Modeling the Impacts of Global Change on Tree Growth and Stand Density of Boreal Forests in Canada
Creators: Xu, Kun
Publisher: name: University of Alberta Library
Year: 2022

<p>Abstract<br>
Global change of environment and human activity has profoundly impacted boreal forests in structure, dynamics and function, imposing serious challenges for maintaining forest growth and yield in Canada. In this thesis, I address three questions important for understanding and predicting dynamics of boreal forests in the face of global change: how to estimate tree biomass under global warming, how to maintain long-term forest plots under intensified fire disturbances, and what is the spatial distribution of tree density in boreal forests. I answered the three questions based on unprecedented datasets compiled from over 30,000 plots established since 1949. My effort comprises three main chapters of my thesis.<br>
In Chapter 2, I assessed the effects of climate on tree allometric biomass equations and proposed to incorporate climatic factors into allometric models. I focused on five major timber species of Canada and built climate-based allometric models by explicitly testing the effect of each climatic factor, e.g., temperature, precipitation, etc. I found that the allometries of three species, i.e., white spruce (Picea glauca), black spruce (Picea mariana) and balsam fir (Abies balsamea), were not sensitive to climate, but the allometric models for trembling aspen (Populus tremuloides) and tamarack (Larix laricina) performed significantly better after incorporating frost-free period and mean annual temperature respectively into their conventional, climate-independent allometric models. Under the modest warming scenario, if the conventional models for trembling aspen and tamarack were still in use in 2030, the aboveground biomass of these two species would be underestimated by 10% in Canada. This chapter suggests the necessity to proactively develop climate-based allometric equations for more accurate and reliable tree biomass estimation.<br>
In Chapter 3, I addressed the challenge of maintaining long-term forest plots in facing intensified fire disturbances in northern forests. Based on 60-year fire burning data of 919 permanent sample plots (PSPs) in Alberta, Canada, I built Cox proportional hazards models to quantify the effects of stand conditions and climate on plot fire hazards. The results showed that 17% of the plots were burned with an average 28.7-year lifespan. I found that plots established more recently suffered higher fire hazards, and plots in the Boreal ecoregion suffered 2.85 and 3.36 times higher risks than those in the Foothills and Rocky Mountain ecoregions, respectively. Higher tree species richness and density of deciduous trees were found associated with lower plot fire hazards, while warming increased fire hazards. Based on the estimated Cox proportional hazards model, I projected plot fire hazards in 2050 to be 1.63 times higher than the current level due to warming. This chapter emphasizes the need to consider intensified natural disturbances, including fire, for the maintenance of long-term forest plots.<br>
In Chapter 4, I attempted to model tree density variation in North American boreal forest by incorporating stand height into an existing biome model. By validating this biome model for density estimation, I identified that it underestimated tree density of 4,367 plots by 32.3%. The tree density model that I developed outperformed the previous biome model as judged by all measures of goodness-of-fit, with only 0.6% underestimation. Based on my model, I estimated there were 351.3 billion trees in the boreal forest of North America, compared to 211.2 billion estimated from the previous model. The underestimation by the previous model was equivalent to a missing of 14.0 trillion kg biomass. I also produced a 1-km resolution boreal tree density map of North America, and projected tree density distribution in 2050. This chapter updates understanding of the role of boreal forests in regulating forest ecosystem functions. It also addresses the urgent need to improve boreal forest models to inform adaptation and mitigation planning.<br>
By modeling biomass allometry for major timber species, fire hazards of long-term forest plots, and tree density distribution across boreal forests, my thesis contributes to data, models and understanding for sustainable management of forests and the impacts of global change on forest ecosystems in Canada.</p>