Abstract
Hydraulic capacitance and water storage form a critical buffer against cavitation and loss of conductivity within the xylem system. Withdrawal from water storage in leaves, branches, stems, and roots significantly impacts sap flow, stomatal conductance, and transpiration. Storage quantities differ based on soil water availability, tree size, wood anatomy and density, drought tolerance, and hydraulic strategy (anisohydric or isohydric). However, the majority of studies focus on the measurement of storage in conifers or tropical tree species. We demonstrate a novel methodology using frequency domain reflectometry (FDR) to make continuous, direct measurements of wood water content in two hardwood species in a forest in Michigan. We present results of a two month study comparing the water storage dynamics between a mature red oak and red maple, two species with differing wood densities, hydraulic architecture, and hydraulic strategy. We also include results pertaining to the use of different probe lengths to sample water content only within the active sapwood and over the entire conductive sapwood and the outer portion of heartwood in red oak. Both species studied exhibited diurnal cycles of storage that aligned well with the dynamics of sap flux. Red maple, a diffuse porous, relatively isohydric species showed a strong dependence on stored water during both wet and dry periods. Red oak, a ring porous relatively anisohydric species, was less reliant on storage, and did not demonstrate a dependence on soil water potential. Comparison between long and short FDR probes in the oak revealed that oaks may utilize water stored in the innermost layers of the xylem when soil moisture conditions are limiting. We found the FDR probes to be a reliable, functional means for continuous automated measurement of wood water content in hardwoods at a fast time scale. Application of FDR technology for the measurement of tree water storage will benefit forest ecologists as well as the modeling community as we improve our understanding and simulations of plant hydrodynamic processes on a large scale.
Highlights
Water storage within the hydraulic systems of trees provides a buffer for the daily demands of transpiration (Cermak and Nadezhdina 2011, Scholz et al 2011, Kocher et al 2013)
We explore the feasibility of deploying frequency domain reflectometry (FDR) probes to measure wood volumetric water content and test hypotheses regarding trunk water storage
We use a combination of meteorological, sap flux, and wood water content measurements to test the effectiveness of FDR sensors for measuring tree water storage and water use dynamics in one red maple and one red oak tree of similar size, height, canopy position and proximity to one another
Summary
Water storage within the hydraulic systems of trees provides a buffer for the daily demands of transpiration (Cermak and Nadezhdina 2011, Scholz et al 2011, Kocher et al 2013). Various studies have demonstrated that 10–50% of daily transpiration may be provided by stem water stores (e.g., Phillips et al 2003, Meinzer et al 2004, Cermak et al 2007, Kocher et al 2013). In most cases where water storage has been measured, the focus has been on tropical or coniferous species (e.g., Waring and Running 1978, Andrade et al 1998, Domec and Gartner 2001, Meinzer et al 2006, Cermak et al 2007), rather than temperate hardwood species (Cermak et al 1982, Wullschleger et al 1996, Hernandez-Santana and Martinez-Fernandez 2008, Kocher et al 2013)
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