PREMISE:The Eocene-Oligocene transition (EOT; 34-33 Ma) was marked by global cooling and increased seasonality and aridity, leading to a shift in North American floras from subtropical forests to deciduous hardwood forests similar to today. This shift is well documented taxonomically and biogeographically, but its ecological nature is less known. METHODS:Using the relationship between petiole cross-sectional area and leaf mass, we estimated leaf dry mass per area (LMA), a functional trait tied to plant resource investment and expenditure, at 22 western North American sites spanning the EOT to determine how the broad restructuring of vegetation during this time was reflected in leaf economics. RESULTS:There was no overall shift in LMA between pre-EOT and post-EOT floras; instead, changes in LMA across sites were mostly driven by a negative correlation with dry-season precipitation and a positive correlation with paleoelevation. These patterns held for both whole sites and subsets of sites containing taxa with similar biogeographical histories (taxa that persisted in the highlands across the EOT or that migrated to the lowlands) and are consistent with most observations in extant floras. CONCLUSIONS:Our data provide a geological context for understanding environmentally paced changes in leaf-economic strategies, particularly linking leaf economic strategies to dry-season precipitation and paleoelevation.

译文

前提:始新世-渐新世过渡期(EOT; 34-33 Ma)以全球降温以及季节性和干旱增加为特征,导致北美植物区系从亚热带森林向落叶硬木森林转变。这种转变在分类学和生物地理学方面都有据可查,但其生态性质鲜为人知。
方法:利用叶柄截面积与叶片质量之间的关系,我们估算了横跨EOT的北美西部22个站点的单位面积叶片干重(LMA),这是与植物资源投资和支出相关的功能性状,以确定这段时期内,植被的广泛重组反映在叶片经济学上。
结果:在EOT之前和EOT之后的菌群之间,LMA没有整体变化。相反,各地点的LMA变化主要是由与旱季降水的负相关和与古海拔的正相关引起的。这些模式适用于包含具有相似生物地理历史的分类单元的整个站点和站点子集(在EOT上在高地上持续存在或迁移到低地的分类单元),并且与现存菌群中的大多数观测结果一致。
结论:我们的数据提供了一个地质背景,用于理解环境经济中叶片经济策略的变化,特别是将叶片经济策略与旱季降水和古海拔联系起来。

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