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.

14 old, unlogged, Picea-dominated stands in the moist cool Sub-Boreal Spruce biogeoclimatic subzone of central British Columbia, Canada, were sampled to describe canopy heterogeneity, regeneration patterns and tree population age structures. These stands are composed of Picea engelmannii × glauca hybrids, Abies lasiocarpa and lesser amounts of Pinus contorta and Populus tremuloides, and had survived 124 -343 yr since the last stand-destroying wildfire. Canopy cover was patchy and highly variable (ranging from 30.5 % to 86.4 %) but was not significantly related to stand age. Vertical canopy structure was less variable, reflecting the shade-tolerance and live crown ratios (length of live canopy expressed relative to tree height) of component species: 18.8 % for Populus, 20.2 % for Pinus, 46.7 % for Picea and 51.4 % for Abies. Individual stands varied considerably in their population structures and in their stand development trajectories, yet some patterns are evident. Survivors of the initial post-disturbance cohort of trees took 51 to 118 yr (mean = 80, s.d. = 20) to establish. Some stands had all tree species present during stand initiation, while other stands indicated early successional roles for Populus and Pinus, or a late successional role for Abies. Abies recruitment, while often slow in the beginning, occurs uniformly throughout the history of most stands, reflecting the high shade-tolerance of this species. Picea is often recruited in high densities early in stand development, and then (after long periods of exclusion) may be displaced by Abies in some stands but maintains itself in others. Minor, single-tree disturbances (due to bark beetles, root rot, and windthrow) were important in accelerating the reinitiation of Picea in the understory. Results thus suggest that stands from this region can be self-perpetuating in the absence of fire. Yet, post-fire tree populations still clearly dominate these spruce-fir forests, for only the oldest stand had greater basal area in the replacement cohort than in the initial cohort.

An evaluation of how coarse woody debris (CWD) changes in quantity and quality during stand development was conducted using a 426-year chronosequence of 71 stands in sub-boreal forests in British Columbia. Additional characteristics of CWD were determined in 14 of the stands. Most stands are fire initiated and input from the predisturbance stand is critical in controlling the amounts and characteristics of CWD within young stands. Log volume declines from over 100 m3/ha in young stands (0-50 years) to just over 60 m3/ha in stands from 51 to 200 years old, and then increases to greater than 140 m3/ha in the oldest (>= 400-year-old) stands. Mean snag basal area is highest (31.6 m2/ha) in young, postfire stands, decreases to a very low value (2.0 m2/ha) in stands 51-100 years old, and then reaches a second maximum (12.1 m2/ha) in stands that are 201-250 years old; it declines slightly in very old stands. The high snag basal area in stands 201-250 years old coincides with the successional transition from lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) to stands dominated by subalpine fir (Abies lasiocarpa (Hook.) Nutt.) and interior spruce (hybrids of Picea glauca (Moench) Voss and Picea engelmannii Parry). Stand age, characteristics of the predisturbance forest, and the disturbance history of stands subsequent to stand initiation all appear to be very important in determining variation in both the quality and quantity of CWD in these sub-boreal forests.

1996. Boreal mixedwood forests may have no "representative" areas: some implications for reserve design. -Ecogrdphy 1 9 162-180.We tested the shifting mosaic steady state theory in a 4 7 3 600 km' landscape within the boreal mixedwood region of Alberta. Canada. The theory predicts that, at some spatial scale. mean vegetation attributes are temponlly stable. It follows that contiguous subregions (stable mosaics) exist whose attributes are similar to regional values. The scale at which this stability occurs may be interpreted as the minimal size for an ecological reserve designed to represent the region. We argue that the appropriate attributes to test the theory in this case are the age and size structures of v.irious forest stand populations. Using forest inventory data. we searched for stable mosaics ("representative areas") at multiple spatial w i l e s . using two direrent spatial decompositions of the study area. Arcas were compred using 2dimensional similarity metrics b i d on goodness-of-fit statistics. We found that the age and size structures of the entire study area could not be replicated at any smaller scale. which disconfirms the theory for the boreal mixedwood. Each of five compositionally distinct populations of forest stands have characteristic age and size structures, and patterns of spatial variability, which we relate to aspects of the regional ecology. Stand age and sin structures have independent spatial variability. We observed multi-scaled heterogeneity in forest composition, which appears to arise partly from infrequent fire episodes when many large fim occur. These episodes are probably not uniform in their erect on forest structure. We evaluate two methods for constructing reserve systems in nonsquilibrium landscapes. using small collections of disconnected subunits. Small areas can readily be built that more closely match landscape properties than any contiguous area of reisonable size. The degree of representativeness and total size of the constructed areas is sensitive to the size of the constituent pieces: in the mixedwood, there is a natural wale of o I 500 km2 which is near-optimal. Reserves constructed in this wdy are unlikely to be temporally stable. We therefore propose a dynamic strategy for miintaining merve systems over time within a managed landscape. Under this "floating reserve" strategy. portions of the system would be periodically replaced in respony to the a@ng of components, unexpected large wale disturbance, or refinements in conservation objectives.