Data from: Trait syndromes among North American trees are evolutionarily conserved and show adaptive value over broad geographic scales

Adaptive syndromes and their evolutionary constraints represent a powerful construct for understanding plant distributions. However, it is unclear how the species requirements to face multiple stressors promotes syndrome formation and to which abiotic stressors these syndromes show adaptive value over broad geographic scales. We combined local occurrence data from the U.S. Forest Inventory and Analysis (FIA) of 219 angiosperm and 85 gymnosperm species living across the conterminous US with phylogenies and trait data to identify tree syndromes, their evolutionary conservatism, and their adaptive value over broad scales. Factor analyses and evolutionary model selection revealed that trees possess functional trait syndromes that are strongly conserved. Major syndromes at the species level differed between angiosperms and gymnosperms. While the two main syndromes in angiosperms were related to cold and drought-waterlogging tolerance, in gymnosperms a trade-off between shade and drought tolerance was the main syndrome followed by a growth-fire resistance syndrome. Additional RLQ and fourth-corner approaches revealed that trait syndromes at the community level were broadly similar to those observed at the species level for angiosperms, although this was less clear for gymnosperms. This suggests that syndrome evolution has played an important role on angiosperm distributions, whereas additional ecological factors explain gymnosperm distributions. Importantly, syndromes show adaptive value, as they were geographically associated with several environmental variables showing structure from continental to local scales, being temperature the main abiotic stressor. Our results indicate that across the conterminous US tree species possess clear syndromes that are subjected to strong evolutionary constraints driving tree species and forest community distribution.

适应性综合征（adaptive syndromes）及其进化限制（evolutionary constraints）是解析植物分布格局的重要研究框架。然而目前尚不清楚，物种应对多重胁迫的生存需求如何促进性状综合征的形成，以及在大地理尺度下，这些综合征对哪些非生物胁迫因子（abiotic stressor）具有适应性价值。本研究整合了来自美国森林资源清查与分析（U.S. Forest Inventory and Analysis, FIA）的物种分布发生数据，涵盖分布于美国本土的219种被子植物（angiosperm）与85种裸子植物（gymnosperm），并结合系统发育数据与功能性状数据，以识别树种的性状综合征、其进化保守性，以及大尺度下的适应性价值。因子分析（factor analyses）与进化模型选择（evolutionary model selection）结果显示，树种拥有高度保守的功能性状综合征。物种水平的主要性状综合征在被子植物与裸子植物之间存在显著差异。被子植物的两大主要综合征分别与耐寒性及耐旱-耐涝性相关，而裸子植物的核心综合征为耐荫性与耐旱性之间的权衡关系，其次为生长-抗火性状综合征。额外的RLQ分析与第四角法（fourth-corner approaches）结果表明，群落水平的性状综合征与被子植物物种水平的性状综合征大体一致，但裸子植物的这一规律并不显著。这意味着性状综合征的演化在被子植物的分布格局中发挥了重要作用，而裸子植物的分布则需由其他生态因子加以解释。值得注意的是，性状综合征具有适应性价值：其地理分布与多个环境变量显著相关，这些变量的空间结构覆盖从大陆尺度到局地尺度的范围，其中温度是最主要的非生物胁迫因子。本研究结果表明，在美国本土范围内，树种拥有明确的性状综合征，且受到强烈的进化限制，这些限制驱动了树种与森林群落的分布格局。