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Abstract As a warmer climate enables an increase in atmospheric humidity, extreme precipitation events have become more frequent in the Northeastern United States. Understanding the impact of evolving precipitation patterns is critical to understanding water cycling in temperate forests and moisture coupling between the atmosphere and land surface. Although the role of soil moisture in evapotranspiration has been extensively studied, few have analyzed the role of soil texture in determining ecosystem‐atmosphere feedbacks. In this study, we utilized long term data associated with ecosystem water fluxes to deduce the strength of land‐atmosphere coupling at Harvard Forest, Petersham, MA, USA. We found a 1.5% increase in heavy precipitation contribution per decade where high‐intensity events compose upwards of 42% of total yearly precipitation in 2023. Intensifying precipitation trends were found in conjunction with a long‐term soil drying at the Harvard Forest despite no significant increase in evapotranspiration over 32 years. This suggests that soil water holding capacity is a key mediating variable controlling the supply of water to ecosystems and the atmosphere. We found that these land surface changes directly impacted the lifted condensation level (LCL) height over Harvard Forest which was found to be increasing at a rate of 6.62 m per year while atmospheric boundary layer (ABL) heights have fallen at a modest rate of 1.76 m per year. This has amplified dry feedbacks between the land surface and the atmosphere such that 80% of observed summers ending in a water deficit also had an anomalously low soil water content in the spring. 

Within seasonal temperate forests, changes in precipitation structure—its form, duration, and seasonal timing—is a dominant characteristic of climate change. While past research has focused primarily on annual precipitation totals, emerging evidence shows that short-duration extreme precipitation can impact ecosystem carbon, water, and biogeochemical cycling when it coincides with key phenological and physiological transitions. These impacts are mediated by the responses of plant and microbial physiology, aboveground–belowground interactions, and lagged feedbacks as organisms and communities adjust to these extremes. This review focuses on shifts within ecosystem water cycling, within tree growth dynamics (carbon uptake and aboveground–belowground allocation and coordination), within soil biogeochemical cycling, from the loss of winter snow, and in forest structure and community composition. Together, these concepts highlight the urgent need to understand how changes in all aspects of precipitation structure reshape the functioning and resilience of mesic temperate forests. 

The northeastern U.S. has experienced a rapid rise in extreme precipitation events and total precipitation due to climate change. Despite higher overall precipitation, long-term near-surface soil moisture at the Harvard Forest in Petersham, MA has decreased since 2010, a pattern also observed in other global temperate forest regions. In this study, we used more than thirty years of ecosystem-atmosphere water and carbon exchange at the Harvard Forest to understand the impact of precipitation extremes during the past decade on ecosystem water and carbon fluxes and the strength of land-atmosphere coupling. We found that in this mesic temperate forest, well-drained post-glacial soils rapidly drain surplus moisture from large rain events, while the remaining moisture necessary to preserve local humidity is quickly lost to evapotranspiration unless frequently replenished by rainfall. This region has also experienced two hot summer droughts during the past decade, causing further hydrological stress with carbon cycle implications. Furthermore, meteorological conditions in the nongrowing season have particularly shifted to warmer, drier conditions that set the stage for more frequent summer soil moisture deficits. In response to this past decade of hydrological extremes, we have observed a dampening of canopy light response curves, indicating lower rates of carbon uptake during the growing season and a parallel decline in ecosystem respiration as soils dry. More frequent dry conditions during key phenological windows, the intense delivery of rainfall during a shorter temporal window in the growing season, and rising summer temperatures and lower humidity have combined to decrease the ecosystem carbon uptake by photosynthesis and created large interannual variation in the strength of the net carbon sink at Harvard Forest during the past decade compared to the prior two decades of this study.