The Rate of Canopy Development Modulates the Link Between the Timing of Spring Leaf Emergence and Summer Moisture

Shifts in phenological timing have important implications for ecosystem processes, with spring leaf emergence as a dominant control of carbon, water, and energy cycling. Phenological events are predominantly determined by weather and climate, therefore dynamic in time and sensitive to climate feedbacks. Improving our understanding of how ecosystems respond to changes in phenological timing will enhance our ability to assess summer soil water availability, since the timing of spring leaf emergence may lead to soil moisture deficits later in the growing season. We leveraged data from five AmeriFlux towers in central and eastern United States to investigate the extent spring leaf emergence (i.e., start of spring, SoS) influences rates at which forest canopies develop and how this impacts summer soil moisture (?(JJA)) variability. Our results indicate that ecosystem processes, specifically gross primary production (GPP) and evapotranspiration (ET), exhibit compensatory responses to varying leaf emergence; with delayed spring-onset, the canopy developed more quickly, resulting in rapid GPP and ET increases, consistent across sites. Nonetheless, early SoS is a relatively good indicator for potential summer soil water deficits, particularly when it occurs together with meteorological conditions (i.e., lower-than-average precipitation, hot summer temperatures) that contribute to soil water deficits. When these meteorological conditions coincide with early SoS, ?(JJA) deficits are exacerbated. To the extent that these extreme conditions occur more frequently under future climate scenarios, the dynamics of spring phenology and hydroclimate may play an increasingly important role in portending the likelihood of summer water deficits, which are projected to become more severe.Plain Language Summary Warmer spring have been occurring over the past several decades resulting in tree leaves emerging earlier in the year. As leaves emerge, transpiration is initiated transporting water that is stored in the soil back to the atmosphere. As transpiration continues to increase with increasing vegetation, more stored water is removed from the ground, and depending upon the frequency and magnitude of precipitation events to replenish this water, the potential for soil water deficits increases, possibly making an early start of spring (SoS) an indicator for summer soil water plant stress. However, the rate of development of forest canopies changes depending on when the leaves emerge such that an early SoS is characterized by more gradual canopy development whereas a later SoS is characterized by more rapid canopy development. The adjustment in rates of canopy development helps modulate the soil water status and ultimately how slow or how quickly the canopy develops exerts control over the time it takes the ecosystem to achieve maximum evapotranspiration.