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.