Seasonally averaged caloric values (kilocalories per gram dry weight) of Populus tremuloides Michx. (trembling aspen) in southern Ontario were 5.17 for leaves, 5.04 for twig bark, and 4.55 for twig wood. The marked difference between leaves and twig wood reflects the differing composition of these tissues. The caloric levels of all the aspen components were highest in May. Levels decreased markedly from May to June, increased to a second peak in August, and then declined again between August and September. The caloric value of twig bark and wood also showed a decline from September to November. Seasonal fluctuations in caloric value of trembling aspen leaves and twigs were large compared with changes reported for other forest tree species and it is suggested that aspen is relatively sensitive to environmental effects.©NRC Canada