https://doi.org/10.1073/pnas.2411721121

Alfaro-Sánchez R., Richardson A. D., Smith S. L., Johnstone J. F., Turetsky M. R., Cumming S. G., Le Moine J. M., Baltzer J. L. (2024) Permafrost instability negates the positive impact of warming temperatures on boreal radial growth, Proceedings of the National Academy of Sciences, Vol 121, Iss 50, Pg e2411721121, https://doi.org/10.1073/pnas.2411721121

In boreal forests, short growing seasons, cold temperatures, and the presence of frozen ground (permafrost) limit tree growth. Climate warming can potentially enhance growth by increasing air and soil temperatures. Here, we found reduced tree growth since the 1980 s at southern latitudes and increased growth at higher, colder latitudes, at least until the 2000 s. Our results showed that recent permafrost warming at the highest latitudes, where permafrost is more prevalent in the landscape, has caused significant stress in tree growth due to ground destabilization. Trees growing in unstable permafrost used their nutrient uptake to remain upright instead of increasing their growth. These findings indicate that boreal forests will not become more productive with climate warming and the resulting permafrost thaw. Climate warming can alleviate temperature and nutrient constraints on tree growth in boreal regions, potentially enhancing boreal productivity. However, in permafrost environments, warming also disrupts the physical foundation on which trees grow, leading to leaning trees or "drunken" forests. Tree leaning might reduce radial growth, undermining potential benefits of warming. Here, we found widespread radial growth reductions in southern latitude boreal forests since the 1980s. At mid latitudes, radial growth increased from ~1980 to ~2000 but showed recent signs of decline afterward. Increased growth was evident since the 1980 s at higher latitudes, where radial growth appears to be temperature limited. However, recent changes in permafrost stability, and the associated increased frequency of tree leaning events, emerged as a significant stressor, leading to reduced radial growth in boreal trees at the highest latitudes, where permafrost is extensive. We showed that trees growing in unstable permafrost sites allocated more nonstructural carbohydrate reserves to offset leaning which compromised radial growth and potential carbon uptake benefits of warming. This higher allocation of resources in drunken trees is needed to build the high-density reaction wood, rich in lignin, that is required to maintain a vertical position. With continued climate warming, we anticipate widespread reductions in radial growth in boreal forests, leading to lower carbon sequestration. These findings enhance our understanding of how climate warming and indirect effects, such as ground instability caused by warming permafrost, will affect boreal forest productivity in the future.

https://doi.org/10.1073/pnas.2411721121