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，是趋同演化（convergent evolution）最典型的例证之一，但其起源与演化机制至今仍待阐明。基于化石记录的相关假说认为，落叶习性最初为应对干旱或黑暗环境而演化，并在寒冷气候扩张过程中为部分演化支（lineages）提供了预适应优势。另一类对立假说则提出，常绿植物最初定居于冰冻环境中，后续才逐步演化出落叶习性。我们对分布于亚洲热带、亮叶林（亚热带山地与暖温带）及凉温带森林的20种荚蒾属（Viburnum）植物的物候模式进行了监测。在亮叶林生境中，所有受试荚蒾均为常绿植物，其展叶节律与雨季起始同步，但叶片衰老的时间差异极大。与之相对，落叶物种的展叶与衰老均呈现严格的同步性，且我们发现落叶习性与年度长期冰冻存在完全的相关性。与此前提出的分步演化假说相悖，本研究结合多项独立证据的一致性佐证，支持同步演化模型（lockstep model）：落叶习性随气候逐步冷却，在原生境中平行演化且与气候变化同步发生。普遍存在的选择压力，加之特定适应性响应的演化可及性提升，或可解释全球生物群系中广泛存在的适应性策略趋同现象。