Vascular anatomy
After we conducted all field measurements, we labelled a piece of each stem segment, stored it in a plastic bag and we took it to the laboratory for vessel diameter and density measuring (n=160). We cut xylem transverse sections freehandedly with a surgical blade, observed them under a light microscope (Moticam 2500, Motic), and photographed them with a digital camera. We recorded vessel density and diameters in a continuous transect from pith and bark of the sample. We calculated the diameter of each vessel by averaging the longest and shortest axis across the lumen (Ewers & Fisher, 1991a) using ImageJ (National Institutes of Health, USA). We then calculated an average for each branch (n > 100 vessels) and an average for each species (five branches). We calculated vessel density as the number of vessels per unit of area tissue in the stem transverse section. We grouped vessels were grouped in 10-µm-diameter classes following the methodology described by Gorsuch et al. (2001), and we calculated the relative contribution of each class to total hydraulic conductivity using the Hagen–Poiseuille law, as the sum of the fourth powers of all vessel radii in the class divided by the sum of the fourth powers of all vessel radii.