Supercooling capacity increases from sea level to tree line in the Hawaiian tree species Metrosideros polymorpha

Population-specific differences in the freezing resistance of Metrosideros polymorpha leaves were studied along an elevational gradient from sea level to tree line (located at ca. 2500 m above sea level) on the east flank of the Mauna Loa volcano in Hawaii. In addition, we also studied 8-yr-old sapl ings grown in a common garden from seeds collected from the same held popul ations. Leaves of low-elevation field plants exhibited damage at -2 degrees C, before the onset of ice formation, which occurred at -5.7 degrees C. Le aves of high-elevation plants exhibited damage at ca. -8.5 degrees C, concu rrent with ice formation in the leaf tissue, which is typical of plants tha t avoid freezing in their natural environment by supercooling. Nuclear magn etic resonance studies revealed that water molecules of both extra- and int racellular leaf water fractions from high-elevation plants had restricted m obility, which is consistent with their low water content and their high le vels of osmotically active solutes. Decreased mobility of water molecules m ay delay ice nucleation and/or ice growth and may therefore enhance the abi lity of plant tissues to supercool. Leaf traits that correlated with specif ic differences in supercooling capacity were in part genetically determined and in part environmentally induced. Evidence indicated that lower apoplas tic water content and smaller intercellular spaces were associated with the larger supercooling capacity of the plant's foliage at tree line. The irre versible tissue-damage temperature decreased by ca. 7 degrees C from sea le vel to tree line in leaves of field populations. However, this decrease app ears to be only large enough to allow M. polymorpha trees to avoid leaf tis sue damage from freezing up to a level of ca. 2500 m elevation, which is al so the current tree line location on the east flank of Mauna Loa. The limit ed freezing resistance of M. polymorpha leaves may be partially responsible for the occurrence of tree line at a relatively low elevation in Hawaii co mpared with continental tree lines, which can be up to 1500 m higher. If th e elevation of tree line is influenced by the inability of M. polymorpha le aves to supercool to lower subzero temperatures, then it will be the first example that freezing damage resulting from limited supercooling capacity c an be a factor in tree line formation.