The functional mechanism behind latitudinal pattern of liana diversity: freezing-thaw embolism reduces the ecological performance of liana species.

1.- There is a strong decrease in liana diversity along latitudinal and altitudinal gradients at global scale, and there is a marked difference in liana diversity between tropical and temperate ecosystems. From these observations it has been proposed that cold temperatures would restrict the liana ecological patterns, because of the vulnerability of their vascular system to freezing-thaw embolism. However, there is scarce empirical evidence that support this idea in natural conditions. 2.- The goal of our study was to establish the functional mechanism behind the loss of liana diversity towards higher latitudes. For this purpose, we evaluated the change in liana performance and the trade-off between safety and efficiency of water transport along a latitudinal gradient in temperate rainforests of South America. We measured the ecological liana performance as the apical growth rate in ten liana species along a latitudinal gradient, and we related it with functional traits associated with the safety (vessel diameter, vessel density, wood density and root pressure) and efficiency of water transport (maximum hydraulic conductivity and percentage of lost conductivity). 4.- We found that lianas species inhabiting the southern (colder) site of the latitudinal gradient have low performance, with a 5-fold decrease in their apical growth rate compared to the northern sites. These would be consequence of a much lower water transport efficiency (26.1-fold decrease) respect to liana species that inhabit northern (warmer) sites, as an outcome of a higher freezing-thaw embolism (37.5% of PLC) and reduction of vessel diameter (3 times narrower). 5.- These results are clear evidence that cold temperature restricts liana performance and would act as a modeler of their latitudinal diversity pattern.