FOREST DYNAMICS DURING EOCENE GREENHOUSE CONDITIONS: ASSESSING ECOSYSTEM RESILIENCE AND DIVERSITY IN THE MEGAFLORAL RECORD OF THE OKANAGAN HIGHLANDS, BRITISH COLUMBIA

Megafloras from localities in the Okanagan Highlands of the southern interior of British Columbia provide a major resource to study paleoenvironmental change during the Paleogene. These ecologically diverse upland sites provide a window on responses of forest communities to environmental change. The early Eocene Falkland flora occurs in an isolated outcrop of thinly bedded lacustrine shale exposed on the southern face of Estekwalan Mountain. An U–Pb date of 50.61 ± 0.16 Ma from ash beds interleaving the fossil-bearing shales places the Falkland megaflora well within the early Eocene Climatic Optimum (52-50 Ma). Initial results are presented using leaf margin analysis (LMA) together with analysis of changes in plant diversity and ecosystem resilience. Over 1400 specimens were recorded during two field seasons, representing 55 morphotypes of gymnosperm and angiosperm leaves, leafy twigs, fruits, flowers, seeds and cones. Based on 38 dicot leaf morphotypes MAT is 9.2 ± 2.2ºC using LMA, comparable to a previously published value (9.4 ± 2.8ºC) for Falkland based on an initial collection of the fossil flora. Using the ‘wet sites' LMA equation of Kowalski & Dilcher, MAT is 11.8 ± 2.6°C. Three stratigraphic units, separated by major ash-beds are recognized within an intensively sampled 2.5 m vertical section within the site. Site stratigraphy was measured relative to an ash layer designated the “datum ash” for the site, providing an arbitrary 0-point. Chi-square analysis of the data shows that there is no significant difference in proportions of leaf margin types and leaf size categories between the three units for dicot leaves. However, there is a highly significant correlation between major taxon type (angiosperm or gymnosperm) and vertical position in the section (Pearson chi-square = 27.141, d.f. = 2, p<0.0001). More specifically the section above the ash layer at 70 cm above a datum ash shows an increased proportion of angiosperms compared to gymnosperms. This change is interpreted as a rapid introduction of angiosperms following a major environmental disturbance. Little change in plant community structure is seen in association with an earlier major volcanic event recorded in the site stratigraphy, suggesting that ecosystem stability (resistance and resilience) was lowered at the time of the second volcanic event.