Gap Dynamics in Boreal Aspen Stands: Is the Forest Older Than We Think?

Abstract

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.

Presettlement forest in southern Ontario: Ecosystems measured through a cultural prism

“…4) are concentrated along the north shore of Lake Ontario on the Oak Ridges Moraine, on the Frontenac Arch of ancient shield rock along the St. Lawrence, and on the Awenda Peninsula at the south end of Georgian Bay. It should be noted that the FRI estimates the predominant age of dominant canopy species, and the forest may actually be older than this (Cumming et al 2000).…”

Divided land base and overlapping forest tenure in Alberta, Canada: A simulation study exploring costs of forest policy

“…The usual model of succession for the mixedwood describes a relatively pure aspen stand establishing after disturbance, with white spruce gradually developing in the understory, leading to a relatively pure white spruce stand after many decades (see Cumming et al 2000, for a current review). However, reforestation standards for the coniferous land base mandate the rapid establishment of a pure coniferous stand.…”

Are Boreal Ovenbirds, Seiurus aurocapilla, More Prone to Move across Inhospitable Landscapes in Alberta's Boreal Mixedwood Forest than in Southern Québec's Temperate Deciduous Forest?

“…Third, although fire return intervals typical of the boreal mixedwood forest are several times longer than the average lifetime of an individual Ovenbird, it seems reasonable to expect that since the last glaciation, Alberta Ovenbird has evolved in a more patchy landscape containing large burned areas with scattered unburned stands, whereas Ovenbird from the temperate forest of southern Québec has evolved in a landscape characterized by small, scattered, non-forest gaps such as small bogs, rocky outcrops, blowdowns, and treefall gaps. Because stands sometimes escape fire and reach old age even in the boreal mixedwood forest (Cumming et al 2000), we assumed that mature and old stands were available to Ovenbird throughout the time period considered.…”

“…Smyth et al (2005) surveyed uplands similar to those occupied by Ovenbird and reported that, on average, live trees cover 26% of the area subject to wildfires. Cumming et al (2000) have also documented the occurrence of gap-phase dynamics in the boreal mixedwood forest of Alberta, with gaps occupying 4-17% of stands surveyed. Thus, the natural disturbance regime of the boreal mixedwood forest does produce a structurally complex mosaic, but we submit that the coarse-grained structure created by fires in boreal Alberta cannot possibly be matched by the natural disturbance regimes that have characterized the Québec region in the recent or distant past.…”