Patterns and mechanisms of understorey vegetation associated with stand development in boreal forests

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.