Photosynthesis And Freezing Avoidance In Ohia (Metrosideros Polymorpha) At Treeline In Hawaii

Metrosideros polymorpha (Ohia), the dominant tree species in Hawaiian forest ecosystems, grows from sea level to treeline (2500 m). Carboxylation efficiency and area-based leaf N content were substantially higher at treeline than at lower elevations while leaf size and instantaneous photosynthetic nitrogen-use efficiency (PNUE) were substantially lower at treeline. For example, PNUE decreased from 45 mu mol CO2 mol N-1 at low elevation to 17.4 mu mol CO2 mol N-1 at high elevation. In contrast, average net CO2 assimilation and integrated PNUE remained relatively constant along the elevation gradient despite suboptimal temperatures and decreased soil nitrogen availability at treeline. These and other homeostatic mechanisms allow M. polymorpha to maintain a relatively high level of growth-related activities at treeline despite frequent near- and below-freezing temperatures. High-elevation populations avoided freezing by supercooling apparently as a result of small leaves, reduced intercellular spaces, and low apoplastic water content in leaves. Ice nucleation temperatures were about -8.5 degreesC for leaves of treeline populations, 3 degreesC lower than those of low elevation populations. Irreversible tissue damage temperature decreased 7 degreesC from sea level to treeline. However, the decrease appeared to be only large enough to allow M. polymorpha trees to avoid leaf tissue damage due to freezing up to the current location of treeline. All of the above leaf traits in high-elevation populations serve to promote carbon gain in a nutrient and temperature limited environment as well as to avoid freezing by supercooling.