Forest plantations have been long-employed to reverse land degradation and support biodiversity, and are now recognized to both take in atmospheric carbon dioxide, reducing the intensity of the greenhouse effect, and moderate local weather. It is important to consider the impact forest aging and management will have on provisioning of these services under climate change and extreme weather events, such as drought. This study encompasses a chronosequence of three Eastern White Pine stands planted in 1939, 1974 and 2002, situated in Turkey Point, Ontario, Canada. The oldest forest received two selective thinning treatments, removing 30% of trees, in 1983 and 2012.
Forest water use efficiency (WUE), which represents the amount of gross ecosystem productivity (GEP) per unit of water released through evapotranspiration (E), was compared among the three sites over 2008-2013. The youngest forest’s annual WUE increased over the study period, surpassing that of the older sites by 2013. When bulk surface conductance (Gs), representing gas exchange, was compared across the sites for the same years, the youngest site had the lowest Gs, particularly during drought. Gs at the oldest forest was highest and the most variable. Statistical analysis showed that across all the sites, E was more responsive to air temperature than atmospheric demand, soil moisture, and incident radiation. This study indicated that younger plantations may be more water-conservative during drought, and that air temperature is important to consider in projections of temperate coniferous forests’ carbon and water exchange.
To assess the impact of the 2012 selective thinning on tree-level and ecosystem-level water use at the oldest forest, sapflow velocity (Js), transpiration (Et) and E were compared between the two stands planted in 1939 and 1974, from 2011 to 2013. A relatively severe drought over the 2012 growing season led to a decline in Et at the unthinned site for that year, however the Et decline was more pronounced at the older, thinned site. From 2011 to 2012, Js increased at the thinned site, converse to the unthinned site – wherein Js was low as expected during drought. Hydraulic redistribution and lag time from sapflow at 1.3 m height to canopy evapotranspiration were seemingly unaffected by the thinning, indicating that low-level selective harvesting was not detrimental to the hydrological functionality of the stand, and may have been beneficial in allowing more soil moisture access per tree. As such, the stand may be better positioned to withstand recurrent dry spells resulting from precipitation variability, as predicted with climate change. / Thesis / Master of Science (MSc)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/17404 |
| Date | 06 1900 |
| Creators | Skubel, Rachel |
| Contributors | Arain, M. Altaf, Earth and Environmental Sciences |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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