Gap dynamics in boreal aspen stands: Is the forest older than we think?

Trembling aspen (Populus tremuloides) in western Canadian boreal forests is generally believed to occur as young, even-aged stands, as part of a fire- dominated landscape. However, the available quantitative estimates of the r ate of disturbance by fire in this region differ markedly. One estimate is consistent with forests much older than are currently thought to exist. The theory of gap dynamics may partly reconcile the discrepancy, by suggesting a mechanism whereby old, uneven-aged aspen stands could develop and persis t. We surveyed for canopy gaps in 43-67 yr old aspen stands in northeastern Al berta, Canada, and found that expanded gaps occupy 3.6-16.6% of stand area, increasing linearly with stand age over the sampled range. Gaps begin to f orm similar to 40 yr after stand initiation, through the accumulated mortal ity of adjacent canopy trees. The densities of aspen (P. tremuloides), bals am poplar (P. balsamifera), and paper birch (Betula papyrifera a) saplings were 2-3 times higher in gaps than in paired control areas under a closed c anopy. Sample plots in order aspen stands in the vicinity had spatially het erogeneous, uneven age structures, consistent with gap dynamics. More exten sive samples of stem-size-structure data and forest-inventory data sets ind icate that this phenomenon is widespread. We conclude that gap dynamics can maintain near-pure deciduous stands in this region, in the absence of shad e-tolerant competitors. a cellular-automata model of aspen-stand dynamics, with spatially random mortality, yields predictions consistent with our eth er results. It follows from the model that stable age structures develop wi thin 250-300 yr, that mean canopy age is a biased estimator of stand age in stands older than 100 yr, and that small-sample maxims have unfavorable sa mpling distributions. Comparable biases may be present in ages estimated fr om aerial photography: significant areas of "young" aspen have age structur es characteristic of simulated old stands. We present less direct arguments that other components of the Alberta boreal forest are also older than is generally thought, and we outline a new model of the regional forest dynami cs. We conclude that vast tracts of boreal forest are now being managed on the basis of an incorrectly estimated age structure and a misconception of their landscape dynamics.