| dc.description.abstract | The understorey vegetation comprises the greatest plant diversity and contributes
substantially to ecosystem functioning and services in boreal forests. Although many studies
have examined patterns of understorey species diversity in relation to stand development
following stand replacing disturbances and overstorey characteristics, the mechanisms driving
these patterns remain largely speculative. Furthermore, despite their ecological importance, the
dynamics of understorey biomass, production and turnover rates following stand-replacing
disturbance and overstorey succession remain poorly understood. The objective of this
dissertation is to improve the understanding of patterns and mechanisms of understorey
vegetation, and their ecological functions with stand development in central boreal forests of
Canada. To achieve this goal, I first studied the effects of coarse woody debris (CWD) decay
class and substrate species on the patterns of epixylic vegetation abundance, diversity and
composition in the boreal forest. Second, I examined the mechanisms underlying patterns of
understorey vegetation by linking resource availability and heterogeneity to understorey species
diversity. Finally, I investigated the dynamics of understorey biomass, production and turnover
rates in the central boreal forests of Canada.
In chapter 2 and 3, the pattern of epixylic vegetation abundance, diversity and
composition on coarse woody debris decay class and substrate species were examined in stands
of varying ages and overstorey compositions types. The percent cover, species richness and
evenness of epixylic vegetation differed significantly with both CWD decay class and substrate
species. Multivariate analysis showed that understorey species composition differed significantly
with decay classes and substrate species and their interactions. My findings suggest that
conservation of epixylic diversity would require forest managers to maintain a diverse range of
CWD decay classes and substrate species. Since stand development and overstorey compositions
influence CWD decay classes and substrate species as well as colonization time and
environmental conditions, our results further suggested that managed boreal landscapes should
consist of a mosaic of different successional stages and a broad suite of overstorey types to
support diverse understorey plant communities.
In chapter 4, the mechanisms for understorey species diversity and cover were studied
using structural equation modeling (SEM) to link time since fire (stand age), light availability
and heterogeneity, substrate heterogeneity and soil nitrogen to understorey vegetation cover and species diversity in boreal mixedwood stands. The best model for total understorey cover showed
a positive direct effect of stand age, and an indirect effect via mean light level and shrub cover,
with a positive total effect; percent broadleaf canopy had a direct negative effect and an indirect
effect via shrub cover. The model for total understorey species richness showed an indirect effect
of stand age via mean light, light heterogeneity, and substrate heterogeneity, with a positive total
effect; percent broadleaf canopy had an indirect effect via light heterogeneity, and substrate
heterogeneity. The models for vascular plants followed similar trends to those for total
understorey cover and species richness; however, there was an opposite indirect effect of light
heterogeneity for both cover and species richness of non-vascular plants. The overall results
highlight the importance of time since colonization, light availability and heterogeneity, substrate
specialization and growth dynamics in determining successional patterns of boreal forest
understorey vegetation.
In chapter 5, the dynamics of understorey biomass, production and turnover rates
following stand-replacing disturbance and throughout forest succession were examined. I found
that herbaceous biomass and production peaked in early stages of stand development, whereas
total, woody and bryophytes biomass and production peaked at intermediate stages of
succession. Herbaceous and woody turnover rates were higher is early stages, and bryophytes
turnover rates were higher at intermediate stages. Understorey total, woody and herbaceous
biomass, production and turnover rates were higher under deciduous broadleaf overstorey, and
those of bryophytes were higher under conifer stands. However, mixedwood stands favoured the
growth of both woody and non-woody plants, and were intermediate between broadleaf and
conifer stands in supporting understorey biomass and production. This study highlights the role
of overstorey succession in long-term forest understorey biomass, production and turnover
dynamics and its importance for modeling total forest ecosystem contribution to the global
carbon cycle.
In summary, this study demonstrated that multiple processes determine changes in
understorey vegetation with stand development in boreal forests and highlight that understorey
vegetation species diversity, and its biomass, production and turnover dynamics are driven by
time since colonization following stand replacing fire, coupled with associated changes in
resource availability and heterogeneity mediated via overstorey succession. This study highlight
that the shifts in forest age structure and composition have strong impact on the dynamics of understorey vegetation and its ecological functions. Therefore management interventions should
aim at maintaining diverse range of stand ages and overstorey types for conserving biodiversity
and their ecological functions in the boreal forest of Canada. | en_US |
