Data from: Convergence, consilience, and the evolution of temperate deciduous forests

The deciduous habit of northern temperate trees and shrubs provides one of the most obvious examples of convergent evolution, but how did it evolve? Hypotheses based on the fossil record posit that deciduousness evolved first in response to drought or darkness and preadapted certain lineages as cold climates spread. An alternative is that evergreens first established in freezing environments and later evolved the deciduous habit. We monitored phenological patterns of 20 species of Viburnum spanning tropical, lucidophyllous (subtropical montane and warm temperate), and cool temperate Asian forests. In lucidophyllous forests, all viburnums were evergreen plants that exhibited coordinated leaf flushes with the onset of the rainy season but varied greatly in the timing of leaf senescence. In contrast, deciduous species exhibited tight coordination of both flushing and senescence, and we found a perfect correlation between the deciduous habit and prolonged annual freezing. In contrast to previous stepwise hypotheses, a consilience of independent lines of evidence supports a lockstep model in which deciduousness evolved in situ, in parallel, and concurrent with a gradual cooling climate. A pervasive selective force combined with the elevated evolutionary accessibility of a particular response may explain the massive convergence of adaptive strategies that characterizes the world’s biomes.北温带乔木与灌木的落叶习性，是趋同进化（convergent evolution）最为显著的例证之一，但其演化机制始终有待厘清。基于化石记录的主流假说认为，落叶习性最初为适应干旱或弱光环境演化而来，并在气候逐步变冷的过程中，为部分演化支赋予了预适应优势。另有替代性假说提出，常绿植物最初在冰冻环境中定植存活，后续才逐步演化出落叶习性。本研究针对分布于亚洲热带、照叶林（lucidophyllous，涵盖亚热带山地与暖温带区域）及冷温带森林中的20种荚蒾属（Viburnum）植物的物候模式开展了系统性监测。结果显示，在照叶林生境中，所有受试荚蒾均为常绿类型，其展叶进程与雨季来临同步，但叶片衰老的时间节点差异极大。与之形成鲜明对比的是，落叶型荚蒾的展叶与衰老进程均呈现出严格的同步性，且本研究发现落叶习性与年度长期冰冻环境存在完全相关关系。与此前提出的逐步演化假说相悖，多项独立证据链的综合佐证支持一种同步演化模型：落叶习性随气候逐渐变冷，在原位平行演化且与降温进程同步发生。一种普遍存在的选择压力，加之特定适应性响应的演化可及性显著提升，或可解释全球生物群系中广泛存在的适应性策略大规模趋同现象。