Drivers and constraints on floral latitudinal diversification gradients

Aim The latitudinal diversity gradient (LDG) is a primary emergent property of the biosphere, yet the cause(s) of this pattern are still debated. Key to many hypotheses is the origins and maintenance of tropical hyperdiversity, and the role of climate in driving low latitude speciation. Here, we analyse patterns of tropical and extratropical floral diversification and migration during the early Palaeogene “greenhouse” interval, to shed further light on the relationship between climatic change, latitude and floral diversity. Location The early Palaeogene, from ~63 to 42 million years ago, of the US Gulf Coastal Plain (GCP) and Colombia. Taxon Terrestrial plants, using pollen and spores as a proxy. Methods We analyse species diversity trends using coverage and sample size‐based interpolation and extrapolation, Chao1 estimated richness, and evenness metrics. Capture–mark–recapture (CMR) modelling is used to estimate origination and extinction probabilities. Origination patterns on the GCP are separated into in situ speciation versus immigration. Results While Colombian (tropical) palynofloral richness and origination rates increased in conjunction with warming, GCP richness remained stable. The single rise in GCP origination rates, coincident with the Palaeocene–Eocene Thermal Maximum, was largely driven by the immigration of Eurasian taxa, rather than in situ origination, which was the case in Colombia. Main conclusions These results show that the relationships among climatic parameters and diversification and dispersal are not straightforward. While temperature may have driven diversification in the tropics, other factors, such as precipitation, insolation or biological interactions, may have constrained diversification in the extratropics. Furthermore, our results suggest that outward dispersal from the tropics was limited in the warm world of the early Palaeogene, with most GCP immigrants being sourced from other extratropical regions. These findings suggest that the tropics and extratropics may have functioned independently at this time.