Impacts of heat and drought on the dynamics of water fluxes in a temperate deciduous forest from 2012 to 2020

Temperate deciduous forests play a significant role in the regional and global water cycles. However, climate change and associated extreme weather events are affecting the growth, water use, and survival of these forests. This study examined nine years of water flux measurements made using the eddy covariance (EC) technique in a temperate deciduous forest in eastern North America from 2012 to 2020 and assessed the impacts of heat and drought events on forest evapotranspiration (ET) and water use efficiency (WUE). Study results showed that the highest annual evapotranspiration (ET) of 521 mm yr -1 was observed in 2020, while the lowest annual ET of 359 mm yr-1 was recorded in 2014. Photosynthetically active radiation (PAR, lmg = 0.33) and air temperature (Tair, lmg = 0.33) were the primary drivers of ET, while vapor pressure deficit (VPD, lmg = 0.41) was the primary regulator of WUE. Over the nine-year study period, observed mean annual ET and precipitation (P) values were 367 +/- 40 mm yr -1 and 1148 +/- 262 mm yr -1, respectively, while mean annual P-ET value was 729 +/- 272 mm yr -1. Annual ET accounted for 38% of P over the study period. Mean growing season WUE, which is an indicator of water utilization by the forest for growth and carbon assimilation, was 3.8 +/- 0.4 g C Kg H2O-1, with the highest WUE of 4.4 g C Kg H2O-1 observed in 2014 (cool year), and the lowest values of 3.0 and 3.1 g C Kg H2O-1observed in 2012 (hot-dry year) and 2020 (dry year), respectively. We found an increasing trend in WUE values from 2012 to 2014 and then a decreasing trend from 2015 to 2020, when every year experienced extreme weather conditions (e.g. hot, dry or hot-dry), except 2019. This long-term flux observation study will help to enhance our understanding of water exchanges and water use in temperate deciduous forests in Eastern North America and develop strategies for managing forest water resources and ensuring water security in the region in changing climate.