Browsing by Author "Gavazzi, Michael J"

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    A comparison of three methods to estimate evapotranspiration in two contrasting loblolly pine plantations: Age-related changes in water use and drought sensitivity of evapotranspiration components
    (Forest Science, 2012-10-02) Domec, Jean-Christophe; Sun, Ge; Noormets, Asko; Gavazzi, Michael J; Treasure, Emrys A; Cohen, Erika; Swenson, Jennifer J; McNulty, Steve G; King, John S
    Increasing variability of rainfall patterns requires detailed understanding of the pathways of water loss from ecosystems to optimize carbon uptake and management choices. In the current study we characterized the usability of three alternative methods of different rigor for quantifying stand-level evapotranspiration (ET), partitioned ET into tree transpiration (T), understory transpiration, interception, and soil evaporation (E S) and determined their sensitivity to drought, and evaluated the reliability of soil moisture measurements by taking into account deep soil moisture dynamic. The analyses were conducted in an early- and in a mid-rotation stand of loblolly pine, the predominant species of southern US forest plantations. The three alternative methods for estimating ET were the eddy covariance measurements of water vapor fluxes (ET EC), the water table fluctuation (ET WT), and the soil moisture fluctuation (ET SM). On annual and monthly scales, the three methods agreed to within 10-20%, whereas on a daily scale, the values of ET SM and ET EC differed by up to 50% and ET SM and ET WT differed by up to 100%. The differences between the methods were attributed to root water extraction below measurement depth and to the sampling at different spatial scales. Regardless of the method used, ET at the early-rotation site was 15-30% lower than that at the mid-rotation site. The dry years did not affect ET at the mid-rotation site but reduced significantly ET at the early-rotation site. Soil moisture trends revealed the importance of measuring water content at several depths throughout the rooting zone because less than 20% of the water is stored in the top 30 cm of soil. Annually, E S represented approximately 9 and 14% of ET EC at the mid-rotation site and the early-rotation site, respectively. At the mid-rotation site, T accounted for approximately 70% of ET EC. Canopy interception was estimated to be 5-10% of annual precipitation and 6-13% of total ET EC. At the early-rotation site, T accounted for only 35% of ET EC. At this site, transpiration from subdominant trees and shrubs represented 40-45% of ET EC, indicating that understory was a significant part of the water budget. We concluded that the eddy covariance method is best for estimating ET at the fine temporal scale (i.e., daily), but other soil moisture and water table-based methods were equally reliable and cost-effective for quantifying seasonal ET dynamics.© 2012 by the Society of American Foresters.
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    Drought and thinning have limited impacts on evapotranspiration in a managed pine plantation on the southeastern United States coastal plain
    (Agricultural and Forest Meteorology, 2018-11) Liu, Xiaodong; Sun, Ge; Mitra, Bhaskar; Noormets, Asko; Gavazzi, Michael J; Domec, Jean-Christophe; Hallema, Dennis W; Li, Jiyue; Fang, Yuan; King, John S; McNulty, Steven G
    © 2018 Managed and natural coastal plain forests in the humid southeastern United States exchange large amounts of water and energy with the atmosphere through the evapotranspiration (ET) process. ET plays an important role in controlling regional hydrology, climate, and ecosystem productivity. However, long-term studies on the impacts of forest management and climatic variability on forest ET are rare, and our understanding of both external and internal drivers on seasonal and interannual ET variability is incomplete. Using techniques centered on an eddy covariance method, the present study measured year-round ET flux and associated hydrometeorological variables in a drained loblolly pine (Pinus taeda L.) plantation on the lower coastal plain of North Carolina, U.S. We found that annual ET was relatively stable (1076 ± 104 mm) in comparison to precipitation (P) (1168 ± 216 mm) during the 10-year study period when the site experienced extreme climate (2007–2008) and forest thinning (2009). At the seasonal time scale, mean ET/P varied between 0.41 and 1.51, with a mean value of 1.12 ± 0.23 and 0.72 ± 0.16 for the growing and dormant seasons, respectively. The extreme drought during 2007–2008 (mean annual P, 854 mm) only resulted in a slight decrease (∼8%) in annual ET owing to the shallow groundwater common to the study area. Although changes in leaf area index and canopy structure were large after the stand was 50% thinned in the fall of 2009, mean annual ET was similar and averaged 1055 mm and 1104 mm before (2005, 2006 and 2009) and after (2010–2015) thinning, respectively. Data suggested that annual ET recovered within two years of the thinning as a result of rapid canopy closure and growth of understory. Further analysis indicated that available energy was the key driver of ET: approximately 69% and 61% of the monthly variations in ET were explained by net radiation during the dormant and growing seasons, respectively. Overall, we concluded that drought and forest thinning had limited impacts on seasonal and annual ET in this energy limited forest ecosystem with shallow groundwater. The results from this study help to better understand regional ecohydrological processes and projecting potential effects of forest management and extreme climate on water and carbon cycles.
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    Hydraulic redistribution of soil water by roots affects whole-stand evapotranspiration and net ecosystem carbon exchange.
    (New Phytol, 2010-07) Domec, Jean-Christophe; King, John S; Noormets, Asko; Treasure, Emrys; Gavazzi, Michael J; Sun, Ge; McNulty, Steven G
    *Hydraulic redistribution (HR) of water via roots from moist to drier portions of the soil occurs in many ecosystems, potentially influencing both water use and carbon assimilation. *By measuring soil water content, sap flow and eddy covariance, we investigated the temporal variability of HR in a loblolly pine (Pinus taeda) plantation during months of normal and below-normal precipitation, and examined its effects on tree transpiration, ecosystem water use and carbon exchange. *The occurrence of HR was explained by courses of reverse flow through roots. As the drought progressed, HR maintained soil moisture above 0.15 cm(3) cm(-3) and increased transpiration by 30-50%. HR accounted for 15-25% of measured total site water depletion seasonally, peaking at 1.05 mm d(-1). The understory species depended on water redistributed by the deep-rooted overstory pine trees for their early summer water supply. Modeling carbon flux showed that in the absence of HR, gross ecosystem productivity and net ecosystem exchange could be reduced by 750 and 400 g C m(-2) yr(-1), respectively. *Hydraulic redistribution mitigated the effects of soil drying on understory and stand evapotranspiration and had important implications for net primary productivity by maintaining this whole ecosystem as a carbon sink.