Concealed truth: Modeling reveals unique Quaternary distribution dynamics and refugia of four related endemic keystone Abies taxa on the Tibetan Plateau.

Understanding the factors driving the Quaternary distribution of Abies in the Tibetan Plateau (TP) is crucial for biodiversity conservation and for predicting future anthropogenic impacts on ecosystems. Here, we collected Quaternary paleo-, palynological, and phylogeographical records from across the TP and applied ecological niche models (ENMs) to obtain a profound understanding of the different adaptation strategies and distributional changes in Abies trees in this unique area. We identified environmental variables affecting the different historical biogeographies of four related endemic Abies taxa and rebuilt their distribution patterns over different time periods, starting from the late Pleistocene. In addition, modeling and phylogeographic results were used to predict suitable refugia for Abies forrestii, A. forrestii var. georgei, A. fargesii var. faxoniana, and A. recurvata. We supplemented the ENMs by investigating pollen records and diversity patterns of cpDNA for them. The overall reconstructed distributions of these Abies taxa were dramatically different when the late Pleistocene was compared with the present. All Abies taxa gradually receded from the south toward the north in the last glacial maximum (LGM). The outcomes showed two well-differentiated distributions: A. fargesii var. faxoniana and A. recurvata occurred throughout the Longmen refuge, a temporary refuge for the LGM, while the other two Abies taxa were distributed throughout the Heqing refuge. Both the seasonality of precipitation and the mean temperature of the driest quarter played decisive roles in driving the distribution of A. fargesii var. faxoniana and A. recurvata, respectively; the annual temperature range was also a key variable that explained the distribution patterns of the other two Abies taxa. Different adaptation strategies of trees may thus explain the differing patterns of distribution over time at the TP revealed here for endemic Abies taxa.