Local and Regional Forest Heterogeneity After the Cretaceous–paleogene Boundary in the Denver  Basin, Colorado, Usa, and Implications for the Origins of Rainforests

Crystal, Victoria Faye

Graduate Thesis Or Dissertation

Local and Regional Forest Heterogeneity After the Cretaceous–paleogene Boundary in the Denver Basin, Colorado, Usa, and Implications for the Origins of Rainforests Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/cj82k7769

Abstract

The recovery of forest ecosystems following the environmental perturbation of the Cretaceous–Paleogene (K-Pg) boundary event are not well understood. Here, I present megafloral data from three new early Paleocene (ca. 65.8 Ma) fossil leaf sites from the western margin of the Denver Basin, Colorado, USA, and compare that to existing megafloral data from other parts of the Basin approximately 80 km to the east. My data show four types of spatial heterogeneity: first, I see pronounced taxonomic differences within individually preserved forest floors (i.e. across ~50 m of lateral extent); second, I see comparable differences among successional forest floor horizons at the same geographic location; third, I see the most floral change among separate megafloral sites in the western portion of the Denver Basin, which I interpret as geographically proximal to the Laramide mountain front; fourth, and finally, I see notable differences between contemporaneous mountain proximal and mountain distal floras within the Denver Basin. These data indicate considerable spatial heterogeneity on local and regional scales immediately following the K-Pg boundary. In addition, results of my analyses indicate high rainfall (average mean annual precipitation (MAP) of approximately 230 cm) in mountain proximal floras in the Basin compared to those further east in the center of the Basin (MAP of approximately 155 cm). Finally, there are leaf morphological traits, including drip-tips, cordate bases and overall large leaf size, that are indicative of rainforests in the mountain proximal floras. Taken with the compositional change across the landscape and MAP estimates for the early Paleocene, these data suggest that the origin of the Castle Rock rainforest, which grew 2.2 million years after the K-Pg boundary in the Denver Basin, could be the result of increased niche space due to local topography and intensified orographic rainfall.

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