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

Conservation targets for viable species assemblages in Canada: are percentage targets appropriate?

Percentage targets for conservation have become a popular tool (advocated in both the scientific literature and the conservation community) for setting minimum goals for the amount of land to be set aside as protected areas. However, there is little literature to support a consistent percentage target that might be widely applied. Moreover, most percentage targets have not taken into account issues of species persistence. A recent study of herbivores in Kruger National Park took into account issues of representation and persistence in setting conservation targets and found that results were consistently about 50% and were unaffected by different permutations of the reserve selection process. Here, we carry out a similar analysis for representation of mammals within sites that are predicted to allow for their persistence, across eight ecologically defined regions in Canada to test whether we see similar consistent patterns emerging. We found that percentage targets varied with the different permutations of the reserve selection algorithms, both within and between the study regions. Thus, we conclude that the use of percentage targets is not an appropriate conservation strategy

Does avian species richness in natural patch mosaics follow the forest fragmentation paradigm?

As one approaches the north-eastern limit of pinyon (Pinus spp.) juniper (Juniperus spp.) vegetation on the Colorado Plateau, USA, woodland patches become increasingly disjunct, grading into sagebrush (Artemisia spp.)-dominated landscapes. Patterns of avian species richness in naturally heterogeneous forests may or may not respond to patch discontinuity in the same manner as bird assemblages in fragmented agricultural systems. We used observational data from naturally patchy woodlands and predictions derived from studies of human-modified agricultural forests to estimate the effects of patch area, shape, isolation and distance to contiguous woodland on avian species richness. We predicted that patterns of species richness in naturally patchy juniper woodlands would differ from those observed in fragmented agricultural systems. Our objectives were to (1) estimate the effect of naturally occurring patch structure on avian species richness with respect to habitat affinity and migratory strategy and (2) assess the concordance of the effects to predictions from agricultural forest systems. We used the analogy between populations and communities to estimate species richness, where species are treated as individuals in the application of traditional capture-recapture theory. Information-theoretic model selection showed that overall species richness was explained primarily by the species area relationship. There was some support for a model with greater complexity than the equilibrium theory of island biogeography where the isolation of large patches resulted in greater species richness. Species richness of woodland-dwelling birds was best explained by the equilibrium hypothesis with partial landscape complementation by open-country species in isolated patches. Species richness within specific migratory strategies showed concomitant increases and no shifts in species composition along the patch area gradient. Our results indicate that many patterns of species richness considered to be fragmentation effects may be general consequences of patch discontinuity and are ubiquitous in naturally heterogeneous systems. There was no evidence for the effects of patch shape and distance to contiguous woodland in juniper woodland, suggesting edge effects and dependence upon regional species pools are characteristics of fragmented agricultural systems. Natural patch mosaics may provide benchmarks for evaluating fragmentation effects and managing forests by mimicking natural landscape patterns