Trees control soil moisture by drawing water through the roots and transpiring them to the air. They also lead to an opposite effect of reducing evaporation from the soil to the air by shading the ground and reducing the wind speed near the surface. Current watershed models simulate soil moisture at regional scales (tens to hundreds of km) and use extremely simplified representations of the interactions between vegetation and soil moisture. The differences between individual crowns and the effects of the canopy structure are not represented in these models. This project aims to improve understanding of the relationships between evaporation, transpiration and soil moisture in heterogeneous forest canopies, and how these relationships affect soil moisture heterogeneity from the tree scale to the ecosystem and regional scales. The project will capitalize on the existing wealth of data and on-going diverse observations at the University of Michigan Biological Station. A combination of detailed observations and state-of-the-art high-resolution modeling tools will be used. Novel radar-based volumetric observation of soil moisture, and in and above canopy micrometeorological measurements will be used, along with a novel integrated modeling approach.
The project will advance our capability to predict the effects of ecosystem structure at multiple scales on the exchanges of energy water and CO2 with the atmosphere, and on the functioning of the regional watershed. Such advancement is particularly important in conditions of changing climate and increased human disturbance to forests, which are expected to increase the spatial heterogeneity of forests and other important natural resources. It will also create a modeling tool that will have a truly transformative value in the hydrological and ecological sciences. Graduate students at Ohio State University and the University of Michigan will be supported through this collaborative project. Undergraduate students will be engaged in the proposed research. "Plant and Water", an educational K-12 community-outreach program will be developed to illustrate the underlying principles of water flow in soil-plant-atmosphere continuum and will train and employ undergraduate students as instructors at Big-Brothers Big-Sisters of Central Ohio Residental Camp Oty'Okwa, as part of the USDA Forest Service's "More Kids in the Woods" initiative, a program for underserved elementary-aged children within central Ohio.
Trees control soil moisture by drawing water through the roots and transpiring them to the air. Current models simulate soil moisture at coarse resolution of tens to hundreds of km and use extremely simplified representations of the relationships between vegetation and soil moisture. The effects of the structure of the trees in the forest canopy and the differences between trees of different species and sizes are not represented well by these models. Predictions of future climate and forecasts on the local weather are dependent on the results of these models. In the Great Lakes region there are vast areas of forest that are going through structural changes. Old, fast growing aspen and birch trees are naturally dying and being replaced by younger, slower growing maple, oak and pine trees. This project aims to improve our understanding of how the structure of the forest, at the scale of individual tree crowns, affects the way the forest as a whole functions - in terms of water exchange from the soil to the air and uptake of carbon dioxide. Our study is conducted at two near-by sites, both at the University of Michigan Biological Station (UMBS) near Pellston MI. One site (Control, Picture 1) is at a 100 year-old forest stand, dominated by aspen and birch. The other site is located at the Forest Accelerated Succession ExperimeT (FASET), where all aspen and birch trees were killed by stem girdling. This site simulates a disturbance with changes structure and composition relative to the control site. We developed a portable canopy lidar (a laser-based sensor) to measure the 3-dimensional structure of the canopy at very resolution (1 m^3) (Picture 2). We also developed computer programs and analytical approaches to process information about canopy structure, process high-frequency measurements of wind and water measurements, and process satellite images of forest canopies. Our methods and computer programs are already being used by other researchers who study aspects of canopy structure in other sites. We installed several meteorological stations within the forest to collect observations about the spatial patterns of soil moisture and humidity in the air inside the canopy (Picture 3). We installed a large array of soil moisture and temperature sensors that tell us how soil moisture is distributed in different depths, at different locations relative to trees of different types. Through our collaboration with researchers from the Department of Electrical Engineering and Computer Science, we deployed near-ground radar (Picture 4) to determine the spatial distribution of soil moisture. Our sensor networks and field campaigns (Picture 5) allowed us to gain insight on water uptake strategies that are used by different forest species; and characterized the differences in the flow and spatial patterns of moisture in the soil under different forest structures. This information has been used to build better models to infer soil water content and its variability. We provide the data from our observations to the public through the Ameriflux website (http://ameriflux.lbl.gov/). We improved two high-resolution models – an atmospheric turbulence model (RAFLES) and a hydrological watershed model (tRIBS+VEGGIE). Both models resolve the movement of water between the soil and the air at a resolution of a few meters, and represent the effects of vegetation on this exchange at the individual tree scale. In RAFLES we improved the way tree stems are represented and coupled this model to an ecosystem model to calculate the amount of heat and water vapor that come out of the leaves at very fast time resolution. In tRIBS+VEGGIE we developed a new approach to resolve the effects of canopy structure and shading patterns on the amount of light that reaches the soil, and we improved the representation of the exchange of water between soil and roots. RAFLES simulations and field observations were used to determine how individual features of canopy structure modify the exchange of water vapor between the vegetation, soil and atmosphere. Modeling with tRIBS+VEGGIE was used to demonstrate the "compensation effect," occurring in dry soil conditions during growing season. When hot air leads to increased evaporation rates, the increased demand for water uptake by evaporating plants is met through an increased root soil moisture uptake in moister regions of the soil with low root biomass (Picture 6). The project provided training of 2 early career faculty, 5 graduate students, and research experience to 3 undergraduate students and one high school teacher. The project provided material for several undergraduate and graduate classes' education, and activities in schools, the OSU Women in Engineering summer camp, and the development of a museum exhibit describing forest modeling for understanding climate change at the North Carolina Museum of Natural Sciences. This project contributed to the publication of 20 scientific papers. Several more are submitted for review or in preparation. The results were also published in more than 50 conference presentations and seminars.
