Moderating night radiative cooling reduces frost damage to Metrosideros polymorpha seedlings used for forest restoration in Hawaii

Winter frosts caused by radiative cooling were hypothesized to limit succes sful reintroduction of Hawaiian plants other than Acacia koa to alien-domin ated grasslands above 1700 m elevation. We determined, in the laboratory, t he temperature at which irreversible tissue damage occurred to Metrosideros polymorpha leaves. We also conducted a field study of this species to dete rmine if (1) leaf damage was correlated with sub-zero leaf temperatures, (2 ) radiative cooling could be moderated by canopies of A. koa, and (3) low s oil temperatures contributed to seedling damage. The last was evaluated by thermally buffering seedlings with water-filled bladders placed at their ba se to keep roots warm, or by installing a radiation shield to reduce early morning transpiration when water uptake from cold soils would be least. Lea f temperatures were monitored between midnight and 7:00 a.m. using fine-wir e thermocouples, and leaf damage was recorded monthly. In the laboratory, s upercooling protected leaves from mild sub-zero temperatures; irreversible tissue damage occurred at about -8 degrees C. In the field, leaf damage was strongly correlated with degree-hours below freezing. Unprotected seedling s suffered the greatest leaf damage. Those sheltered under A. koa trees rar ely experienced temperatures below -3 degrees C, and damage was minimal. Sh aded and thermally buffered seedlings suffered less damage than unprotected plants, probably due to elevated leaf temperatures rather than improved wa ter relations. Using A. koa or artificial devices to reduce radiative cooli ng during winter nights should enhance establishment of M. polymorpha in hi gh-elevation rangeland.