Physiological Tolerance Mechanisms of Boreal Forest Tree Species to Climatic and Anthropogenic Stress Factors
Creators: Fleurial, Killian G.
Publisher: name: University of Alberta Library
Year: 2024

<p>Boreal plant and tree stress responses under climate change and anthropogenic disturbances have become an important research area due to a strong climate warming signal and large-scale anthropogenic pollution. This thesis investigates tree physiological response mechanisms to a variety of stressors that are particularly relevant to the reforestation and reclamation of boreal forests disturbed by oil sands development.<br>
In my first research chapter I use an experimental approach to observe tree growth and physiological traits under hypoxic or phytotoxic environments using 6 common boreal species that are known to have different sensitivities to hypoxia. The expectation tested is that species that are more sensitive to hypoxia will be compromised disproportionately by additional phytotoxic compounds. However, although chapter results show that root hypoxic conditions further reduced the survival, growth, and physiology of all plants exposed to oil sands tailings water, it was the more salt tolerant species that were disporportionaely affected. Specifically, hypoxia disrupted a salt sequestration mechanism in trembling aspen (Populus tremuloides) and may have inhibited an osmoregulative mechanism in black spruce (Picea mariana) needles. My results suggest that the preparation of reclamation sites impacted by tailings water should involve efforts to enhance soil aeration to minimize detrimental effects on plants.<br>
Then, to detect physiological responses to specific phytotoxic compounds, in my second research chapter I expose trembling aspen seedlings to a variety of “reconstituted tailings waters” by manipulating the tailings water composition and removing individual potentially phytotoxic compounds. For this experiment trembling aspen (Populus tremuloides) was used as it is widely employed in the reclamation of oil sands mining sites because of its early successional life history, relative tolerances to salinity, and high growth rates. Here, the results corroborated an inference from the first research chapter; that trembling aspen Na tolerance mechanisms at the root level involves the maintenance of membrane selectivity of K over Na and that it is a prerequisite for survival and growth in saline conditions. Furthermore, a pH dependent effect of naphtenic acids on plant roots was inadvertently revealed with results that suggest that lowering the pH of tailings might not be more favorable for plant growth if significant amounts of naphthenic acids are present. Thus, if tailings water is to be adjusted to a more neutral pH, extracting naphthenic acids or minimizing their contact with roots would be important for improving revegetation success.<br>
Low non-negative root temperatures often elicit similar effects on plants as salinity, or drought, yet the subject remains understudied and underexposed. Thus, in my third research chapter I comment on recent findings on the topic and look to examine some of the ways trees may have adapted to avoid or tolerate lower root temperatures effects while maintaining growth and productivity. While lower soil temperatures restrict root hydraulic conductivity, and thus water uptake, often triggering stomatal closure, an increase in transmembrane aquaporin water transporters could partly compensate. Furthermore, though hydraulically risky, it may be that reduced stomatal control and a “willingness” to accept lower xylem water potentials is a strategy used to maintain productivity.<br>
Finally, in my fourth research chapter I further explore the importance of a tradeoff between hydraulic risk and productivity by experimentally investigating needle anatomy, foliar water uptake, and aquaporin expression under variable simulated drought conditions. By using branch samples collected from a white spuce (Picea glauca) provenance trial representing seed origins from throughout the range of the species the results can subsequently be related to the climate of provenance origin. Provenances with source climates characterized by cold and dry climatic conditions from the Yukon and northern Alberta had thinner Casparian strips and hypodermis layers and lost more water during needle dehydration. Foliar water uptake uptake was highest in northern and central Alberta (Boreal Plains), These anisohydric characteristics appear to imply that northwestern white spruce populations are anatomically maladapted to drought. However, this lack of radial foliar hydraulic resistance, along with regulated aquaporin expression, can also allow drought limited western provenances to take advantage of foliar wetting even when precipitation does not meaningfully wet the soil. In these populations foliar water uptake may be a coping strategy that has an overall net benefit to the plant’s water balance.</p>