Plant Water Deficits, Osmotic Properties, and Hydraulic Resistances of Hawaiian Dubautia Species from Adjacent Bog and Wet-Forest Habitats

Abstract

Functional responses of two closely related Dubautia species from a mosaic of Hawaiian bogs and wet forest were compared to help explain their differential distributions. Dubautia paleata is largely restricted to saturated bogs, while D. raillardioides is restricted to the surrounding, better-drained wet forest. Minimum diurnal tissue water potentials of D. paleata are significantly lower than those of D. raillardioides, despite the moister condition of bog soil. The tissue osmotic potential at full hydration of D. paleata is significantly lower than that of D. raillardioides. As a result, the tissue water potential at which turgor reaches zero for D. paleata is significantly lower than that of D. raillardioides. Dubautia paleata is thus able to maintain positive turgor to lower water potentials than D. raillardioides. Lack of a lowered, in D. raillardioides may therefore contribute to exclusion of that species from the bog habitat. Preliminary data suggest a significantly greater hydraulic resistance for D. paleata than for D. raillardioides, probably due to higher root resistance caused by the reduced condition of the waterlogged bog substrate. The difference in hydraulic resistance could help account for the contrasting water deficits of the two species.