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.