BUILDING A FOREST FROM THE LEAVES: ISOTOPIC METHODS FOR RECONSTRUCTING FOREST STRUCTURE OF THE PAST

Although the age and location is disputed, the rise of the first closed-canopy forest is likely linked with the expansion of angiosperms in the late Cretaceous or early Cenozoic. The relationship between canopy closure and warming is of interest to paleobotanists and paleoclimatologists alike, because forests significantly impact both regional and global climates. Forests regulate water cycling on continents and maintain the warm, moist conditions to which they are best adapted. The carbon isotope “canopy effect” reflects the extent of canopy closure, and is well documented in δ¹³C values of leaves in modern forests. To test the extent of canopy closure among the oldest documented angiosperm tropical forests, we analyzed isotopic characteristics of leaf fossils from the Guaduas and Cerrejón Formations. The Guaduas Fm. (Maastrichtian) contains some of the earliest angiosperm fossils in the Neotropics, and both leaf morphology and pollen records at this site suggest open-canopy structure. The Cerrejón Fm. (Paleocene) contains what are believed to be the first recorded fossil leaves from a closed-canopy forest. We analyzed the carbon isotope content of fossil leaves (n = 199) representing ten families of eudicots (Apocynaceae, Bombaceae, Euphorbaceae, Fabaceae, Lauraceae, Malvaceae, Meliaceae, Menispermaceae, Moraceae, Sapotaceae). We interpreted extent of canopy coverage based on the range of δ¹³C values. The narrow range of δ¹³C values in leaves from the Guaduas Fm. (2.7‰) is consistent with an open canopy. For one site in the Cerrejon Fm. a significantly wider range in values (6.3‰) suggests a closed-canopy, which contrasts with a narrow range (3.3‰) from a site representing a lacustrine facies. The narrow range is consistent with a forest edge, as suggested by the taxa in the leaf assemblage, and consistent with modern studies that show leaves in lake sediments tend to be biased toward plants living at the lake edge. Such environments do not experience closed-canopy conditions, and express the isotopic characteristics associated with an open canopy. This study confirms the identity of the first closed-canopy forest in the geologic record via isotopic analysis and indicates this forest structure developed sometime following the K-Pg mass extinction, and prior to Eocene hyperthermal events, including the PETM.