Abstract：
Although climate change is expected to drive tree species toward colder and wetter regions of their distribution, broadscale empirical evidence is lacking. One possibility is that past and present human activities in forests obscure or alter the effects of climate. Here, using data from more than two million monitored trees from 73 widely distributed species, we quantify changes in tree species density within their climatic niches across Northern Hemisphere forests. We observe a reduction in mean density across species, coupled with a tendency toward increasing tree size. However, the direction and magnitude of changes in density exhibit considerable variability between species, influenced by stand development that results from previous stand-level disturbances. Remarkably, when accounting for stand development, our findings show a significant change in density toward cold and wet climatic conditions for 43% of the species, compared to only 14% of species significantly changing their density toward warm and arid conditions in both early- and late-development stands. The observed changes in climate-driven density showed no clear association with species traits related to drought tolerance, recruitment and dispersal capacity, or resource use, nor with the temperature or aridity affiliation of the species, leaving the underlying mechanism uncertain. Forest conservation policies and associated management strategies might want to consider anticipated long-term species range shifts alongside the integration of contemporary within-distribution density changes.
摘要：
尽管预计气候变化将推动树种向较冷和较湿润的分布区域发展，缺乏广泛的经验证据。一种可能性是森林中过去和现在的人类活动掩盖或改变了气候的影响。这里,使用来自73个分布广泛的物种的200多万棵监测树木的数据，我们量化了北半球森林气候生态位内树种密度的变化。我们观察到不同物种的平均密度降低，再加上树木大小的增加趋势。然而,密度变化的方向和幅度在物种之间表现出相当大的变异性，受先前林分水平扰动导致的林分发展的影响。值得注意的是,在考虑展台发展时，我们的发现表明，43%的物种在寒冷和潮湿的气候条件下密度发生了显著变化，相比之下，在早期和后期发展的林分中，只有14％的物种将其密度显着改变为温暖和干旱的条件。观测到的气候驱动密度变化与耐旱性相关的物种性状没有明显关联，招募和分散能力，或资源使用，也不考虑物种的温度或干旱程度，留下的潜在机制不确定。森林保护政策和相关的管理策略可能要考虑预期的长期物种范围变化以及当代分布内密度变化的整合。
