Climate change could lead to landscape-level tipping points
By Summit Voice
SUMMIT COUNTY — A comprehensive new study shows that about 18 percent of forests in the Southwest have already been affected by global warming. The region’s forests are more susceptible to temperature changes than any other, and if current climate predictions hold true, they will experience more frequent and severe forest fires, higher tree death rates, more insect infestation, and weaker trees, according to the study published this week in the Proceedings of the National Academy of Sciences. The scientific article is part of a special PNAS feature edition called “Climate Change and Water in Southwestern North America.”
“Our study shows that regardless of rainfall going up or down, forests in the Southwest U.S. are very sensitive to temperature –– in fact, more sensitive than any forests in the country,” said first author Park Williams, postdoctoral researcher in the Department of Geography at UC Santa Barbara. “Forests in the Southwest are most sensitive to higher temperatures in the spring and summer, and those are the months that have been warming the fastest in this area.”
After starting with gradual changes, the impacts can lead to tipping points, resulting in dramatic landscape-level impacts.
“Such big, fast changes in Southwest forest vegetation could have significant effects on a wide range of ecosystem goods and services, from watershed protection and timber supplies to biodiversity and recreation,” said co-author Craig D. Allen of the U.S. Geological Survey. “These emerging vulnerabilities present increasingly clear challenges for managers of southwestern forests to develop strategies to mitigate or adapt to the coming changes, in order to sustain these forested ecosystems and their benefits into the future.”
Past forest studies have shown that warmer temperatures are associated with wildfires and bark beetle outbreaks. “We found that up to 18 percent of forest area in the Southwest –– millions of acres –– has experienced mortality due to severe wildfires and bark beetle outbreaks in the last 20 years,” said Williams.
Co-author Joel Michaelsen, a professor of geography at the University of California Santa Barbara, said, “In order to carry out this research project, Park Williams assembled a very comprehensive data set of over 1,000 tree ring chronologies from all across the United States.” Michaelsen is a dendroclimatologist –– a scientist who studies climate using analysis of tree rings.
“Instead of using the chronologies to reconstruct past climate patterns, as is usually done in dendroclimatic work, the relationships between growth and climate were used to study possible impacts of future climate change on forest health,” said Michaelsen. “One noteworthy finding was that tree growth throughout the Southwestern U.S., while quite sensitive to precipitation variations, is also negatively impacted by warmer temperatures. This is an important result, because predictions of future warming in the region are considerably more certain than any predictions of precipitation change.”
Based on the tree-ring research, historic patterns show that the landscape often responds gradually to climate change at first, with, increasing insect outbreaks and fire activity, until a threshold is reached. Then there can be dramatic landscape changes, such as tree die-offs or episodes of broad-scale fire or erosion. The stressors that contribute to tree mortality tipping points can develop over landscape and even sub-continental scales.
Co-author Christopher Still, an associate professor of geography at UCSB, said, “These predicted large-scale changes in forest cover and composition (i.e., types of tree species present) will have large implications for everything from snowpack and the river flows that our society depends on, to the intensity and frequency of fires, to the visual appearance of these landscapes that drives much of the tourism in this region.”
Forests help retain rainwater and keep it from flowing down mountains immediately, noted Williams in explaining the importance of forests to landscapes and rivers. “When forests disappear,” he said, “water runs downhill more quickly and takes the upper layers of soil with it.”
According to Williams, the erosion makes it harder for future generations of trees to establish themselves and makes it more difficult for people to capture storm water as it flows from the mountains. In addition, erosion increases the amount of sediment flowing in rivers and settling in lakes, and causes water to remain in the forest long after the rain.
Filed under: Environment, forest fires, forests, global warming, Summit County Colorado Tagged: | climate change, Environment, forests, global warming, Proceedings of the National Academy of Sciences, Southwest, southwestern forests, Summit County Colorado, Summit County News, United States, United States Geological Survey, University of California Santa Barbara, wildfire