Rapid warming has impacted the northern ecosystem so significantly that scientists are concerned the region's vegetation is losing the ability to recover from climate shocks, suggests a new study.
Their findings revealed that due to frequent disturbances like wildfires that raze down vegetation and persistent drought and deforestation that starve both the land and wildlife, the resilience of many plant communities in southern boreal forests - or their ability to recover after these events - significantly decreased over time. This may affect the Arctic carbon budget, foreshadowing a future where the region is likely to become a carbon source instead of remaining a carbon sink due to its limited capacity to absorb atmospheric carbon dioxide in the coming decades.
This is because Arctic and boreal regions have warmed several times faster than other places around the globe and further warming is expected in the near future, said Yue Zhang, lead author of the study and a graduate student in earth sciences at The Ohio State University.
"When we talk about the response of forests to climate change, most of the time we're thinking about the tropical rainforest," said Zhang. "But remote boreal forests are really important in terms of their vast extent, large carbon storage and potential to mitigate climate change."
The study was recently published in Nature Ecology & Evolution.
To better understand how the region's ecosystem changed because of increased warming, researchers used historical data from NASA's Arctic-Boreal Vulnerability Experiment (ABoVE) program to remotely sense subtle changes in greenness in Alaska and western Canada between 2000 and 2019. They were able to estimate the time-varying speed of vegetation recovery from small fluctuations or large losses, even in areas where large losses have not happened yet.
The study found that while plant resilience in the southern boreal forests notably decreased, even in regions with greening trends, resilience was thought to have increased in most of the Arctic tundra. Besides fires, other factors like heat and drought could have contributed to declines in plant resilience in the south, and changes in nutrient availability could have helped vegetation thrive in the rest of the Arctic.
While the release of food may benefit plant growth and resilience, the reality is that the rising temperatures that make this possible could also cause Arctic permafrost to melt more quickly than it already is, releasing from the ground as much carbon as 35 million cars emit in a year and hastening the arrival of climate tipping points.
It is uncertain now how much of the carbon will be absorbed by plants and how much will contribute to further warming, said Zhang.
"That's pretty concerning, because while greening may indicate that productivity and carbon uptake in these regions is increasing now, resilience decline indicates that it may not be sustainable in the longer term," she said.
According to the study, these shifts are indications that the entire ecosystem is in danger, as a large fraction of southern boreal forests is losing its stability, potentially leading to widespread forest loss and biome shifts.
Greening regions that experience resilience decline at the same time might also signal that the region is struggling to take a few last deep breaths before significant forest loss, said Yanlan Liu, senior author of the study and an assistant professor of earth sciences at Ohio State. This means that while the region could absorb significant amounts of carbon in the short term, scientists expect that if resilience continues to decline, the Arctic boreal ecosystem may not be as effective in mitigating climate in the long term as previously thought.
"Temperature records show this region is warming up to two to four times faster than the global average," said Liu. "This is a hot spot of vegetation change where studying it can tell us about the ecosystem stability and what it's capable of tolerating before it transitions into an alternative state through pervasive forest loss."
The study further revealed that warm and dry areas with high elevation and dense vegetation cover were among the hotspots of resilience decline. Yet because many climate models currently lack consensus on how vegetation change and carbon dynamics contribute to the other, this team's work will help enhance such models by informing scientists of where vegetation changes are likely to occur.
Ultimately, said Zhang, their method revealed more nuanced changes in the health of the region's vegetation, beyond previously reported greening and browning trends. This method also provides researchers a tool to identify potential vegetation loss in other regions in the coming decades.
With plans to continue trying to accurately predict ecosystem changes, researchers note their results warrant more field investigations aimed at better characterizing and understanding the resilience of the region.
"Scientists need to learn to quantify climate-induced risks through diverse lenses," said Liu. "On top of satellite remote sensing, we need more ground observations to help us identify ways to leverage these findings to inform future resources and risk management strategies."
The study was supported by NASA and the Ohio Supercomputer Center. Co-authors include Kaiguang Zhao from Ohio State, Jonathan Wang from the University of Utah, and Logan T. Berner and Scott J. Goetz from Northern Arizona University.
