Out of the tropics and adaptation in the Asian subtropics of Dendrobium section Dendrobium (Orchidaceae)

The evolutionary processes contributing to plant biodiversity in the tropics are a matter of active discussion, as there are remaining questions about lineages' diversification patterns and adaptations after migration from the tropics to the subtropics. Here, we investigate these issues using the epiphytic orchids Dendrobium section Dendrobium. We reconstructed the spatio-temporal dynamics of this group based on whole plastome and nrITS data, analyzed macroevolutionary patterns, explored adaptive traits of subtropical lineages and predicted their distribution over time. We found that D. sect. Dendrobium originated in the Asian tropics in the middle Miocene and independently dispersed into the Asian subtropics at least four times since the early Pliocene. The net diversification rate decreased continuously due to temperature decline in the Asian tropics, while the net diversification rate of subtropical lineages increased with niche and trait rates rising. Smaller stem and leaf sizes of D. sect. Dendrobium were morphological adaptations in the Asian subtropics, and were mainly related to temperature seasonality. Importantly, the northern distribution limits of D. sect. Dendrobium mostly overlapped with the Asian subtropical evergreen broad-leaved forests since the Pliocene. This study represents an important case for understanding plant adaptations as they move out of the tropics.