Persistent Evapotranspiration in a Shallow Boreal Shield Peatland Despite Summer Drought: Implications for Ecohydrological Resilience

ABSTRACT Peatlands generally act as critical long‐term carbon sinks, but there is concern that this ecosystem function is threatened by climate warming and increased drought. While shallow peatlands have recently been proposed as climate change sentinels for deeper peatlands, their evapotranspiration (ET) dynamics during drought remain understudied. We assessed 9 years of growing season surface energy balance and water exchange in a shallow peatland and rock barrens landscape in the Boreal Shield of Ontario, Canada. Actual ET in the shallow study peatland (mean depth 0.49 m) was approximately 41.2% of potential ET. In dry years, we observed a shift in energy partitioning from latent to sensible heat, higher surface albedo and a significant decrease in the ratio of actual to potential ET. During dry periods, higher albedo could increase daily losses of incoming solar energy by as much as 1.27 MJ m −2 . However, despite these mechanisms that serve to limit water loss, total ET remained surprisingly consistent between wet and dry years until severe moisture limitation was reached. This delayed evaporative shutdown may be driven by the unique hydrophysical properties of shallow peatlands: lower specific yield leads to rapid water table decline, while highly decomposed upper peat layers sustain capillary connectivity and allow water loss to persist until the water table drops below the peat profile. Advective energy inputs from adjacent rock barrens may have also contributed to continued ET. Ultimately, because ET becomes highly restricted only once the water table disappears from large areas of the site, this delayed response cannot prevent severe net moisture deficits. We expect that the limited storage capacity and persistent ET increase vulnerability of shallow peatlands in this ecozone to drought stress and severe burning in the event of wildfire, thus threatening their long‐term carbon storage function.