Elizabeth Arango-Ruda, M. Altaf Arain (2022). Effect of extreme weather events on the water fluxes, energy partitioning, and water-use efficiency of an evergreen conifer forest in southern Ontario, Canada. Proceedings of the GWF Annual Open Science Meeting, May 16-18, 2022.

Temperate forests play a significant role in the global water and energy cycles. Interactions between forest, energy, and water fluxes are important due to the impact they have on carbon storage, cooling of terrestrial surfaces, and water yield. Mono-specific forest management practices influence evapotranspiration (ET) and the energy partitioning response to extreme weather conditions. Therefore, quantification of evapotranspiration and energy fluxes (water use efficiency) under heat and drought stresses is important for understanding the cooling effect of this managed mono-specific forest under a changing climate. This work presents eighteen years of ET and energy balance components at an 83-year-old white pine managed forest in southern Ontario, Canada. The mean ET over the study period was 446 (±47) mm yr-1. ET showed a positive trend from 2003 to 2010, after which it decreased smoothly. It was not until 2017 that it showed increasing values again before reaching the highest level in 2010 (537 mm), a year with moderately dry, hot conditions. ET was sensitive to active photosynthetic radiation and air temperature and tended to decrease with increasing atmospheric dryness partly due to the inhibitor effect of VPD on transpiration (T) by constraining stomatal closure in plants trees. Prolonged dry periods with increased Ta significantly reduced ET (i.e., 2016). Water table depth (WTD) generally increased with spring recharge, then gradually declined over the growing season. The dry and warm conditions were reflected in WTD (i.e., 2010 and 2012, respectively). The water use efficiency was 0.4 g C kg-1 d-1, indicating high evapotranspiration and carbon storage rates. The energy partitioning was consistent throughout the years. During the growing season, the net radiation peaked, especially in years characterized by hot conditions. The site exhibited higher sensible heat (H) in the early growing season (spring), shifting to latent heat (LE) in the summer when the tree leaves were entirely out. The ground heat flux was small, particularly in 2005 (1.02 W m-2). Evapotranspiration, energy partitioning, and water use efficiency vary according to factors highly influenced by hot and dry periods, leading to significant implications on the water and energy cycles. This response depends upon the multiple interactions inherent to the forest (i.e., forest age, sapwood area dynamics, understory plants, biodiversity, etc.). Our results highlight the importance of extreme weather events such as drought and heat on water fluxes and the coupling of forest carbon and water, which has enormous implications for ecosystem functioning and sustainable management. However, how this affects the global hydrological cycle, freshwater resources, and global climate remains uncertain.

AOSM2022 First Author: Elizabeth Arango-Ruda Additional Authors: M. Altaf Arain