Bottom-up controls of flammability in the Canadian boreal forest at multiple temporal and spatial scales
Creators: Campos Ruiz, Rodrigo T.
Publisher: name: University of Alberta Library
Year: 2023

<p>Wildfires are the most common disturbances occurring in the Canadian boreal forests. Their incidence and characteristics are controlled by different factors at different temporal and spatial scales. At every scale, vegetation is an essential factor fueling fires. Despite its importance, some aspects of vegetation flammability in a biological context remain poorly understood. In this work, I explore the role of vegetation as a bottom-up control of fire and its interaction with climate at different spatial and temporal scales, in the Canadian Boreal Forest. <br>
First, I examined the flammability of living jack pine (Pinus banksiana Lamb.) needles as characterized by their physicochemical features (moisture, chemical, and form) as a function of leaf age and time of the growing season. I measured their curvature, form coefficient, surface-area-to-volume ratio and calculated their content of carbon, nitrogen, starch, sugars, lipids and terpenes. I employed a modified cone calorimeter and open flame to measure their ignitability, combustibility, sustainability, and consumability of the pine needles. The results indicated that needles under a year old are the least flammable; they were rounder, more voluminous, with higher moisture and nitrogen content. One- and two-year-old needles are more ignitable, combustible, and consumable; they are drier, more curved, thinner, longer, and contain more carbon and terpenes than younger needles. Regardless of age, needles release more energy during the first half of the growing season (June-July) when starch and lipids are at their highest concentrations. Overall, the results suggest that needle moisture content by itself had a minor influence on their flammability compared to their form and chemical content.<br>
Second, I built a model to estimate the development and potential regeneration failure of a conifer forest under scenarios of vegetation resistance to reburn and climate change. Young forests are less likely to reburn, mainly due to fuel limitations, and they have the potential to reduce fire activity. On the other hand, climate change enhances fire activity through increasing temperatures, droughts, and the frequency of extreme fire events. To evaluate the effect of resistance and climate change on age structure and forest loss, I built a state-and-transition model parametrized with a Markov Chain Model. The model structure was based on 157 years of fire history in a conifer-dominated forest in northern Quebec. I created scenarios from the combination of different levels of resistance to burning and climatic change, plus two behaviors of interannual variation (irregular vs smooth). After forecasting for 50 years, I found that: 1) the youngest forests increase in frequency, while older ones disappear for short periods. 2) resistance to reburn varies latitudinally affecting the degree of forest loss, 3) forest loss without reburning resistance would be two times higher, 4) reburning resistance may counteract the effects of the projected climate change reducing forest loss from 1.5 to two times, but 5) occasional events of extreme fire conducting conditions may overwhelm this resistance. This study emphasizes the significance of incorporating the negative feedback from vegetation on fire occurrence to enhance the accuracy of estimating the future of the boreal forest.<br>
Finally, I assessed the impact of human influence on the fire regime in two areas of the boreal forest in north Alberta. Humans do not only alter the number of fires and area burned directly by setting fires, but they also suppress them. Indirectly, humans intentionally modify the landscape as a preventive measure, or unintentionally through other activities (e.g., agriculture, urban development, forest harvesting). To determine the type and extent of these changes on the fire regime, I compared trends and cyclical patterns of fire activity of two areas with contrasting human influence from 1970 to 2015 (Wood Buffalo National Park and Lower Athabasca Plains) and examined their climate-fire associations. Human influence affects fire activity by 1) limiting area burned and reversing the increase in the number of fires caused by climate change, 2) shifting the peak of fire activity from the summer to the spring, 3) modifying the association between fire and climate, and 4) shortening the cyclical patterns of fires. In conclusion, humans have altered the temporal and spatial patterns of fire activity creating a novel fire regime in areas under their influence.</p>