Plant community ecology and climate on an upland volcanic landscape during the Early Eocene Climatic Optimum: McAbee Fossil Beds, British Columbia, Canada

Abstract

The McAbee Fossil Beds, in south-central British Columbia, Canada, provide a record of forest ecosystems within a volcanically-active, upland landscape during the Early Eocene Climatic Optimum (EECO). To assess plant community ecology and climate within this environment, palynology and census-style macrofossil collections were investigated from two shale horizons separated in time by likely 103–104 years. An ensemble of leaf physiognomic estimates indicates a persistent warm (mean annual temperature [MAT] ≈ 10–12 °C) and mesic (mean annual precipitation ≈ 1000 mm∙yr−1) climate between the horizons. The plant community was similar in both horizons, comprising angiosperms (e.g., Ulmus, Fagus, Alnus, Betula, and Carya) and gymnosperms (e.g., Metasequoia, Pinus, Picea, and Larix). Plant diversity and the distribution of dicot leaf mass per area (MA) values were consistent between the two horizons, despite intervals of varying tuff thickness and frequency between the horizons. Differences in macrofossil abundances of Ulmus, Fagus, and Pinus, and a lack of such differences in their respective pollen abundances might be explained by local patch dynamics, influenced by spatial heterogeneity resulting from topographic diversity and disturbance. MA estimates suggest dicots were predominately deciduous, and the distribution of MA is similar to a modern forest in Panama influenced by seasonal precipitation. The diversity of dicots at McAbee, despite relatively lower mean annual temperatures, is comparable to fossil plant quarries of similar volume from early Eocene sites in Patagonia (e.g., Laguna del Hunco) and the Denver Basin with relatively warmer MATs (17–25 °C). This is best explained by a combination of predominately frost-free winters, high productivity, and an abiotic environment characterized by spatial heterogeneity and high rates of change. The sampled paleofloras of this study showcase an EECO plant community, likely representing an ecological solution to volcanic disturbance over millennial time scales.