Allocation of nitrogen and carbon in leaves of Metrosideros polymorpha regulates carboxylation capacity and δ13C along an altitudinal gradient

1. Metrosideros polymorpha (O'hia), the dominant tree species in Hawaiian forest ecosystems, grows from sea level to treeline (2500 m). Consistent changes in its morphology and anatomy occur along this altitudinal/temperature gradient. Patterns of variation in photosynthetic gas exchange, leaf nitrogen content, nitrogen-use efficiency, delta(13)C, and morphological and anatomical characteristics were determined across the elevational gradient. In addition, on-line carbon isotope discrimination studies of high and low elevation 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 unit area (LMA), less negative foliar delta(13)C and (2) progressively smaller 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 parts per thousand at low elevation to - 24 parts per thousand at high elevation) and was strongly correlated with foliar N and LMA. Foliar delta(13)C was also correlated with variations in the ratio of intercellular to ambient 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 relatively large internal resistance to CO2 diffusion did not differ between high and low elevation populations, despite differences in LMA. Less negative values of delta(13)C at high elevations and corresponding lower values of p(i)/p(a) were associated with increased carboxylation efficiency and N content on a unit leaf area basis. 5. Two major homeostatic responses in M. polymorpha plants along elevational/temperature gradients were observed: (1) maintenance of similar photosynthetic rates per unit leaf surface area despite suboptimal conditions for CO2 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.