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Harvard Forest Symposium Abstract 2006
- Title: Evaluating the sensitivity of Acer rubrum L. sap flux to environmental stress under two root-zone soil water limitation scenarios
- Primary Author: Justin Pettijohn (Boston University)
- Additional Authors: Michael Daley (Boston University); Nathan Phillips (Boston University); Guido Salvucci (Boston University)
- Abstract:
Evapotranspiration (ETa), or the combined processes of land surface evaporation and transpiration, is a fundamental stage of the hydrologic cycle. Unfortunately, our ability to constrain ETa is limited as direct measurements of ETa, unlike rainfall or streamflow, are difficult to obtain. As such, indirect methods of quantifying ETa are preferred since regional estimations of ETa require prohibitive instrumentation or highly parameterized and data intensive land surface models (e.g., involving temporally and spatially-varying soil moisture, soil hydraulic properties, and vegetation properties). Bouchet’s Complementary Relationship (CR) is one such indirect method of estimating ETa from the more easily estimated potential evapotranspiration (ETp), as seen in Figure "a". Whereas ETp conceptually includes a transpiration component, the treatment of vegetation in existing CR applications varies from neglecting it to indirectly accounting for it through recalibration of empirical, Dalton-type wind functions. Due to the overwhelming neglect of vegetation in previous CR models, we implemented a field campaign at Harvard Forest to constrain the physiological feedback mechanisms regulating CR. Specifically, we hypothesized that a CR-type relationship should occur between red maple (Acer rubrum L.) actual transpiration and red maple potential (water-unlimited) transpiration. Daily averages of whole-tree transpiration for our cooccurring irrigated and reference maple network were estimated using heat-dissipation sap flux sensors. Soil moisture, temperature and matric potential parameters were measured using Campbell Scientific sensors. Our results suggest that irrigated and reference maple transpiration fluxes do not exhibit fully CR-type behaviors. While the reduction in leaf-level stomatal stress due to root-zone irrigation increases maple transpiration, additional environmental stresses (e.g., vapor pressure deficit) prohibit the irrigated transpiration signal from following a typical CR-type ETp (see Figure "c"). Finally, we demonstrate how an unstressed Penman-Monteith ETp estimation (unlike the irrigated transpiration) satisfies the conditions of CR, as seen on Figure "c".
- Research Category: Forest-Atmosphere Exchange
