Hydrologists and ecologists alike have often conceptualized transpiration and photosynthesis as depending on root-zone soil moisture, thereby neglecting the movement of water through the plant. But this movement is not instantaneous, and in reality these fluxes respond to the plant’s internal water status (often studied through water potential), which could be decoupled from soil moisture. We study – using remote sensing, models, and stand-scale data – how this flow of water through plants affects the dynamics of evapotranspiration and photosynthesis. Key themes in this research include the sensitivity of stomatal closure to vapor pressure deficit (which is expected to increase under future, hotter droughts) and the role of xylem and stomatal traits. As an example of the latter, we created the first global map of isohydricity, a measure of leaf water response to soil water stress.
Microwave remote sensing can be used to measure both soil moisture and vegetation water content. While traditionally, the vegetation signal was only used as a correction to improve soil moisture retrieval, the remote sensing community has slowly realized these vegetation datasets are also potentially informative! Our work focuses on the applications of vegetation water content determined from radar or radiometry. For most applications, we use vegetation optical depth (VOD) determined from passive spaceborne radiometers, which is proportional to vegetation water content. The group has both developed new VOD datasets (see Datasets page) and also uses VOD datasets produced by others that have longer time records (e.g. the LPDR dataset produced at the University of Montana). We create new datasets and use these datasets for a variety of application domains, including fire, drought-driven tree mortality, and more.
Many types of local variability affect water and carbon fluxes - factors like differences in behavior between different species, competition between different species, variability in soil properties, topography, etc… For regional or global studies, it is all but impossible to represent many of these local sources of variability, both because it’s computationally infeasible and because not enough information about the landscape is available. Our group instead studies the ‘effective ecosystem-scale’ ecosystem traits that account for all the small-scale variability and represent differences in behavior at intermediate stand-scales. These traits are either determined directly from remote sensing, or through model-data fusion, whereby the parameters that best match a set of observed fluxes at larger scales are determined. We study the effects of trait variability at regional scales both in the context of plant hydraulics. We also often use the CARbon DAta MOdel fraMework (CARDAMOM), a terrestrial carbon cycle model that uses model-data fusion in combination with several remote sensing data streams.
Despite exhibiting among the largest water and carbon fluxes on earth, tropical ecosystems are generally under-studied relative to those in the mid-latitudes, as in situ measurements are logistically more difficult than in developed countries. We are particularly interested in the wet tropics across the under-studied old-world continents. Past and current work focuses on understanding the seasonal variability of water uptake and evapotranspiration in the Congo river basin. We also study the interplay between anthropogenic hydrologic change (e.g. canal building and restoration), land use change, and fire in Insular Southeast Asia.