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

Global patterns of Cretaceous forest composition and productivity are analyzed using a comprehensive fossil wood database (n = 2238). To ascertain forest composition, records were classified by botanical affinity, plotted on georeferenced paleomaps, and analyzed with ArcGIS tools. Results confirm previous conjecture that araucarioid and podocarpoid conifers were globally codominant in Early Cretaceous time, especially in humid tropical and paratropical 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 associated 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 diversified 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. Comparison with modern data shows that Cretaceous forest productivity was significantly elevated (x2) in mid- and high paleolatitudes, implying a poleward displacement of the temperate zone by >15 degrees. Our data provide quantitative verification of Cretaceous climate-vegetation models and improve the understanding of the long-term effects of future global warming.