Variation in foliar δ¹³C in Hawaiian Metrosideros polymorpha: a case of internal resistance?

Sun leaves of Metrosideros polymorpha were collected in 51 sites on 9 lava flows that represented gradients of elevation, precipitation, substrate age, and substrate texture on Mauna Loa volcano, Hawai'i. Leaf mass per unit leaf area increased with increasing elevation on all flows, while foliar nitrogen concentration decreased with increasing elevation and increased with increasing substrate age. Foliar δ¹³C became less negative with increasing elevation on the wet east-side lava flows, but not the dry northwest-side flows; it did not reflect patterns of precipitation or presumed water availability. δ¹³C was very strongly correlated with leaf mass per area across all of the sites. Limited gas-exchange information suggested that calculated c_i/c_a did not decrease with elevation in association with less-negative δ¹³C, and photosynthesis per unit of nitrogen was significantly reduced in high-elevation plants. These results are consistent with a substantial internal resistance to CO₂ diffusion in the thick Metrosideros polymorpha leaves in high elevation sites.