Stem water storage capacity and efficiency of water transport: their functional significance in a Hawaiian dry forest

We investigated the contribution of internal water storage and efficiency of water transport to the maintenance of water balance in six evergreen tree species in a Hawaiian dry forest. Wood-saturated water content, a surrogate for relative water storage capacity, ranged from 70 to 105%, and was inversely related to its morphological correlate, wood-density, which ranged between 0.51 and 0.65 g cm(-3). Leaf-specific conductivity (k(L)) measured in stem segments from terminal branches ranged for 3 to 18 mmol m(-1) s(-1) MPa(-1), and whole-plant hydraulic efficiency calculated as stomatal conductance (g) divided by the difference between predawn and midday leaf water potential (psi(L)), ranged from 70 to 150 mmol m(-2) s(-1) MPa(-1). Hydraulic efficiency was positively correlated with k(L) (r(2) = 0.86). Minimum annual psi(L) ranged from -1.5 to -4.1 MPa among the six species. Seasonal and diurnal variation in psi(L) were associated with differences among species in wood saturated water content, wood content, wood density and k(L). The species with higher wood-saturated water content were more efficient in terms of long-distance water transport, exhibited smaller diurnal variation in psi(L) and higher maximum photosynthetic rates. Smaller diurnal variation in psi(L) in species with higher wood-saturated water content, k(L) and hydraulic efficiency was not associated with stomatal restriction of transpiration when soil water deficit was moderate, but avoidance of low minimum seasonal psi(L) in these species was associated with a substantial seasonal decline in g. Low seasonal minimum psi(L) in species with low k(L), hydraulic efficiency, and wood-saturated water content was associated with higher leaf solute content and corresponding lower leaf turgor loss point. Despite the species-specific differences in leaf water relations characteristics, all six evergreen tree species shared a common functional relationship defined primarily by k(L) and stem water storage capacity.