Roles of climate niche conservatism and range dynamics in woody plant diversity patterns through the Cenozoic

Aim: Large-scale biodiversity patterns are often discussed in the context of ‘out of the tropics’ (OTT) dynamics and/or tropical niche conservatism (TNC), but empirical evidence of these processes remains very limited. The aim of this study was to infer diversification processes and mechanisms of Cenozoic diversity dynamics using woody angiosperms. Location: East Asian continental islands and global. Time period: Cenozoic. Major taxa studied: Woody plants. Methods: We compiled Cenozoic fossil and modern records for woody angiosperm genera to reconstruct biodiversity patterns. To evaluate the relative importance of TNC and OTT, we investigated extinction/survival patterns and latitudinal range dynamics for each genus, in combination with their clade age (mean family age), cold tolerance and per-genus species richness. Results: We found diversity decreasing with latitude in modern-day flora, but not in the warmer periods of the Neogene and Palaeogene. The percentage of genera surviving decreased with latitude through the Cenozoic. Older genera with less cold tolerance and/or species-poor genera went extinct locally at high latitudes in response to post-Pliocene global cooling. Evolutionarily younger temperate genera dispersed from the extratropics to lower latitudes and the Southern Hemisphere after the Neogene. Main conclusions: The latitudinal diversity gradient (LDG) has rapidly steepened post-Pliocene through: (a) selective extinction in higher latitudes of old, less-diversified tropical genera with low freezing tolerance, and (b) equatorward distributional shift of temperate genera. Both these processes were driven by the high-latitude cooling. Such major roles of TNC and OTT, wherein temperate genera in the Northern Hemisphere expanded through the tropics into higher latitudes of the Southern Hemisphere across climatic boundaries without losing their temperate presence, are in contrast to OTT processes in marine systems. Cenozoic patterns of terrestrial woody angiosperm biodiversity indicate the importance of TNC and high-latitude processes, including the extinction of tropical genera and range contraction or shift of temperate genera.