Leaf physiognomic data from the 45-44 Ma John Day Gulch Flora, Clarno Formation, indicate that central Oregon experienced a frost-free, warm subtropical climate with high, non-seasonal rainfall during the middle Eocene. These data were used to test two hypotheses to explain climatic cooling trends during the middle Eocene. The traditional model (Wolfe, 1992), proposes that the John Day Gulch Flora grew during a late Eocene "Late Eocene Cool Interval", which was followed by a return to near-tropical conditions in the latest Eocene. An alternative hypothesis (Myers, 1996; Manchester et al, 1998) proposes that the John Day Gulch Flora a series middle to late Eocene cooling steps. Morphologic/anatomical (physiognomic) analysis of the leaves of extant woody dicot assemblages provides quantitative information about climate that can be applied to ancient leaf floras. Two leaf physiognomic approaches, Climate Leaf Analysis Multivariate Program - CLAMP (Wolfe, 1993) and Leaf Litter Analysis (Greenwood, 1992), were used to estimate the paleoclimate of the middle Eocene John Day Gulch Flora. The CLAMP method and the Leaf Litter Analysis yield essentially identical MAT estimates of 17.5oC and 17.8oC, respectively (+/- ~ 1oC), compared to the ~13oC CLAMP MAT estimated from the flora by Wolfe (1992). This leaf physiognomic MAT estimate, combined with new age information, indicate that the John Day Gulch assemblage grew under climate conditions similar to estimates from other assemblages of the Clarno Flora, and refute the hypothesis that John Day Gulch assemblage grew during a late Eocene cool interval.

Key words: CLAMP, Clarno Flora, middle Eocene, paleobotany, paleoclimate