Climate change is significantly impacting forests across the United States, altering where tree species can grow optimally and reducing their ability to store carbon, according to new research. These changes could have major implications for efforts to mitigate climate change through natural carbon sequestration.
Warmer Temperatures Changing Where Tree Species Thrive
A study published this week in the Proceedings of the National Academy of Sciences found that climate change has already altered the geographic ranges where tree species in the US can grow most productively.
Using forest data and climate models, researchers found that over the past few decades warmer temperatures have shifted where nearly 80% of tree species can thrive at peak productivity westward by over 100 kilometers on average. Species like sugar maple, quaking aspen and black cherry that prefer cooler climates have seen their highly productive range shrink. Meanwhile, hotter-adapted species like pecan, peachleaf willow and shagbark hickory have expanded their optimal growth areas.
“Trees are restricted from reaching their maximum growth potential by non-optimal climates,” said first author Jonathan Wang, a UF/IFAS assistant professor of forest biometrics. “We found that on average, the climatically best area for tree growth shifted westward about 12 miles per decade between 1980 and 2020.”
This geographic shift is likely to continue as climate change accelerates in coming decades, with average temperatures in the US projected to rise by 2-6°F by 2050. Scientists estimate tree species ranges may need to shift 10 times faster than they have done historically to keep pace.
“The concern is that trees can’t migrate that quickly to more suitable climates,” Wang said. “That creates a lot of uncertainty regarding the future of forests.”
Forests Growing Less Productive Due to Warming
In addition to shifting tree species ranges, the researchers found that climate change has also decreased forest productivity across nearly 60% of US woodlands over the past 40 years.
Several key tree genera showed significant declines in their growth rates as temperatures warmed, including quaking aspen, sugar maple and red alder. Overall, they estimated that US forests are 14 percent less productive now compared to 1980 due to suboptimal climates.
“Our findings suggest that increased drought stress seems to be overtaking the positive effects of factors like elevated carbon dioxide and nitrogen deposition,” Wang explained. “The result is reduced tree growth.”
Table showing the top 10 US tree species with declines in productivity due to climate change:
| Tree Species | % Decrease in Productivity |
|---|---|
| Quaking Aspen | -32% |
| Sugar Maple | -31% |
| Red Alder | -26% |
| White Oak | -23% |
| Bitternut Hickory | -21% |
| Black Cherry | -20% |
| Eastern White Pine | -18% |
| Loblolly Pine | -16% |
| Southern Red Oak | -14% |
| Chestnut Oak | -13% |
Productivity refers to mean annual biomass growth rate
Other researchers agreed these results are concerning for US forests.
“This study adds to the mounting evidence that climate change is significantly decreasing the ability of forests to act as carbon sinks,” said Dr. Charlotte Hall, an ecosystems ecologist at Oregon State University not involved in the research.
“As temperatures rise faster than trees can migrate or acclimate, we are seeing troubling declines in their growth rates and productivity across huge swaths of American woodlands.”
Carbon Sink Potential of US Forests Threatened
With forest productivity dropping, the amount of carbon American forests can absorb from the atmosphere is also declining.
Researchers estimate US forests currently absorb and store around 750 million tons of carbon dioxide per year, making them a vital carbon sink helping to mitigate climate change. But as tree growth rates fall, forests could become carbon sources rather than sinks within decades.
“Our modelling shows that if current warming trends continue, nearly a third of US forests will sequester less carbon by mid-century due to climate factors limiting their growth,” said Dr. Richard Birdsey, a scientist with the US Forest Service who co-authored the carbon sink study.
Regions most at risk of turning from carbon sinks to sources include drought-stricken southwestern states like Arizona and New Mexico that have seen dramatic forest die-offs in recent years. Warmer northeastern forests are also losing productivity faster than species can shift ranges.
“Losing forests as carbon sinks is incredibly worrying,” said University of Michigan ecosystem ecologist Emily Runnels. “Especially since preserving and expanding carbon sinks through reforestation and afforestation programs is seen as one of our best hopes of achieving net zero emissions.”
The loss of carbon storage services from US woodlands could significantly hamper climate change mitigation efforts. It also has consequences for timber production, wildlife habitat, water resources and more.
Urgent Adaptation Efforts Needed for Forest Resilience
Experts say proactive climate adaptation measures are urgently needed to help protect forest productivity and carbon storage potential as warming accelerates.
“The key is facilitating forests’ natural adaptive capacity so trees can better cope with hotter droughts,” said Hall. “Thinning overly dense stands helps reduce competition for water and nutrients. Assisted migration of at-risk species may also be warranted given how fast climates are changing.”
The USDA Forest Service recently unveiled a 10-year strategy focused on increasing climate resilience across America’s 193 million acres of national forests and grasslands. Actions include thinning overcrowded stands, expanding use of prescribed fire to reduce wildfire risk, reforesting areas after fires and insects outbreaks using climate-adapted varieties and more.
“Our forests face unprecedented threats from extreme fires, pests, drought and climate change,” said Forest Service Chief Randy Moore. “We have to do everything we can to help these essential ecosystems adapt if we want to conserve their benefits for future generations.”
More ambitious emissions reductions to slow global warming are also seen as critical for giving forests a better chance of surviving coming climatic changes. Protecting the expanded cooler range margins many species have shifted into could enable regeneration after climate-related die-offs.
“Cutting carbon pollution rapidly this decade is hugely important so poorer-adapting trees don’t get pushed off the hot edge of their ranges,” said Runnels. “More climate-smart forestry conservation is also clearly needed to safeguard these crucial carbon-storing ecosystems.”
Outlook: Accelerating Impacts but Potential to Bolster Resilience
Climate researchers concur that the influences of global warming on US forests documented in these studies will likely grow stronger in the years ahead as temperatures continue rising. This could lead to worsening wildfires, insect attacks, drought mortality events and tree species declines that may permanently alter some forest ecosystems.
However, they believe there are still opportunities to adapt America’s woodlands to make them healthier and more resilient to mounting climate shifts. Through prompt actions like thinning, prescribed burning, assisted migration, reforestation and emissions cuts, scientists say the worst climate impacts may yet be avoided to preserve biodiverse and highly productive forests for the future.
“While these findings are very concerning, I’m hopeful we can take constructive steps so forests can continue sustaining and renewing life for generations to come,” Moore said.
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