Many biodiversity hotspots are located in montane regions, thus, understanding the underlying mechanisms driving species assembly along elevational gradients is of major interest in ecology and biogeography. Here, we assess spatial patterns and climatic drivers, and the effects of clade age, on patterns of phylogenetic structure of ferns along the world's longest elevational gradient in the central Himalaya. We used correlation and regression analyses to relate metrics of phylogenetic structure reflecting both shallow (tip-weighted) and deep (basal-weighted) evolutionary histories of ferns, and their two major groups reflecting different ages (polypods representing a young clade, all other ferns representing old clades), in fifty 100-m vertical bands to climatic factors representing different aspects of climatic conditions (mean climate, stressful climate and climate seasonality). Variation partitioning analysis was used to determine the relative importance of each group of climatic factors on phylogenetic structure. We find that the composition of fern assemblages along the Himalayan elevational gradient in Nepal shows strong signatures of evolutionary processes. In a simplified way, species-rich assemblages at mid-elevations are likely the result of recent radiations in combination with low extinction rates, whereas species-poor assemblages at low elevations are composed of numerous lineages with limited radiations, and those at high elevations of few lineages, also with limited signature of recent radiations. Variables related to temperature and climatic extremes tended to play a more important role than precipitation- and seasonality-related variables, respectively, in driving fern phylogenetic structure. Combining the results of ferns and angiosperms suggests that there are a few generally consistent evolutionary processes that apply to all plant groups (e.g. niche conservatism and environmental filtering), but that the specific outcomes of these processes vary with elevation, clade age and taxon.

中文翻译：

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世界上最长海拔梯度的蕨类植物系统发育结构的空间格局和气候驱动因素

许多生物多样性热点位于山区，因此，了解沿海拔梯度驱动物种聚集的潜在机制是生态学和生物地理学的主要兴趣所在。在这里，我们评估了空间格局和气候驱动因素，以及进化枝年龄对喜马拉雅中部世界最长海拔梯度沿线蕨类植物系统发育结构模式的影响。我们使用相关性和回归分析来关联系统发育结构的指标，反映蕨类植物的浅层（尖端加权）和深层（基础加权）进化历史，以及反映不同年龄的两个主要群体（多足动物代表一个年轻的进化枝，所有其他蕨类植物代表古老的进化枝），在 50 个 100 米的垂直带中代表气候条件不同方面的气候因子（平均气候，紧张的气候和气候季节性）。变异划分分析用于确定每组气候因子对系统发育结构的相对重要性。我们发现尼泊尔喜马拉雅海拔梯度沿线的蕨类植物组合的组成显示出强烈的进化过程特征。以一种简化的方式，中海拔地区物种丰富的组合可能是近期辐射与低灭绝率相结合的结果，而低海拔地区物种匮乏的组合由众多辐射有限的谱系组成，而高海拔地区的很少有谱系，近期辐射的特征也有限。与温度和极端气候相关的变量往往分别比与降水和季节性相关的变量发挥更重要的作用，在驱动蕨类植物系统发育结构。结合蕨类植物和被子植物的结果表明，有一些普遍一致的进化过程适用于所有植物群（例如生态位保守性和环境过滤），但这些过程的具体结果因海拔、进化枝年龄和分类单元而异。