OOS 43-3 - Poseidons of the Forest: Species-specific Governance of Forest Water Cycling

Plant phenology wields control over water cycling, as transpiration is initiated when leaves emerge in the spring. Advanced spring leaf emergence is mainly driven by increasing temperatures, but species sensitivities to temperature fluctuations vary. An early start of spring (SoS) corresponds to enhanced spring evapotranspiration (ET), but the magnitude of ecosystem-scale ET depends on the timing of phenological events at the species level. Here, we quantify the SoS (i.e., differences in leaf emergence) impacts on springtime water fluxes. using a combination of flux tower data, two independent records of species-level phenology (i.e., Phenocam and ground-based visual observations), and thermal dissipation sapflow estimates of transpiration all collected in the footprint of the Morgan-Monroe State Forest AmeriFlux site (US-MMS). Phenophases across a 100 m x 100 m square transect were monitored weekly for-12 species (~120 individuals), including sugar maple (ACSA), tulip poplar (LITU), sassafras (SAAL), and white oak (QUAL) from February – November 2020-2023. Sapflux probes (N~32) were installed on QUAL, LITU, ACSA, and SAAL throughout the eddy covariance tower footprint to estimate species-specific and ecosystem fluxes. We explore 1) correlations between Phenocam- and ground-based observation- derived SoS, 2) variability in SoS across species to ask what extent the seasonal variations of ET and species-specific sapflow correspond to phenological SOS.

First, we found a strong agreement between species-specific ground-based observations and camera-derived green chromatic coordinate (GCC) for springtime phenophases, which provides support for our species-specific approach to Phenocam image processing. Additionally, we found consistent patterns in species-level phenology that help explain overall patterns the relationship between springtime ET and SoS. Across the multi-year records, LITU leaves consistently emerge before ACSA, SAAL, and QUAL, increasing the potential for LITU to drive much of the springtime ET. Sapflow observations further support LITU driving springtime ET, as LITU expressed ~35% and 120% greater fluxes relative to ACSA and QUAL, respectively. Relative to QUAL, higher overall flux rates coupled with 12 to 9-day leads in transpiration for LITU and ACSA, respectively, provide a physiological advantage for harvesting light and soil nutrients. However, LITU affinity towards early leaf emerges exposes them to the risk of late season frost which will in turn diminish water transport abilities, ultimately reducing the magnitude of ecosystem-scale ET when they suffer leaf damage, which feedbacks on the local climate by altering the ratio of sensible and latent heat.