IMPACT OF CLIMATE VARIABILITY AND FOREST MANAGEMENT REGIMES ON WATER AND ENERGY FLUXES IN TEMPERATE FORESTS IN THE GREAT LAKE REGION

Forest ecosystems cover about 30% (42 million km²) of the Earth's land surface and 25% of these forests are located in temperate climate zone. Forests play a crucial in global carbon cycle and provide numerous goods and services. Due to warmer temperatures caused by increasing greenhouse gas emissions, forest ecosystems have become an important player in withdrawing carbon dioxide from the atmosphere and providing natural climate solutions. While the role of forests in carbon sequestration is well-recognized, less emphasis has been placed on their role in water and energy exchanges, particularly in the context of climate change and extreme weather events. Understanding these exchanges is crucial for assessing the cooling potential of forests through carbon sequestration and evapotranspiration, which significantly influence the carbon and water cycles, respectively. This study analyzed long term eddy covariance measurements of water and energy fluxes from 2012 to 2021 at five forest sites in Southern Ontario, Canada, which are part of the Turkey Point Environmental Observatory. These temperate forests, which grow under similar climatic and edaphic conditions, included four conifer forests comprising three white pine plantations of different ages (83, 48, and 20 years old) and an 81-year-old red pine plantation forest that underwent four different variable retention harvesting (VRH) treatments and a >90-year-old naturally regenerated but managed deciduous forest. The analysis of evapotranspiration (ET) and water use efficiency (WUE) in the deciduous forest revealed impacts of extreme weather events and environmental variables. At daily timescale, ET was primarily controlled by photosynthetically active radiation (PAR) and air temperature (Tair). The mean annual ET was 419 ± 45 mm year-1 from 2012 to 2020. The highest annual ET of 521 mm occurred in 2020, a year characterized by hot and dry conditions, while the lowest annual ET of 359 mm was recorded in 2014, a wet year with more cloudy conditions. Concurrent hot and dry conditions generally increased ET. On average, ET represented 38% of precipitation (P) for the study period. The highest annual WUE (the ratio of Gross Ecosystem Productivity (GEP) to ET) of 4.4 g C kg H₂O⁻¹ was observed in the cool year of 2014, whereas the lowest values of 3.0 and 3.1 g C kg H₂O⁻¹ were found in the hot and dry year of 2012 and the dry year of 2020, respectively. WUE increased from 2012 to 2014, then decreased from 2015 to 2020, where each year (except 2019) experienced extreme weather conditions (e.g., hot, dry, or hot-dry). Dry conditions were defined as periods when the Relative Extractable Water (REW) was below 0.4, while hot conditions were identified when the daily maximum temperature (Tmax) reached or exceeded 27.5°C. This temperature threshold corresponds to the 90th percentile of daily Tmax over the 30-year reference period (1971–2000), based on data from the ECCC weather station in Delhi, Ontario. The results showed the impacts of consecutive and concurrent extreme weather events on the forest water and carbon cycle and the WUE. Overall, WUE was mainly regulated by vapour pressure deficit (VPD). ET and WUE in the conifer (white pine) forests showed differences among stands due to their ages. The mean annual ET values were 465 ± 41, 466 ± 32, and 403 ± 21 mm yr⁻¹ in the 83-, 48-, and 20-year-old stands, respectively, from 2008 to 2021. The two oldest forests exhibited higher annual ET values than the youngest forest and the deciduous stand. On average, ET accounted for 43% of total annual precipitation in the two older forests and 38% in the youngest forest, with the ET/P ratio in the latter being similar to that of the deciduous forest. The mean annual WUE values were 3.4 ± 0.4, 3.6 ± 0.4, and 4.0 ± 0.8 g C kg H₂O⁻¹ in the 83-, 48-, and 20-year-old stands, respectively, indicating overall higher WUE compared to the mean annual WUE of 3.3 ± 0.4 g C kg H₂O⁻¹ in the deciduous forest. Similar to the deciduous forest, Tair emerged as the dominant factor controlling ET and WUE across all three conifer stands of varying ages, but specifically at the monthly timescale. The oldest conifer forest exhibited lower sensitivity to drought, suggesting its higher resilience to dry conditions, likely due to its well-established rooting system and more open and diverse species composition in the understory due to management practices. Moreover, a decline in the ET/P ratio was observed in all three conifer stands over three consecutive drought years from 2015 to 2017. WUE during these drought years increased, with the youngest stand generally exhibiting the highest WUE. Such an increase in WUE was also observed in the deciduous forest over the same period. Lastly, the analysis of sap flow velocity (SV) in the 81-year-old red pine trees was conducted across four different VRH treatments: 33% dispersed basal area retention (33D), 55% dispersed retention (55D), 33% aggregated retention (33A), and 55% aggregated retention (55A), along with an unharvested control (CN) plot. The results illustrated that the 55D treatment was the most optimal forest management practice to promote forest growth, as indicated by the higher transpiration rates. PAR was identified as the primary driver of daily sap flow across VRH treatments, followed by Tair. However, vapor pressure deficit (VPD) assumed greater importance on hourly timescale. Overall, studies conducted in this dissertation have enhanced our understanding of water and energy exchange dynamics in temperate conifer and deciduous forests, particularly in response to interannual variability and extreme weather events. The studies have also contributed to valuable insights into the impact of forest management practices on the resilience of the forests to drought events. This work will help to develop strategies for enhancing forest growth, carbon uptake and water use efficiency, which are vital for forest ecosystems adapting to climate change. These findings will aid stakeholders in adopting effective forest management regimes to promote sustainability and resilience to climatic stresses in forest ecosystems.

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