Photosynthesis and freezing avoidance in Ohia (Metrosideros polymorpha) attreeline in Hawaii

Metrosideros polymorpha (Ohia), the dominant tree species in Hawaiian fores t ecosystems, grows from sea level to treeline (2500 m). Carboxylation effi ciency and area-based leaf N content were substantially higher at treeline than at lower elevations while leaf size and instantaneous photosynthetic n itrogen-use efficiency (PNUE) were substantially lower at treeline. For exa mple, 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 assimilati on and integrated PNUE remained relatively constant along the elevation gra dient despite suboptimal temperatures and decreased soil nitrogen availabil ity at treeline. These and other homeostatic mechanisms allow M. polymorpha to maintain a relatively high level of growth-related activities at treeli ne despite frequent near- and below-freezing temperatures. High-elevation p opulations 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 populati ons. Irreversible tissue damage temperature decreased 7 degreesC from sea l evel 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 t o the current location of treeline. All of the above leaf traits in high-el evation populations serve to promote carbon gain in a nutrient and temperat ure limited environment as well as to avoid freezing by supercooling.