Environment and Natural Resources Institute, University of Alaska Anchorage, Anchorage, Alaska, 99508, USA.
Department of Biological Sciences, University of Alaska Anchorage, Anchorage, Alaska, 99508, USA.
Ecology. 2019 Dec;100(12):e02878. doi: 10.1002/ecy.2878. Epub 2019 Sep 24.
Trees growing near the Arctic treeline have long been used to reconstruct past climates. However, recent studies have shown deterioration of historically strong positive correlations between air temperature and tree growth (known as "divergence"). Divergence has important implications for confidence in paleoclimate reconstructions and ecosystem-atmosphere carbon exchange. Studies in the Brooks Range of northern Alaska showed that white spruce in the west increased growth in response to late 20th century warming, whereas those in the east failed to show a growth increase. In an earlier study across four watersheds in the Brooks Range, we tested and rejected the hypothesis that divergence in the easternmost watershed reflects moisture limitation of growth. Here, using 16 sites distributed across the same four watersheds, we tested an alternative hypothesis, that greater nutrient limitation in the east may have impeded positive growth responses to warming. Climate comparison across the four Brooks Range study watersheds revealed that, although the easternmost watershed generally had a drier growing-season climate, the most consistent difference was that winter air temperature and both winter and summer soil temperatures were much colder in the central and eastern watersheds. Soil nutrient availability, foliar nutrient concentrations, and tree growth were all generally lower in the central and eastern than in the western watersheds. Foliar phosphorus concentration was the best predictor of spatial variation in branch extension growth-a finding that is somewhat inconsistent with the theory that forest productivity on young, glacially derived soils should be strongly nitrogen limited. Experimental fertilization yielded the greatest growth increase in the eastern, an intermediate response in the central, and the smallest growth increase in the western watershed, generally mirroring trends in soil temperature, soil nutrient availability, foliar nutrient concentrations, and growth of control trees. Our results confirm that growth in the easternmost watershed is more nutrient limited and suggest that phosphorus limitation may be at least as important as nitrogen limitation of growth. We hypothesize that cold soil effects on tree access to nutrients might explain divergence in the eastern Brooks Range and elsewhere near the Arctic treeline, particularly in areas with cold winters and widespread permafrost.
在北极林线附近生长的树木长期以来一直被用于重建过去的气候。然而，最近的研究表明，空气温度与树木生长之间的历史上很强的正相关关系（称为“发散”）正在恶化。发散对古气候重建和生态系统-大气碳交换的置信度有重要影响。在阿拉斯加北部布鲁克斯山脉的研究表明，西部的白云杉对 20 世纪后期的变暖做出了增加生长的反应，而东部的白云杉则没有表现出生长增加。在布鲁克斯山脉的四个流域的早期研究中，我们测试并否定了这样一种假设，即最东部流域的发散反映了生长的水分限制。在这里，我们使用分布在相同的四个流域中的 16 个地点，测试了一个替代假设，即东部的养分限制可能更大，从而阻碍了对变暖的积极生长反应。对四个布鲁克斯山脉研究流域的气候比较表明，尽管最东部的流域通常具有更干燥的生长季节气候，但最一致的差异是，中央和东部流域的冬季空气温度以及冬季和夏季土壤温度都要低得多。土壤养分供应、叶片养分浓度和树木生长在中央和东部流域普遍低于西部流域。叶片磷浓度是枝扩展生长空间变化的最佳预测因子——这一发现与森林生产力在年轻的冰川衍生土壤上应受强烈氮限制的理论有些不一致。实验施肥在东部流域产生了最大的生长增加，在中部流域产生了中等的响应，在西部流域产生了最小的生长增加，通常与土壤温度、土壤养分供应、叶片养分浓度和对照树的生长趋势相吻合。我们的结果证实，最东部流域的生长受养分限制更大，并表明磷限制可能至少与氮限制一样重要。我们假设，土壤对树木获取养分的冷效应可能解释了布鲁克斯山脉东部以及北极林线附近其他地区的发散现象，特别是在冬季寒冷和广泛存在多年冻土的地区。
