Multidecadal evapotranspiration measurements in a temperate forest demonstrate a sustained impact of warmer winters and a transient effect of biotic disturbance

The water cycle in temperate forests is changing due to natural succession, climate change, and disturbances from both natural and anthropogenic causes, particularly invasive insect pests. Biotic disturbances disrupt expected successional trajectories, altering canopy structure and increasing uncertainty in predicting hydrological responses, especially evapotranspiration (ET) patterns. This study examined how biotic insect disturbance interacts with climate change to influence ET and the multi-decadal land–atmosphere water cycle at a northeastern U.S. Forest site where local water supply systems are dependent on surface water. Using long-term eddy covariance data, we quantified ET at two Harvard Forest towers with distinct forest types: one with a hardwood forest footprint (EMS; 1993-2023) and a second with an evergreen eastern hemlock forest impacted by the invasive hemlock woolly adelgid insect since 2010 (HEM; 2005-2023). At EMS, non-growing-season potential evapotranspiration (PET) and ET increased by approximately 13% and 39%, respectively, whereas no significant PET trend was detected at HEM. Changes in ET during the growing season were not significant at the EMS tower site. At HEM, annual ET initially declined during hemlock mortality by the hemlock woolly adelgid and subsequently recovered as deciduous vegetation became established. At HEM, ET was more strongly associated with net ecosystem exchange (NEE) and gross primary productivity (GPP) than at EMS, consistent with closer coupling between carbon and water fluxes at the disturbed site. These findings reveal how climate change and invasive insects leave distinct signatures on the water cycle.