Allometric Relationships of Branch Water-Storage Capacity and Capacitance in Four European Trees Species

Water storage capacity and capacitance in trees regulate hydration levels, providing water reserves during drought. However, the effects of varying traits, tissue fractions and of different water pools on the allometry of branch-/sample-level properties have not been systematically investigated. We analyse the relationships between branch size and branch capacity and capacitance with respect to wood density, xylem vulnerability to embolism, and tissue fractions. The analysis was performed using data from four tree species sampled from 12 to 15 sites across Europe. We show that of the three phases of the water release curve, the second phase (dominated by elasticity) was significantly influenced by leaf and bark proportions, the sapwood/heartwood ratio and xylem vulnerability to embolism for capacity and/or capacitance. However, the first (dominated by capillarity) and the third phase (characterised by embolism) were not influenced by the morpho-physiological properties measured. Our results indicate that branch capacity and capacitance are allometrically related (slope < 1) to branch dry mass, leaf area and total water content, indicating that normalising by these size measures does not completely remove size-dependency. We conclude that the only means of obtaining size-independent water storage traits directly applicable in comparative and modelling studies is by normalising by water quantity per phase.