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 s pecies compared with wet forests. We characterized spatial and temporal par titioning of soil water resources among seven indigenous and one invasive d ry forest species to determine whether the degree of partitioning was consi stent 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 d elta D values ranged from -20 parts per thousand near the surface to -48 pa rts per thousand at 130 cm depth. Metrosideros polymorpha, an evergreen spe cies, and Reynoldsia sandwicensis, a drought-deciduous species, had xylem s ap delta D values of about -52 parts per thousand, and appeared to obtain t heir water largely from deeper soil layers. The remaining six species had x ylem FD values ranging from -33 to -42 parts per thousand, and apparently o btained water from shallower soil layers. Xylem water delta D values were n egatively correlated with minimum annual leaf water potential and positivel y correlated with leaf solute content, an integrated measure of leaf water deficit. Seasonal patterns of leaf production ranged from dry season decidu ous at one extreme to evergreen with near constant leaf expansion rates at the other. Species tapping water more actively from deeper soil layers tend ed to exhibit larger seasonality of leaf production than species relying on shallower soil water sources. Individuals of Myoporum sandwicense were mor e spatially isolated than would be expected by chance. Even though this spe cies apparently extracted water primarily from shallow soil layers, as indi cated by its xylem FD values, its nearly constant growth rates across all s easons may have been the result of a larger volume of soil water available per individual. The two dominant species, Diospyros sandwicesis and Nestegi s sandwicensis, exhibited low leaf water potentials during the dry season a nd apparently drew water mostly from the upper portion of the soil profile, which may have allowed them to exploit light precipitation events more eff ectively than the more deeply rooted species. Character displacement in spa tial and temporal patterns of soil water uptake was consistent with the rel atively high diversity of woody species in Hawaiian dry forests.