The common expectation that aboveground biomass (AGB) decreases along elevational gradients due to temperature reduction is challenged for some recent studies showing that tropical montane forests in Africa, South America, and Southeast Asia contain similar AGB values their lowland counterparts. One possible driver of high AGB on tropical montane forests is biogeographic history. If temperate originated tree species colonize highlands after mountain uplift and are pre-adapted to coldness, these tree species can maintain larger size and AGB on mountain forests.
The size of tropical originated tree species decreases with elevation, while the size of temperate originated species is higher in highlands.
We used herbarium records from tropical tree species distributed in tropical Andes obtained from BIEN database. Then, we generated a tree community matrix across the tropical Andes, cutting in latitude each degree and separating elevational bands each 500m. Then, we classified each species in tropical and temperate originated species based on the current distribution of each botanical family that these belong. Also, we obtained maximum tree diameter and maximum tree height from TRY database from each species. We used the genus or family average if one species is not reported in the database. Finally, we estimated if tree size decreases with elevation and expected and the effect of biogeographic origin on this relationship.
We found that tree size tends to decrease with elevation, and this relationship is stronger using tropical trees alone. The size of temperate originated species does not change along elevation, supporting the idea that these species maintain larger tree size on tropical highlands and affect the variation of AGB on tropical elevational gradients. In contrast, the size of tropical trees decreases with elevation as expected if coldness influences the performance of large trees. Then, we suggest that temperate tree species that migrate to tropical mountains can influence the current structure of tropical montane forests.
The evolutionary history is a driver of the montane forest structure and can be useful to understand the response of these forests to climate change.
Andean biogeography, Tropical-Temperate mixing; Tree size