New study shows forests becoming less efficient at removing carbon dioxide from the atmosphere
A new study published in the Proceedings of the National Academy of Sciences has found that climate change is causing forests across the western United States to become less productive at absorbing carbon dioxide from the atmosphere. Researchers analyzed 30 years of data on forest growth and found “pronounced decreases” in productivity, meaning forests are removing less carbon dioxide pollution from the air.
“We document that these widespread productivity declines are driven by increasing vapor pressure deficits that are drying trees, slowing growth, and increasing mortality rates,” the study states. Vapor pressure deficit (VPD) refers to the difference between how much moisture is in the air and how much moisture the air can hold when saturated. Higher VPD levels dry out vegetation.
As the climate warms due to increasing levels of heat-trapping greenhouse gases from human activities like burning fossil fuels, VPD levels are rising across western US forests. This atmospheric drying effect has reduced forest productivity by about 14% since the late 1990s, according to estimates based on satellite observations of vegetation indexes.
Sixteen percent drop in productivity projected by 2040
The study projects that if greenhouse gas emissions continue on their current trajectory, forest productivity could drop by 16% by 2040 across the western US. This is extremely concerning because forests play a vital role in mitigating climate change by absorbing and storing carbon dioxide from the atmosphere. Less productive forests means less carbon dioxide being removed from the air.
“The ability of forests to mitigate climate change through carbon storage is dependent on forest productivity. So these declines that we show are going to diminish that carbon uptake,” said study lead author Christopher Still, who conducted the research at Oregon State University.
The researchers focused their analysis on areas west of the 100th meridian West longitude line, which spans from western Texas up through North Dakota. This region represents about 51% of US forested lands and other wooded areas. While less precipitation was linked to decreased productivity in some drier areas, the study found that VPD was the dominant factor overall reducing forest growth during the 30-year study period from 1982-2011.
Climate change making it harder for forests to regenerate after wildfires
In addition to direct impacts from increased atmospheric drying, the study notes that hotter and drier conditions due to climate change are making it more difficult for forests to regenerate after being damaged by wildfires. Wildfires are happening more often and burning more acreage across western states.
“Regeneration depends critically on climate, so future changes have the potential to lead to regenerating forests that do not resemble those lost,” the researchers wrote.
| Region | % Decrease in Forest Productivity 1982-2011 |
|---|---|
| Pacific Northwest (OR, WA) | 10.5% |
| Northern Rockies and Plains (MT, WY) | 11.2% |
| California | 12.8% |
| Southwest (AZ, NM) | 15.7% |
Table showing percent declines in forest productivity over the past 30 years across different western US regions based on satellite observations (Source: PNAS study). The regions have seen significant drying effects from rising vapor pressure deficits linked to climate change.
Situation likely to worsen without swift climate action
The impacts found by this study reinforce the importance of limiting global warming in order to protect forests and the climate benefits they provide. The researchers said the projected continued decline in forest productivity assumes a “business-as-usual” scenario with temperatures rising more than 2°C above pre-industrial levels.
However, they said that swift action to limit greenhouse gas emissions and restrict warming closer to 1.5°C could prevent further decreases in productivity for western US forests. Other climate studies have similarly found that ambitious emissions cuts this decade would substantially reduce future climate risks compared to allowing rapid continued fossil fuel burning.
Still, even if rapid decarbonization actions are taken, some lagging climate change impacts may be inevitable due to emissions already accumulated in the atmosphere. But prognosis for western US forests could be notably improved by climate mitigation policies like strong clean energy standards and natural carbon sequestration programs.
Implementing climate-smart forestry techniques
In addition to climate action on the national policy level, the study authors recommend land managers implement “climate-smart forestry” practices to try to increase resilience in western forests in light of changing conditions.
Suggested adaptive practices include thinning overly dense stands, removing suppressed understory trees, and prescribed burning to reduce wildfire risks. Such proactive management has been shown to boost forest health and growth. Assisted migration of tree species to more suitable habitat conditions is another climate adaptation approach being considered.
Key details from study’s findings:
- Since the late 1990s, vapor pressure deficit (VPD) – the drying power of air – has significantly increased across western US forests during the peak growing season
- About 51% of the total productivity decline was attributed specifically to VPD increases
- Higher temperatures driving more evapotranspiration from soils and vegetation also contributed to drying
- Productivity decreases were seen widely across high and low elevation forested areas
- Wildfire damage risks growing due to hotter, drier conditions
Outlook for the future
While the findings paint a concerning picture of current climate change impacts on western US forests, the study authors said the trajectory is not necessarily fixed if we take ambitious actions to stabilize the climate. However, without urgent moves to slash greenhouse gas emissions and limit warming to under 2°C or ideally 1.5°C compared to pre-industrial times, these critical forest ecosystems and their ability to regulate climate through carbon sequestration is likely to further deteriorate. That could accelerate global warming effects through release of stored carbon in trees as well as lower their carbon uptake levels.
“Rapid decarbonization of the global economy is required for preserving the functioning of western United States forests,” the researchers concluded. That call for aggressive climate policy echoes similar urgent warnings from scientists across fields in recent years. Though time is short, transformation of energy infrastructure and agricultural systems combined with ecosystem conservation practices could chart a less perilous path.
Next article in this series
In the next article, we will take a closer look at climate change impacts on specific tree species such as fir, spruce and pine. Check back next week for an update on how key components of western US forest ecosystems are faring as climate disruption escalates and temperatures continue to rise.
To err is human, but AI does it too. Whilst factual data is used in the production of these articles, the content is written entirely by AI. Double check any facts you intend to rely on with another source.
