| dc.description.abstract | The scientific understanding of boreal forest stand development and vegetation
dynamics has been well documented with respect to stand-initiating disturbances;
however, the role of subtler disturbance events has received less attention. Two scenarios
that warrant the study of small-scale disturbance in the boreal forest of Ontario include i)
forest management practices and changing climatic factors that range in spatial scale
from local to global are predicted to skew age class distribution of forests towards
mature, oldgrowth stages of development, and ii) during these stages of forest
development, small-scale disturbance is hypothesized to be the major disturbance
mechanism to influence vegetation dynamics. Many principles and methods of the
research that has been conducted on vegetation dynamics and small-scale disturbance in
deciduous temperate and tropical forests can be applied to the boreal forest as a study
system. This thesis attempts to understand vegetation dynamics during intermediate
stages of boreal forest development, with an emphasis on the role of small-scale
disturbance.
The thesis is presented in four chapters that, i) develop the rationale for studying
small-scale disturbances in boreal forests, ii) characterize the extent of small-scale
disturbance in trembling aspen forests, and ii) quantify the effect of small-scale
disturbance on understorey plant communities. Analyses varied considerably among
chapters. Multivariate ordination techniques and average basal area change
measurements were used to assess stand and species dynamics of 217 forests stands
sampled across northeastern Ontario. Relationships between small-scale disturbance with
time since stand initiation (60 - 120 years) of ten trembling aspen were assessed using linear models. Juvenile tree recruitment patterns and community changes (woody and
herbaceous plant species) in gaps and understorey environments of five trembling aspen
stands were assessed using general linear models and various multivariate methods.
Consistent patterns in boreal forest stand development include early dominance of
shade intolerant species such as Populus tremuloides and Pinus banksiana, followed by
increasing basal area of shade tolerant conifer species such as balsam fir, black spruce,
and white cedar. Important declines, between 80 and 100 years since stand initiation, in
the basal area of species that dominate the initial post-disturbance cohort point towards
the occurrence of small-scale disturbance (i.e. mortality of mature canopy trees). The
occurrence of such disturbance warrants the investigation of i) how important small-scale
disturbance actually is, and ii) the effect that it has on the vegetation dynamics of
understorey plant communities.
Results from chapter two clearly show that small-scale disturbances are an
important and conspicuous element of forest stand development when trembling aspen
are the dominant tree species. The most important factors that lead to tree mortality
include infection by wood decay pathogens and wind. The increasing number of gap
makers per gap, and number of gap maker decay classes per gap with time suggests that
gap expansion, resulting from punctuated disturbance events is an important mechanism
of gap formation. Snapped gap makers, followed by standing dead and uprooted gap
makers were the most important structural attributes of gaps, and their relative
importance remained similar with time.
Despite the importance of small-scale disturbance during 95-120 years since stand
initiation, juvenile tree recruitment patterns and plant community response to gap environments of different sizes and ages was negligible. There was no detectable
difference in stem density and growth of all important juvenile tree species (including
Abies balsamea, P. tremuloides, Picea mariana, Picea glauca, and Betula papyrifera)
when compared among gap environments and the closed canopy understorey condition.
Similarly, understorey plant communities showed no difference when compared among
different gap types. High across-stand variability in community composition limited the
ability to detect consistent compositional trends with respect to gap size and age.
However, regardless of gap type, slight increases in species richness and abundance were
detected for understorey woody species of heights between 2 and 5 m.
The effect of small-scale canopy tree mortality on understorey plant communities
in trembling aspen dominated forests is minimal. This is in contrast to predictions that
were made based on expectations from the role of small-scale disturbance dynamics in
temperate and tropical forests. Based on the results of this thesis, future studies of boreal
forest development and understorey vegetation dynamics should consider i) the
establishment mechanisms of shade tolerant conifer species such as A. balsamea during
early stages of forest development, ii) minimal gap sizes that facilitate growth of shade
intolerant tree species that are taller than two metres, and iii) appropriately replicate
different understorey community types. | |
