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.

中文翻译：

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始新世-渐新世过渡期的叶片经济战略与旱季降水和古海拔相关

前提始新世-渐新世过渡（EOT；34-33 Ma）的特点是全球变冷以及季节性和干旱增加，导致北美植物群从亚热带森林转变为类似于今天的落叶阔叶林。这种转变在分类学和生物地理学上都有很好的记录，但其生态性质鲜为人知。方法 利用叶柄横截面积和叶质量之间的关系，我们估计了跨 EOT 的 22 个北美西部地点的单位面积叶干质量 (LMA)，这是一种与植物资源投资和支出相关的功能性状，以确定广泛的在此期间植被的重组反映在叶经济学中。结果 EOT 前和 EOT 后菌群之间的 LMA 没有整体变化；反而，不同地点的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驱动的。这些模式适用于包含具有相似生物地理历史的类群（在整个 EOT 的高地中持续存在或迁移到低地的类群）的整个站点和站点子集，并且与现存植物群中的大多数观察结果一致。结论我们的数据为理解叶片经济策略中的环境节奏变化提供了地质背景，特别是将叶片经济策略与旱季降水和古海拔联系起来。这些模式适用于包含具有相似生物地理历史的类群（在整个 EOT 的高地中持续存在或迁移到低地的类群）的整个站点和站点子集，并且与现存植物群中的大多数观察结果一致。结论我们的数据为理解叶片经济策略的环境节奏变化提供了地质背景，特别是将叶片经济策略与旱季降水和古海拔联系起来。这些模式适用于包含具有相似生物地理历史的类群（在整个 EOT 的高地中持续存在或迁移到低地的类群）的整个站点和站点子集，并且与现存植物群中的大多数观察结果一致。结论我们的数据为理解叶片经济策略的环境节奏变化提供了地质背景，特别是将叶片经济策略与旱季降水和古海拔联系起来。