Alfaro-Sánchez Raquel, Richardson Andrew D, Smith Sharon L, Johnstone Jill F, Turetsky Merritt R, Cumming Steven G, Le Moine James M, Baltzer Jennifer L
Northern Forestry Centre, Canadian Forest Service, Natural Resources Canada, Edmonton, AB T6H 3S5, Canada.
Department of Biology, Wilfrid Laurier University, Waterloo, ON N2L 3C5, Canada.
Proc Natl Acad Sci U S A. 2024 Dec 10;121(50):e2411721121. doi: 10.1073/pnas.2411721121. Epub 2024 Dec 2.
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.
气候变暖可以缓解北方地区树木生长所受的温度和养分限制，有可能提高北方地区的生产力。然而，在永久冻土环境中，变暖也会破坏树木生长的物理基础，导致树木倾斜或形成“醉汉”森林。树木倾斜可能会减少径向生长，削弱变暖带来的潜在益处。在这里，我们发现自20世纪80年代以来，北纬地区的北方森林普遍出现径向生长减少的情况。在中纬度地区，径向生长从约1980年到约2000年有所增加，但之后出现了近期下降的迹象。自20世纪80年代以来，高纬度地区的生长增加很明显，那里的径向生长似乎受温度限制。然而，永久冻土稳定性的近期变化以及相关的树木倾斜事件频率增加，成为了一个重大压力源，导致高纬度地区（永久冻土广泛分布）的北方树木径向生长减少。我们表明，生长在不稳定永久冻土地区的树木会分配更多的非结构性碳水化合物储备来抵消倾斜，这损害了径向生长以及变暖带来的潜在碳吸收益处。醉汉树中这种更高的资源分配是为了构建富含木质素的高密度反应木，以维持垂直位置。随着气候持续变暖，我们预计北方森林的径向生长将普遍减少，导致碳固存降低。这些发现增进了我们对气候变暖和间接影响（如永久冻土变暖导致的地面不稳定）如何在未来影响北方森林生产力的理解。
