Climate Change is Causing World's Coldest Forests to Shift Northward

Rising temperatures are driving Earth's coldest forests to relocate northward, according to new research from Northern Arizona University, raising concerns about biodiversity, increasing wildfire danger, and the growing implications of climate change on northern communities.

The article, "Satellite observations document trends consistent with a boreal forest biome shift," was published Thursday in Global Change Biology by Logan Berner, assistant research professor in the School of Informatics, Computing, and Cyber Systems (SICCS), and Scott Goetz, Regents' professor and director of the GEODE Lab.

Northward Shift

The boreal forest is a nearly 9,000-mile-long band of cold-tolerant conifer trees that runs through northern North America and Eurasia; it covers nearly a quarter of the Earth's forest area and is the world's coldest (albeit primarily quickly warming) forest biome.

The researchers analyzed where and why vegetation greened and browned during recent decades using 40 years of fairly fine (30m) resolution satellite images and numerous geographic climate-related datasets of the boreal forest. Greening refers to increased rates of vegetation development, which can occur when climate change encourages tree and shrub growth, as seen near the arctic and alpine tree lines. When hotter and drier conditions limit tree development and kill trees, "browning" signifies reduced rates of vegetation growth and perhaps vegetation death.

"There is accumulating evidence that climate change is forcing boreal trees and shrubs to expand into arctic and alpine tundra, while trees near the warm southern borders of the boreal forest become more stressed and die," Berner said. "These dynamics could result in a progressive northward shift in the boreal forest biome's geographic span, although the extent to which such shifts are already happening is unknown."

What they discovered wasn't particularly shocking. Warmer temperatures caused enhanced vegetation development and allowed trees and bushes to extend into arctic and alpine tundra across much of the boreal forest's icy northern edges.

Conversely, as a result of hotter, drier circumstances that increased tree stress and death, vegetation grew browner throughout areas of the biome's warm southern edges. Surprisingly, vegetation was more likely to turn greener in places with rich soil nitrogen, indicating that soil nutrient availability is a major constraint on boreal vegetation's reaction to climate change, according to Berner.

"The boreal forest ecosystem has changed dramatically in recent decades," Goetz said, "and these changes are frequently linked to increased fire disturbance." "To pick out the effect of climate change, we deliberately concentrated on places that had not previously been affected by fire. This research confirmed our predictions: forests are becoming more productive in the cooler northern and higher elevation areas, while they are becoming less productive in the warmer and more southerly parts as a result of hot air masses and dryness. We fully expect this trend to continue, if not intensify, in the coming years."

Changes in vegetation may have an impact on plant and animal biodiversity, particularly for species such as caribou and moose, which have unique feeding habits (e.g. deciduous shrubs and trees). In the boreal-tundra ecotone, several wildlife species are important food supplies for subsistence populations.

Changes in vegetation have an impact on the stability of carbon-rich permafrost soils and the absorption of solar energy by the land surface, potentially speeding up climate change. Furthermore, increased tree mortality could have far-reaching consequences for forest products, as well as accelerate the erosion of semi-continuous and intermittent permafrost.

"Fundamentally, human-caused greenhouse gas emissions are warming the Earth's climate, which is causing the boreal forest to shift northward, as well as affecting other ecosystems throughout the world," Berner added. "To mitigate the negative effects of climate change, significant reductions in greenhouse gas emissions, particularly those related to fossil fuel consumption and deforestation, are required. In addition, northern towns must prepare for possible changes in vegetation that could affect resource availability (e.g., animals, timber) and wildfire risk."

(Source: Northern Arizona University)