Seasonal variation of branch respiration of a treeline forming (Betula ermanii Cham.) and a montane (Fagus crenata Blume) deciduous broad-leaved tree species on Mt. Fuji, Japan

In this study we focus on four particular aspects in the field of ecological studies of woody tissue respiration: 1) the application of the non-destructive soda lime technique to the in situ measurement of CO2 efflux rates from branches or stems of standing trees all year round; 2) the investigation of respiration rates of branches of similar diameter but different positions in the canopy; 3) the comparative study of seasonal variation and long-term thermal acclimation of branch respiration along an altitudinal transect up to the treeline; 4) the comparison of branch respiration between a treeline forming (Betula ermanii) and a montane (Fagus crenata) deciduous broad-leaved tree species in order to evaluate the relevance of woody tissue respiration to tree survival in low temperature climates. No qualitative differences in respiratory behaviour between the treeline forming B. ermanii and the montane F crenata were found in this study. Significantly higher respiration rates of upper crown branches of both tree species were due to biogenic factors which were spatio-temporally unevenly effective within the crown. In early spring. branch respiration rose independently of xylem temperature. This was caused by an increase in metabolic activity of branch parenchymatous tissues associated with the phenological development of the buds. During summer, the CO2 efflux of upper crown branches reached up to sixfold higher rates than that of lower crown branches. This was associated with secondary thickening being more pronounced in the upper crown. During winter, higher rates of maintenance respiration in the upper crown reflected different physiological states of branches, probably with respect to protein metabolism involved in processes of cold resistance of live branch tissues. As shown in this study. thermal acclimation of branch respiration is occuring at a similar level as was reported for leaves of herbaceous alpine plants. The tendency for a lower activation energy for respiration towards low temperature climatic conditions points to a physiologically driven increase in respiratory capacity in branches of B. ermanii and F crenata. In treeline forming tree species a decrease in activation energy may contribute to keep respiratory activity relatively unaffected by large daily temperature fluctuations of more than 25 degreesC during the cold season. In this study, no evidence was found that branch respiration may act as an ecophysiologically limiting parameter for the survival of Betula ermanii at its present upper distribution limit in the Japanese Alps.