Allocation of nitrogen and carbon in leaves of Metrosideros polymorpha regulates carboxylation capacity and delta C-13 along an altitudinal gradient

1. Metrosideros polymorpha (O'hia), the dominant tree species in Hawaiian f orest ecosystems, grows from sea level to treeline (2500 m). Consistent cha nges in its morphology and anatomy occur along this altitudinal/temperature gradient. Patterns of variation in photosynthetic gas exchange, leaf nitro gen content, nitrogen-use efficiency, delta(13)C, and morphological and ana tomical characteristics were determined across the elevational gradient. In addition, on-line carbon isotope discrimination studies of high and low el evation M. polymorpha were performed. 2. Observed trends with increasing altitude were: (1) progressively higher carboxylation efficiency, leaf N content on an area basis, leaf mass per un it area (LMA), less negative foliar delta(13)C and (2) progressively smalle r leaf size. Net CO2 assimilation (A) expressed on an area basis, leaf dry mass and N content per leaf remained relatively constant along the gradient . 3. Foliar delta(13)C became less negative with increasing elevation (- 30 p arts per thousand at low elevation to - 24 parts per thousand at high eleva tion) and was strongly correlated with foliar N and LMA. Foliar delta(13)C was also correlated with variations in the ratio of intercellular to ambien t partial pressure of CO2 (p(i)/p(a)), as determined by field gas-exchange studies. 4. Results from on-line fractionation experiments suggested that the relati vely large internal resistance to CO2 diffusion did not differ between high and low elevation populations, despite differences in LMA. Less negative v alues of delta(13)C at high elevations and corresponding lower values of p( i)/p(a) were associated with increased carboxylation efficiency and N conte nt on a unit leaf area basis. 5. Two major homeostatic responses in M. polymorpha plants along elevationa l/temperature gradients were observed: (1) maintenance of similar photosynt hetic rates per unit leaf surface area despite suboptimal conditions for CO 2 assimilation at high elevation and (2) similar N content per leaf despite lower soil N availability at high elevations. These homeostatic mechanisms allow M. polymorpha to maintain a relatively high level of growth-related activities at high elevation, despite limiting environmental conditions.