Frost resistance and ice nucleation in leaves of five woody timberline species measured in situ during shoot expansion

Frost resistance and ice nucleation temperatures of leaves, from bud swelling until after full expansion, were measured in situ for five major woody timberline species with recently developed field freezing equipment. Frost resistance determined in situ on leaves of attached twigs was significantly higher than values determined on detached leaves in laboratory tests (e.g., the temperature at which incipient frost damage was observed (LT(i)) was 1.2 degreesC higher for detached leaves than for attached leaves of Picea abies (L.) Karst.). Frost resistance of leaves of all species changed significantly during shoot expansion (e.g., changes of 7.2 and 11 degreesC for Rhododendron ferrugineum L. and Larix decidua Mill., respectively). Expanding leaves (between 0 and 60% of full expansion) were the most sensitive to frost, with LTi values ranging from -3.4 degreesC in R. ferrugineum to -6.3 degreesC in L. decidua. Among the studied species, P. abies and R. ferrugineum were the most frost sensitive throughout the shoot elongation period. In situ freezing patterns of leaves of attached twigs also differed from those of leaves of excised twigs. During leaf expansion, two distinct freezing exotherms were always registered in situ. The first freezing event (E1, high-temperature exotherm) was recorded at -1.5 +/- 0.2 degreesC and reflected extracellular ice formation. Exposure of leaves to temperatures at which E1 occurred was, in all cases, noninjurious. The low-temperature exotherm (E2) mostly coincided with frost damage, except for some stages of leaf expansion in R. ferrugineum and P. abies, indicating that in situ freezing exotherms were not accurate estimators of frost damage in these species.