Temporal and spatial partitioning of water resources among eight woody species in a Hawaiian dry forest

Lowland dry forests are unique in Hawaii for their high diversity of tree species compared with wet forests. We characterized spatial and temporal partitioning of soil water resources among seven indigenous and one invasive dry forest species to determine whether the degree of partitioning was consistent with the relatively high species richness in these forests. Patterns of water utilization were inferred from stable hydrogen isotope ratios CFD of soil and xylem water, zones of soil water depletion, plant water status, leaf phenology, and spatial patterns of species distribution. Soil water delta D values ranged from -20 parts per thousand near the surface to -48 parts per thousand at 130 cm depth. Metrosideros polymorpha, an evergreen species, and Reynoldsia sandwicensis, a drought-deciduous species, had xylem sap delta D values of about -52 parts per thousand, and appeared to obtain their water largely from deeper soil layers. The remaining six species had xylem FD values ranging from -33 to -42 parts per thousand, and apparently obtained water from shallower soil layers. Xylem water delta D values were negatively correlated with minimum annual leaf water potential and positively correlated with leaf solute content, an integrated measure of leaf water deficit. Seasonal patterns of leaf production ranged from dry season deciduous at one extreme to evergreen with near constant leaf expansion rates at the other. Species tapping water more actively from deeper soil layers tended to exhibit larger seasonality of leaf production than species relying on shallower soil water sources. Individuals of Myoporum sandwicense were more spatially isolated than would be expected by chance. Even though this species apparently extracted water primarily from shallow soil layers, as indicated by its xylem FD values, its nearly constant growth rates across all seasons may have been the result of a larger volume of soil water available per individual. The two dominant species, Diospyros sandwicesis and Nestegis sandwicensis, exhibited low leaf water potentials during the dry season and apparently drew water mostly from the upper portion of the soil profile, which may have allowed them to exploit light precipitation events more effectively than the more deeply rooted species. Character displacement in spatial and temporal patterns of soil water uptake was consistent with the relatively high diversity of woody species in Hawaiian dry forests.