The biogeographic origin of species may help to explain differences in average tree height and aboveground biomass (AGB) of tropical mountain forests. After the Andean uplift, small‐statured trees should have been among the initial colonizers of the highlands (new cold environment) from the lowland tropics, since these species are pre‐adapted to cold conditions with narrow vessels that are relatively resistant to freezing. If the descendants of these small‐statured clades continue to dominate tropical highland forests, there will be a high co‐occurrence of close relatives at high elevations. In other words, this scenario predicts a systematic decline in tree size, AGB, and phylogenetic diversity with elevation. In contrast, the colonization of Andean forests by some large‐statured clades that originated in temperate regions may modify this expectation and promote a mixing of tropical and temperate clades, thereby increasing the phylogenetic diversity in tropical highland forests. This latter scenario predicts an increase or no change of tree size, AGB, and phylogenetic diversity with elevation. We assessed how the historical immigration of large‐statured temperate‐affiliated tree lineages adapted to cold conditions may have influenced the composition and structure of Andean forests. Specifically, we used 92 0.25‐ha forest inventory plots distributed in the tropical Andes Mountains of Colombia to assess the relationship between the phylogenetic diversity and AGB along elevational gradients. We classified tree species as being either “tropical affiliated” or “temperate affiliated” and estimated their independent contribution to forest AGB. We used structural equation modeling to separate the direct and indirect effect of elevation on AGB. We found a hump‐shaped relationship of phylogenetic diversity, AGB, and tree size with elevation. The high phylogenetic diversity found between 1,800–2,200 m above sea level (asl) was due to the mixing of highland floras containing many temperate‐affiliated species, and lowland floras containing mostly tropical‐affiliated species. The high AGB in highland forests, which contrasted with the expected decline of AGB with elevation, was likely due to the significant contribution of temperate‐affiliated species. Our findings highlight the lasting importance of biogeographic history on the composition and structure of Andean mountain forests.

Summary: Climate warming is causing compositional changes in Andean tropical montane forests (TMFs). These shifts are hypothesised to result from differential responses to warming of cold‐ and warm‐affiliated species, with the former experiencing mortality and the latter migrating upslope. The thermal acclimation potential of Andean TMFs remains unknown. Along a 2000 m Andean altitudinal gradient, we planted individuals of cold‐ and warm‐affiliated species (under common soil and irrigation), exposing them to the hot and cold extremes of their thermal niches, respectively. We measured the response of net photosynthesis (A_net), photosynthetic capacity and leaf dark respiration (R_dark) to warming/cooling, 5 months after planting. In all species, A_net and photosynthetic capacity at 25°C were highest when growing at growth temperatures (T_g) closest to their thermal means, declining with warming and cooling in cold‐affiliated and warm‐affiliated species, respectively. When expressed at T_g, photosynthetic capacity and R_dark remained unchanged in cold‐affiliated species, but the latter decreased in warm‐affiliated counterparts. R_dark at 25°C increased with temperature in all species, but remained unchanged when expressed at T_g. Both species groups acclimated to temperature, but only warm‐affiliated species decreased R_dark to photosynthetic capacity ratio at T_g as temperature increased. This could confer them a competitive advantage under future warming.