Macro-Climatic Distribution Limits Show Both Niche Expansion and Niche Specialization among C4 Panicoids

Grasses are ancestrally tropical understory species whose current dominance in warm open habitats is linked to the evolution of C4 photosynthesis. C4 grasses maintain high rates of photosynthesis in warm and water stressed environments, and the syndrome is considered to induce niche shifts into these habitats while adaptation to cold ones may be compromised. Global biogeographic analyses of C4 grasses have, however, concentrated on diversity patterns, while paying little attention to distributional limits. Using phylogenetic contrast analyses, we compared macro-climatic distribution limits among ~1300 grasses from the subfamily Panicoideae, which includes 4/5 of the known photosynthetic transitions in grasses. We explored whether evolution of C4 photosynthesis correlates with niche expansions, niche changes, or stasis at subfamily level and within the two tribes Paniceae and Paspaleae. We compared the climatic extremes of growing season temperatures, aridity, and mean temperatures of the coldest months. We found support for all the known biogeographic distribution patterns of C4 species, these patterns were, however, formed both by niche expansion and niche changes. The only ubiquitous response to a change in the photosynthetic pathway within Panicoideae was a niche expansion of the C4 species into regions with higher growing season temperatures, but without a withdrawal from the inherited climate niche. Other patterns varied among the tribes, as macro-climatic niche evolution in the American tribe Paspaleae differed from the pattern supported in the globally distributed tribe Paniceae and at family level.草本植物（禾草类）的祖先均为热带林下物种，如今它们在温暖开阔生境中的主导地位与C4光合途径（C4 photosynthesis）的演化密切相关。C4禾草可在温暖且水分胁迫的环境中维持较高的光合速率，该光合综合征被认为可推动其生态位向此类生境转移，但其对寒冷环境的适应能力或因此受到削弱。然而，过往针对C4禾草的全球生物地理学研究多聚焦于多样性分布格局，却极少关注其分布边界极限。本研究采用系统发育对比分析（phylogenetic contrast analyses）方法，对约1300份黍亚科（Panicoideae）禾草的大气候分布极限进行了比较；该亚科涵盖了已报道的禾草类C3-C4光合途径转变事件的4/5。我们分别在亚科尺度以及黍族（Paniceae）、雀稗族（Paspaleae）两个类群尺度下，探讨了C4光合途径的演化是否与生态位扩张、生态位转变或生态位保守存在相关性。本研究对比了生长季温度、干旱度以及最冷月平均温度三类气候极值指标。我们验证了所有已报道的C4物种生物地理分布格局，但研究发现这些格局的形成同时涉及生态位扩张与生态位转变两类过程。在黍亚科内，针对光合途径转变唯一具有普遍性的响应，是C4物种向生长季温度更高的区域实现生态位扩张，但并未脱离其祖先继承的气候生态位。其余分布格局则存在类群间差异：美洲本土的雀稗族的大气候生态位演化模式，与全球广布的黍族以及亚科尺度下的演化模式均显著不同。