Warming Tipping Point for tree growth in boreal permafrost landscapes

Boreal ecosystems are warming at three to four times the global average due to Arctic amplification. At these higher latitudes, where plant growth is constrained by low temperatures, climate warming is expected to shift the tree line northward and enhance vegetation productivity.

Permafrost thaw is also a major driver of climate-induced landscape changes in the north, significantly impacting tree growth and productivity. Approximately 80% of the boreal biome lies within the permafrost region. With continued global warming, permafrost temperatures will rise, leading to increased thaw rates and a reduction in permafrost extent.

Some studies suggest that permafrost thaw may benefit the functioning of overlying forests, primarily due to warmer soils, deeper permafrost tables, and access to newly released resources previously trapped in the frozen ground. However, the combined effects of climate change on growth trajectories in boreal trees remain uncertain. Indeed, satellite and ground-based vegetation studies, including tree-ring analyses, reveal substantial inconsistencies across the boreal and Arctic biomes, with some regions showing accelerated growth and greening, while others exhibit reduced growth and browning.

Here, we assembled a network of tree-ring data from sites with a historical record of permafrost thaw, spanning a climatic gradient in the boreal-subarctic Canadian region, to analyze tree growth patterns and identify their primary drivers—temperature, moisture, or permafrost changes.

Our findings revealed that the positive response of tree growth to warmer temperatures shifted in recent decades, with no significant positive temperature response at any studied site after 2007. Sensitivity to moisture also varied, showing exclusively negative impacts of higher vapor pressure deficit and precipitation on tree growth. Overall, tree growth exhibited a steady increase across the climatic gradient, peaking between 1993 and 2007, followed by a decline after 2007.

Nearly all permafrost monitoring sites examined showed consistent increases in permafrost thaw since 2007, with more pronounced ground destabilization occurring at lower latitudes within the climatic gradient. We found that permafrost thaw generally had a negative impact on tree growth. These reductions in growth were linked to ground destabilization caused by seasonal and long-term changes in ice-rich permafrost, which led to trees tilting off-vertical. Tree leaning triggered the formation of reaction wood, which alters radial growth as the trees counteract the physical instability of the permafrost.

Our results indicate that continued climate warming will drive widespread reductions in radial growth in boreal forests, leading to decreased carbon sequestration capacity.