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Tree population dynamics of some old sub-boreal spruce stands Kneeshaw, Daniel D.


Disturbance at the scale of single trees shapes the development of boreal forest ecosystems, especially in 'overmature' or 'old-growth' stages, despite the acknowledged role of fire as a catastrophic agent of stand reinitiation. This study has reconstructed the population dynamics of fourteen Sub-Boreal Spruce stands (composed of Picea engelmanni x glauca, Abies lasiocarpa and lesser amounts of Pinus contorta and Populus tremuloides) since the last stand destroying wildfire. Identification of the post-disturbance cohort and subsequent recruitment provided a means of assessing the relative role of small-scale (single-tree) disturbance and large-scale, catastrophic disturbance on species composition and stand development. The results suggest that SBS stands are self-perpetuating, and that although Picea may disappear from some stands it is maintained within forests of this zone. Presumably due to its high shade tolerance, Abies recruitment (when present) occurred uniformly throughout the stand's history. Picea ingress was associated with more exacting conditions, and it can return in sufficient numbers to perpetuate itself after long periods of exclusion. Minor disturbances were important in accelerating the reinitiation of Picea below the forest canopy. In many stands Abies was the most abundant species but this study suggests that although Picea regeneration often occurred in lower numbers it is maintained as a dominant overstory species due to poorer Abies recruitment to larger size classes. In many of the stands there was evidence of an earlier Populus component that may have played an important role in conifer establishment after wildfire. The faster growing Populus may ameliorate harsh environmental conditions to the benefit of conifer regeneration. This process may be especially important in the Sub-Boreal Spruce zone where long periods of colonization (on average 75 years but up to 100 years) are common. Another part of this project focused on defining the later stages of forest stand development in terms of tree population biology. Old-growth was recognized as the stage when the rate of tree regeneration and mortality, and thus age structure, are influenced more by single tree processes than by the stand initiating disturbance (Hayward 1991). The tree population structures and old-growth attributes described for SBS stands suggest that this definition was a useful and objective ecological tool for ranking the later stages of stand development. In the SBS structural data was found appropriate for identifying the later stages of stand development through multivariate analysis. Attributes found to be strongly correlated with stand development, such as the number of large downed logs and snags, reflect the importance of single-tree mortalities to the old-growth stage.

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