Physiological and anatomical responses of Acacia koa (Gray) seedlings to varying light and drought conditions

True leaves of Acacia koa (Gray) seedlings play a vital role in the early stages of seedling establishment, particularly in the Hawaiian archipelago where regeneration occurs under very heterogeneous light and moisture conditions. Given the importance of understanding seedling regeneration in the highly disturbed forests of Hawaii, we studied the growth, biomass allocation, physiological, and anatomical characteristics of A. koa seedlings with true leaves in response to three levels of light (low, intermediate, and high sunlight) and two levels of moisture regimes (100% field capacity, 20% field capacity) in a greenhouse experiment. With increasing light intensity, seedlings exhibited greater growth, biomass accumulation, intrinsic water-use efficiency, more foliar C, and higher stomatal density. Their leaflets were thicker, containing more palisade and spongy parenchyma. Low light intensity ameliorated drought-related effects of carbon assimilation on A. koa seedlings, while seedling growth of drought-stressed seedlings was reduced under intermediate and high sunlight. Our results suggest that true leaves of A. koa seedlings are adaptive when light is the only limiting environmental variable. The low adaptive capacity of true leaves to drought stress under high light intensity negatively affected seedling performance, which possibly explains environmental conditions under which phyllodes are likely to emerge.