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 rate 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 persist. We surveyed for canopy gaps in 44–67 yr old aspen stands in northeastern Alberta, 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 form ∼40 yr after stand initiation, through the accumulated mortality of adjacent canopy trees. The densities of aspen (P. tremuloides), balsam poplar (P. balsamifera), and paper birch (Betula papyrifera) saplings were 2–3 times higher in gaps than in paired control areas under a closed canopy. Sample plots in older aspen stands in the vicinity had spatially heterogeneous, uneven age structures, consistent with gap dynamics. More extensive samples of stem‐size‐structure data and forest‐inventory data sets indicate that this phenomenon is widespread. We conclude that gap dynamics can maintain near‐pure deciduous stands in this region, in the absence of shade‐tolerant competitors. A cellular‐automata model of aspen‐stand dynamics, with spatially random mortality, yields predictions consistent with our other results. It follows from the model that stable age structures develop within 250–300 yr, that mean canopy age is a biased estimator of stand age in stands older than 100 yr, and that small‐sample maxima have unfavorable sampling distributions. Comparable biases may be present in ages estimated from aerial photography: significant areas of “young” aspen have age structures 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 dynamics. 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.