Water supply regulates structure, productivity, and water use efficiency of Acacia koa forest in Hawaii

We studied changes in stand structure, productivity, canopy development, gr owth efficiency, and intrinsic water use efficiency (WUE=photosynthesis/sto matal conductance) of the native tree koa (Acacia koa) across a gradient of decreasing rainfall (2600-700 mm) with increasing elevation (700-2000 m) o n the island of Hawaii. The stands were located on organic soils on either smooth (pahoehoe) or rough (aa) lava flows. In the greenhouse, we also exam ined growth and WUE responses to different water regimes of koa seedlings g rown from seeds collected in the study area. We tested the hypotheses that (1) stand basal area, aboveground net primary productivity (ANPP), leaf are a index (LAI), and growth per unit leaf area decreased with decreasing rain fall, (2) WUE increased with decreasing rainfall or water supply, and (3) W UE responses were caused by stomatal limitation rather than by nutrient lim itations to photosynthesis. The carbon isotope composition of phyllode tiss ues (delta(13)C) was examined as an integrated measure of WUE. Basal area a nd LAI of koa stands on both pahoehoe and aa lava flows, and ANPP on aa lav a flows decreased with elevation. Basal area, LAI, and ANPP of koa in mixed stands with the exotic tropical ash (Fraxinus udhei) were lower compared t o single-species koa stands at similar elevations. Along the gradient, phyl lode delta(13)C (and therefore WUE) increased with elevation from -30.2 to -26.8 parts per thousand. Koa in mixed stands exhibited higher (less negati ve) delta(13)C than in single-species stands suggesting that koa and tropic al ash competed for water. In the greenhouse, we observed the same trend ob served in the field, as phyllode delta(13)C increased from -27.7 to -24 par ts per thousand as water supply decreased. Instantaneous gas exchange measu rements in the greenhouse showed an inverse correlation of both maximum (mo rning) photosynthesis (A) and conductance (g) with delta(13)C values and, a lso, a good agreement between instantaneous (Alg) and integrated measures o f WUE. Phyllode delta(13)C was not correlated with foliar concentrations of N or other nutrients in either the field or the greenhouse, indicating tha t differences in delta(13)C were caused by stomatal limitation rather than by nutrient-related changes in photosynthetic capacity. This study provided evidence that long-term structural and growth adjustments as well as chang es in WUE are important mechanisms of koa response to water limitation.