Thesis Defence: Unexpected water uptake under drought conditions and thinning treatments in young and overstocked lodgepole pine (Pinus contorta) forests

Emory Ellis, supervised by Dr. Adam Wei, will defend their thesis titled “Unexpected water uptake under drought conditions and thinning treatments in young and overstocked lodgepole pine (Pinus contorta) forests” in partial fulfillment of the requirements for the degree of Master of Science in Earth and Environmental Sciences.

An abstract for Emory’s thesis is included below.

Please email adam.wei@ubc.ca to obtain a zoom link for this defence.

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ABSTRACT

As drought and prolonged water stress become more prevalent in dry regions under climate change, understanding and preserving water resources have become the focal point of many conversations. Forest regeneration after deforestation or disturbance can lead to over-populated juvenile stands with high water demands and low water use efficiency. Forest thinning improves tree health, carbon storage, and water use while decreasing stand demands in arid and semi-arid regions. However, little is known about the impacts of dense regenerating stands on seasonal variation in depth-to-water uptake nor the magnitude of the effect of growing season drought conditions on water availability, and existing reports are highly variable by climatic region, species, and thinning intensity. In this study, from a site in the Okanagan Valley in the interior of British Columbia, Canada, stable isotope ratios of hydrogen (δ2H) and oxygen (δ18O) were determined for water collected from the soil profile at varying depths and from lodgepole pine (Pinus contorta) branches under different degrees of thinning (control: 27,000 stems per ha; moderately thinned: 4,500 stems per ha; heavily thinned: 1,100 stems per ha). Samples were taken over the course of a growing season and data were analyzed using the MixSIAR Bayesian mixing model to partition the relative contributions of source water from various depths. Results indicate that lodgepole pine trees shift their depth-to-water uptake depending on water availability under drought conditions and rely more heavily on older precipitation events, such as winter snowpack, that percolate through the soil profile when shallow soil water becomes less accessible. Interestingly, forest thinning did not cause a significant change in depth-to-water uptake. Our findings concur with other results in the literature by indicating that although lodgepole pine are drought tolerant and have dimorphic root systems, they did not shift from deep water sources when shallow water became more available at the end of the growing season.