Phenology was monitored in natural stands of subalpine larch (Larix lyallii Parl.) in parts of four decades. Bud-burst date varied by about six weeks from year to year, and leaf-fall date by a month, suggesting that the importance of photoperiod in these processes has been over emphasized, even in the case of leaf-fall. Most of the variation in bud-burst date is accounted for by spring temperature, while date of leaf-fall depends on both spring and summer temperatures. This rather curious observation might be explained in terms of heat-mediated leaf aging. Early bud-burst, due to warm temperatures in spring, results in early leaf-fall, and warm summers also tend to terminate the life of the leaves sooner, from which it might be inferred that longevity is governed by a heat-sum above a certain threshold temperature. In controlled environments it was shown that the threshold temperature for bud-burst is very low, less than 1.5OC, and the heat-sum requirement is less than 75 degree-days above this threshold. This is the cause of the extremely early flushing date of subalpine larch relative to that of it's common associates.