Seasonal droughts are becoming more prevalent in recent years as a result of climate change and associated fluctuations in precipitation events. Forested ecosystems react very differently to changes in the hydrological cycle in different regions. This study investigates how atmospheric water fluxes react to changes in meteorological variables and subsurface hydrological conditions in a 90-year old temperate deciduous forest in southern Ontario, Canada, using eddy covariance flux data over five years (2012-2016).
Results show that although drought occurrences in 2012, 2015 and 2016 put stress on the forest ecosystem, with significantly varying climate and soil conditions, no substantial impact on forest evapotranspiration appeared to occur. Annual eddy-covariance mean evapotranspiration of the study was 382 ± 46 with a growing season average of 353 ± 36. Highest annual evapotranspiration (422 mm) occurred in 2012 with 389 mm occurring during the growing season while the lowest annual evapotranspiration (341mm) occurred in 2014 with 316mm occurring in the growing season. Air temperature and vapour pressure deficit had the dominant control on evapotranspiration as expected, with the highest sensitivity occurring during drought years. Soil water potential (SWP), as the soil moisture stress proxy, reached maximum values during drought years, but was not found to have a significant control with evapotranspiration. SWP peaked when evapotranspiration values reach maximums combined with warm air temperatures, progressively increasing when precipitation input was minimal. Our results indicate that evapotranspiration rates in this forest ecosystem were predominantly determined by atmospheric controls when accessibility of deep soil water was possible. These findings suggest that temperate deciduous forests were able to sustain ET during low to moderate intensity droughts and observed over the study period due to their ability to access deep soil moisture stores during periods of stress. However, these forests may experience significant declines in ET if severe or multi-year drought conditions arise, similar to those that occurred in the last two years of this study. These conclusions will help to elucidate how ET in deciduous forests will respond to future climate regimes in this region. / Thesis / Master of Science (MSc) / With seasonal droughts occurring more frequently from climate change, water required by tree species may reach limitations to sustain atmospheric demand. In particular, temperate deciduous forests, that are common in the northern hemisphere, may be affected due to predicted future climate scenarios. It is important that we understand how they will respond to future climate conditions. Understanding how forest evapotranspiration is affected by various meteorological and hydrological parameters will help gauge how forests will cope with limiting water resources in the future. An assessment of a temperate deciduous forest in southern Ontario, which is part of the Turkey Point Flux Station, was conducted in this study over 5 years (2012-2016). Over this period the forest experienced varying drought intensities. Results show that air temperature and vapour pressure deficit were the dominant controls on forest ET during times of water limitations. The forest was able to withstand low-moderate droughts by accessing deep soil water stores to keep up with the extreme demand. This study suggests that temperate deciduous forests in the region are well adapted to drought stress and they may be able to cope with similar intensity droughts in the future by maintaining water flow.
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/22657 |
| Date | January 2017 |
| Creators | Burns, Brandon |
| Contributors | Arain, Altaf, Earth and Environmental Sciences |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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