on avian habitat, bird species that reside in wooded areas are under stress from human-caused changes to the composition of the forest.
“Reducing forest loss has been the main focus of conservation policy to date, which is well justified because it has a strong negative effect on biodiversity,” said Matt Betts of the OSU College of Forestry.
“But the effects of changing the composition and age of forest via timber management have traditionally been very difficult to measure at large scales and thus have been largely ignored. Our work shows population declines in many bird species in eastern Canada are due to habitat loss caused by forestry activities.”
The results of the global team lead by Betts were released today in Nature Ecology & Evolution.
The scientists examined how clearcutting and subsequent thinned or replanted single tree species affected bird habitat and long-term trends in bird populations. They also examined how forest degradation, loss of biological complexity, and reduction in complexity affected bird habitat.
The Acadian Forest in Canada’s maritime provinces was the subject of the investigation. According to the study, breeding habitat loss affected 66% of the 54 most prevalent bird species in the forest from 1985 to 2020 and was closely correlated with the destruction of older forests.
Seven species in total showed habitat declines of more than 25%, with the golden-crowned kinglet and Blackburnian warbler seeing the biggest reductions.
Long-term bird population decreases were highly correlated with habitat loss, particularly for species found in ancient forests. Over the course of the 35-year investigation, the researchers calculate that between 33 and 104 million birds perished as a result of forest degradation.
The researchers discovered that nine bird species in the study region have dropped at rates greater than 30% over the previous ten years, which is required for classification as vulnerable under Canadian endangered species legislation.
Old forest declined by 39% over the period we observed. Over the same period, forest cover actually increased by a net 6.5%. That pattern of an extensive harvest of old forest, followed by rapid regeneration of young forest and then subsequent harvest before maturity is attained, seems to be common in many forest regions of North America and northern Europe.
Matt Betts
“Due to increased global demand for wood, more and more of the Earth’s surface is being used for timber extraction,” said Betts, the lead scientist for the HJ Andrews Long-term Ecological Research Program. “This shows up on remote sensing as both forest loss and forest gain, but unfortunately the ‘gain’ is often vastly simplified young forest. Our paper presents a new way to quantify these sorts of changes.”
In order to analyze trends in forest and bird populations, Betts and colleagues from Cornell University, the University of Rhode Island, the University of New Brunswick, Google, and numerous Canadian and American agencies combined satellite imagery, breeding bird survey data, and species distribution modeling.
According to Betts, the Acadian Forest, which is renowned for the variety of its tree species, has been degrading for the past at least three decades. The Acadian Forest has been clearcut on more than 3 million hectares since 1985, and a large portion of that land is currently dominated by a single tree species or a combination of early successional species.
“Old forest declined by 39% over the period we observed,” Betts said. “Over the same period, forest cover actually increased by a net 6.5%. That pattern of extensive harvest of old forest, followed by rapid regeneration of young forest and then subsequent harvest before maturity is attained, seems to be common in many forest regions of North America and northern Europe.”
According to him, managed forests are more likely to be dominated by one or two commercially viable tree species and to be younger than those that would naturally arise in response to disturbances like windstorms or fires. That might have effects on species connected to old-growth or mature forests.
In light of recent findings by the Intergovernmental Panel on Biodiversity and Ecosystem Services that suggest the planet is facing a biodiversity crisis, Betts said quantifying the effects of forest degradation is especially crucial. Betts also cites research published in Science that demonstrates bird populations have been experiencing sharp declines across North America.
“Clearly the research by Betts and collaborators identifies one of the critical smoking guns of avian declines,” said Peter Marra, the director for Georgetown University’s Institute for Environment and Sustainability and a co-author of the recent Science paper on bird declines. “We’ve assumed once a natural forest is cut down, as long as you plant more trees all the rest of the plants and animals will fill back in. The new research shows that’s not the case.”
According to Betts, deforestation and subsequent permanent conversion to another land-cover type accounted for less than 2% of all habitat loss among the 54 bird species studied in the Acadian Forest.
“Overall, our findings indicate broad-scale declines in forest birds of the Acadian Forest, and for most species, abundance is strongly associated with habitat amount,” he said. “We expect that similar consequences for biodiversity are in place for intensively managed forests in other parts of the world as well. If all you look at is forest cover, you’ll miss the more subtle but critically important role of forest age and type in maintaining biodiversity.”
The Environment and Climate Change Canada Climate Nature Fund supported this research. The collaboration included scientists from the New Brunswick Department of Natural Resources and Energy Development; Canada’s National Wildlife Research Centre; the Ontario Ministry of Natural Resources and Forestry; and Google Earth Engine.
