https://doi.org/10.1002/hyp.70134

Evapotranspiration (ET) from temperate forests plays a significant role in the regional and global water cycles. However, extreme
weather events such as heat and drought are affecting the water use and water use efficiency (WUE) of these forests. Climate
change impacts may be more severe in plantation forests where the age of the forest plays a significant role, causing differences
in their responses to environmental stresses. This study presents 14 years (2008–2021) of water flux data measured using the
eddy covariance technique in an age sequence (83, 48 and 20 years as of 2021) of eastern white pine (Pinus strobus L.) forests
in the Great Lakes region in southern Ontario, Canada. The mean annual ET was 465 ± 41, 466 ± 32 and 403 ± 21 mm year −1 in
the 83-­, 48-­and 20-­year-­old stands, respectively, with the highest annual water flux observed in the 83-­year-­old stand, which
was similar to that of the 48-­year-­old stand. Mean annual gross ecosystem productivity (GEP) was 1585 ± 100, 1660 ± 115 and
1634 ± 331 g C m−2 year−1in the 83-­, 48-­and 20-­year-­old stands, respectively, while mean annual WUE was 3.4 ± 0.4, 3.6 ± 0.4
and 4.0 ± 0.8 g C kg H 2O year −1 in the respective stands. Lower ET and relatively higher GEP resulted in the highest WUE in the
youngest stand, even though the highest GEP was observed in the middle-­aged stand. Air temperature (Tair) was the dominant
control on ET, GEP and WUE in all three different-­aged stands, while drought, characterised as the relative extractable water
(REW) in the soil, had a significant impact on ET in the late summer. The results of this study further showed that forest age
significantly influenced how forests responded to drought stresses. The younger stand was more efficient in carbon seques-
tration and water use despite exhibiting greater sensitivity to water stress and higher drought coupling. The long-­term eddy
covariance measurements analysed in this study have helped to enhance our understanding of water exchange processes in the
temperate conifer forest ecosystems in Eastern North America. Specifically, this work contributes to a better understanding of
how different-­aged forests respond to extreme weather events, aiding in the development of new strategies for managing water
resources and ensuring water security in the region under a changing climate.

https://doi.org/10.1002/hyp.70134