Browsing by Author "Wang, Jiejie"

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    A nested frailty model for bivariate recurrent events: a Poisson modelling approach
    (University of New Brunswick, 2018) Wang, Jiejie; Renjun Ma; Yan, Guohua
    Survival analysis is used to study the time until the occurrence of an event of interest. Some events of interest can occur more than once in a subject. These events are termed recurrent events. In this thesis, we consider survival analysis of bivariate recurrent events in which each subject may experience two distinct types of recurrent events. For example, in the peritonitis dialysis study conducted in Taichung Veterans General Hospital in Taiwan, both Gram-positive and Non-Gram-positive peritonitis are observed on 575 patients over time. Each of these two types of peritonitis may occur more than once in a patient. Clearly these two types of recurrent events are clustered by subject. In addition, the recurrent events of each type are further clustered by the type of events. To characterize those clustering effects in our analysis, we incorporate two levels of nested frailties into Cox survival models to analyze bivariate recurrent events jointly. There are many different approaches to the estimation of nested frailty Cox survival models in the literature. In this thesis, we propose a Poisson modelling approach to the estimation of our nested frailty Cox Survival models for bivariate recurrent events. This approach enables us to develop an optimal model estimation based on orthodox best linear unbiased predictor of frailties in an auxiliary frailty Poisson model. An important feature of this approach is that the principal results depend only on the first and second moments of the unobserved frailties. Our approach deals with an unspecified baseline hazard function. In addition, the treatment of ties and stratification is explicitly incorporated in our approach in the same way as in the standard Cox model. The usefulness of our proposed method is illustrated through analysis of peritonitis dialysis data and a simulation study.
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    Climate change impacts on forest growth and mortality in eastern North America
    (University of New Brunswick, 2024-02) Wang, Jiejie; D’Orangeville, Loïc; Taylor, Anthony
    Forests in eastern North America play a critical role in regulating global atmospheric carbon fluxes and sustaining one of the world’s largest lumber and newsprint industry. Unfortunately, tree growth and mortality are expected to be affected by climate change with temperature increases and declines in water availability. These shifts may pose a significant threat to future forest health, wood fiber supply, and carbon stock. However, the impacts of climate change on growth and background mortality remain controversial and overlooked, mainly because of limited data availability, methodological biases, and regional variability in forest dynamics. In this thesis, I compiled an unprecedented network of tree growth and mortality records from permanent sample plot network distributed across Canada and the United States, spanning a wide climatic gradient, to disentangle the climatic controls on growth and mortality. My results reveal a large, positive effect of warming on tree growth, leading to projected short-term increases in tree growth in the Canadian boreal forest, along with increases in tree mortality that may potentially cancel out some of the gains in forest productivity under climate change and affect net wood supply and carbon sequestration. Comparing the growth response to climate between overstory and understory temperate and boreal tree species, I find divergent patterns which suggest potential biases in current estimates of forest dynamics under climate change. Finally, I develop a novel, empirical approach to improve the climate-growth equations used in forest simulation models which reveals underestimated climate optimums for many species in eastern Canada. The results of this thesis, although not accounting for future changes in disturbance regimes and other forest properties such as composition or structure, should help inform forest management under climate change.
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    Estimates of genetic parameters for growth traits and of effects of climatic transfer and edaphic conditions for select families New Brunswick black spruce
    (University of New Brunswick, 2017) Wang, Jiejie; Krasowski, Marek; Weng, Yuhui
    The knowledge of genetic parameters estimate for black spruce in New Brunswick (NB) is limited. In this thesis, the estimates of genetic parameters for growth traits (bole volume, height, and diameter at breast height) and for stem forking (SF) in black spruce 35-year old family tests in NB are presented. These include heritability estimates (individual heritability and family mean heritability), type-B genetic correlation, age-age genetic correlation, and trait-trait genetic correlation. The growth of bole volume (V), tree height (HT) and diameter at breast height (DBH) are highly genetically controlled while SF is not. Type-B genetic correlations are moderately high for the growth traits, implying the weak environment × genetic interactions within the geographic region. Age-age genetic correlations are strong for the growth traits but weak for SF. Trait-trait genetic correlations for V and HT, as well as V and DBH are strong and positive, but are weak for V and SF. The potential effects of climatic transfer and of edaphic conditions on height growth of black spruce families are explored. Both the mean annual temperature and accumulated growing degree-days are important for height growth of black spruce in NB. Height growth is better on planting sites cooler than seed-source locations. Higher air temperatures resulting from changing climate will likely reduce black spruce height growth. Transfer difference in precipitation has little critical effect on height growth of black spruce from local NB sources planted in the province. In practice, temperature variable seems the most important climatic variable compared with other variables when deploying black spruce families. Transferring seed sources to relatively cooler sites will optimize height growth of black spruce in the province. Medium-rich soils of moderate moisture are the best for height growth of local black spruce in NB.