The coldest forests on our planet are shifting northwards, due to climate change - and the shift could cause new dangers.
Researchers have warned that the movement of conifer forests to the north is visible on satellites and could lead to increased wildfires and new risks to biodiversity.
The researchers also warn that the change could have knock-on effects which could accelerate climate change.
The boreal forest is a belt of cold-tolerant conifer trees that stretches nearly 9,000 miles across northern North America and Eurasia.
The boreal forest accounts for almost a quarter of the Earth's forest area and is the coldest forest - though mostly rapidly warming.
Logan Berner, assistant research professor in the School of Informatics, Computing, and Cyber Systems (SICCS) at Northern Arizona University, says: "There is emerging evidence that climate change is causing boreal trees and shrubs to expand into arctic and alpine tundra, while at the same time causing trees to become more stressed and die along the warm southern margins of the boreal forest.
"These dynamics could lead to a gradual northward shift in the geographic extent of the boreal forest biome, but the extent to which such changes are already underway has remained unclear."
The researchers used 40 years of satellite observations and various geospatial climate-related datasets of the boreal forest and assessed where and why vegetation greened and browned during recent decades.
"Greening" indicates higher rates of vegetation growth, which can happen when climate warming promotes growth of trees and shrubs.
"Browning" indicates lower rates of vegetation growth and potentially vegetation death, such as when hotter and drier conditions suppress tree growth and kill trees.
Vegetation became greener across much of the cold northern margins of the boreal forest; warmer conditions led to increased vegetation growth and enabled trees and shrubs to expand into arctic and alpine tundra.
Vegetation became browner along parts of the warm southern margins of this biome as a result of hotter, drier conditions increasing tree stress and death.
Co-author Scott Goetz, Regents' professor and director of the GEODE Lab, said, "The boreal forest ecosystem is changing in many ways over recent decades, and those changes are often linked with increasing fire disturbance.
"Here we intentionally focused on areas that were not recently disturbed by fire so we could tease out the effect of climate change.
“Our hypotheses about what would happen were verified by this analysis - forests are getting more productive in the cooler northern and higher elevation areas, and they're getting less productive as a result of hot air masses and drying in the warmer and more southerly areas. We fully expect that will continue and probably intensify in the years to come."
Changes in vegetation could affect both plant and animal biodiversity, especially species like caribou and moose, which have specific foraging preferences like deciduous shrubs and trees.
Changes in vegetation also impact the stability of carbon-rich permafrost soils and absorption of solar energy by the land surface in ways that could accelerate climate warming.
Increasing tree mortality could have widespread implications for forest products while also leading to further degradation of semi-continuous and sporadic permafrost.
Berner said, "Fundamentally, greenhouse gas emissions from human activities are causing Earth's climate to warm, which in turn is leading the boreal forest to shift northward, as well as impacting other ecosystems across the planet"
"To minimise adverse impacts of climate change, efforts are needed to dramatically reduce greenhouse gas emissions, especially related to fossil fuel consumption and deforestation.
“Furthermore, northern communities need to plan for potential changes in vegetation that could impact resource availability (e.g. wildlife, timber) and wildfire risk."