Cretaceous forest composition and productivity inferred from a global fossil wood database

Abstract

ABSTRACTGlobal patterns of Cretaceous forest composition and produc-tivity are analyzed using a comprehensive fossil wood database (n = 2238). To ascertain forest composition, records were classifi ed by botanical affi nity, plotted on georeferenced paleomaps, and ana-lyzed with ArcGIS tools. Results confi rm previous conjecture that araucarioid and podocarpoid conifers were globally codominant in Early Cretaceous time, especially in humid tropical and paratropi-cal biomes, but drastically reduced in numbers and range during the Late Cretaceous. Cupressoid conifers, which were most common in seasonally dry mid-latitudes, and pinoid conifers, which were asso-ciated with temperate conditions at higher northern latitudes, also declined at the same time, though less markedly. Spatial analysis suggests that the loss of conifer forests (especially araucarioids) was linked to the rise of co-occurring angiosperms. Our data also show that while angiosperms explosively diversifi ed in mid-Cretaceous time, they did not become forest dominants until the latest Cretaceous (25 m.y. later), by which time the modern relictual pattern of conifer distribution had been established. To ascertain forest productivity, mean tree-ring width data were obtained from direct measurements and literature reviews (n = 284) and plotted by paleolatitude. Com-parison with modern data shows that Cretaceous forest productivity was signifi cantly elevated (×2) in mid- and high paleolatitudes, imply-ing a poleward displacement of the temperate zone by >15°. Our data provide quantitative verifi cation of Cretaceous climate-vegetation models and improve the understanding of the long-term effects of future global warming.INTRODUCTION