Morphological and physiological adjustment to N and P fertilization in nutrient-limited Metrosideros polymorpha canopy trees in Hawaii

Leaf-level studies of Metrosideros polymorpha Gaud. (Myrtaceae) canopy tree s at both ends of a substrate age gradient in the Hawaiian Islands pointed to differential patterns of adjustment to both nutrient limitation and remo val of this limitation by long-term (8-14 years) nitrogen (N), phosphorus ( P) and N + P fertilizations. The two study sites were located at the same e levation, had similar annual precipitation, and supported forests dominated by M. polymorpha, but differed in the age of the underlying volcanic subst rate, and in soil nutrient availability, with relatively low N at the young site (300 years, Thurston, Hawaii) and relatively low P at the oldest site (4, 100,000 years, Kokee, Kauai). Within each site, responses to N and P f ertilization were similar, regardless of the difference in soil N and P ava ilability between sites. At the young substrate site, nutrient addition led to a larger mean leaf size (about 7.4 versus 4.8 cm(2)), resulting in a la rger canopy leaf surface area. Differences in foliar N and P content, chlor ophyll concentrations and carboxylation capacity between the fertilized and control plots were small. At the old substrate site, nutrient addition led to an increase in photosynthetic rate per unit lear surface area from 4.5 to 7.6 mu mol m(-2) s(-1), without a concomitant change in leaf size. At th is site, leaves had substantially greater nutrient concentrations, chloroph yll content and carboxylation capacity in the fertilized plots than in the control plots. These contrasting acclimation responses to fertilization at the young and old sites led to significant increases in total carbon gain o f M. polymorpha canopy trees at both sites. At the young substrate site, ac climation to fertilization was morphological, resulting in larger leaves, w hereas at the old substrate site, physiological acclimation resulted in hig her leaf carboxylation capacity and chlorophyll content.