Hydraulic Redistribution of Soil Water in a Drained Loblolly Pine Plantation: Quantifying Patterns and Controls over Soil-to-Root and Canopy-to-Atmosphere Interactions Jean-Christophe Domec, Asko Noormets, John S. King, Ge Sun, Steven G. McNulty, Michael J. Gavazzi, Emrys Treasure and Johnny L. Boggs. jdomec@ncsu.edu North Carolina State University / U.S. Forest Service This research is funded by the USDA Forest Service Southern Global Change Program and located on Weyerhauser private land

and their biophysical regulations. A network of sites has been established in the United States to measure and model water and carbon fluxes across different forest types and management regimes. Most of these sites are located in natural upland forests, but little attention has been given to coastal plain ecosystems. (Hargrove et al. 2003) The conversion of wetlands to intensively managed forest lands in eastern North Carolina is widespread and the consequences on water balance are not well studied. Objective 1: quantification of soil water partitionning, tree transpiration (E), and evapotranspiration (ET) of forested wetlands, and their biophysical regulations.

Above canopy measurements: eddy fluxes, meterological parameters Site Descriptions: 15 year-old Loblolly pine plantations are located in the lower coastal plain North Carolina (precipitation 1400 mm/yr) Above canopy measurements: eddy fluxes, meterological parameters Sub-canopy measurements: Tree sap-fluxes Soil measurements: water content, predawn (soil) water potentials Frequency domain capacitance

Seasonal tree water loss vs. total evapotranspiration Mean daily from May to Nov.: ET = 3.58 (± 0.11) mm d-1 ; (0.14 in) Etree = 2.16 (± 0.06) mm d-1 (0.08 in) Mean daily from May to Dec: ET = 2.55 (± 0.08) mm d-1 (0.10 in) Etree = 1.86 (± 0.05) mm d-1 (0.07 in)

Seasonal tree water loss vs. total evapotranspiration Mean daily: ET = 3.58 (± 0.11) mm d-1 ; Etree = 2.16 (± 0.06) mm d-1

Daily tree water loss vs. total evapotranspiration

Canopy conductance responses to VPD (how open are the leaf pores) Decrease in stomatal sensitivity to VPD as soil dries?

72 45 Responses of stomatal conductance and transpiration to water vapor pressure deficit as a function of the relative extractable water (REW).

Seasonal daily water dynamic

ET represented 90-95% of total soil water depletion Seasonal water loss based on volumetric soil water depletion (soil H2O depletion), or on eddy covariance measurements (evapotranspiration) ET = 3.58 (0.11) mm d-1 (0.14 in) H2Osoil = 3.80 (0.10) mm d-1 (0.15 in) ET represented 90-95% of total soil water depletion Total water use peaked between 4 and 6.5 mm/day during the growing season

Seasonal soil water dynamic Precipitation inputs = 502 mm (19.8 in) Soil H2O depletion = 650 mm (25.6 in) Soil water content varied with soil depth After periods of more than 5 days without rain, water extraction shifted to the deeper layers

Hydraulic Redistribution (HR) The passive movement of water from moist to drier portions of the soil profile via roots (Richards and Caldwell 1987; Burgess et al. 1998) Objective 2: Does Hydraulic lift/redistribution occur in forested wetlands? If yes, what is its impact on soil moisture ?

HYDRAULIC REDISTRIBUTION DAY DRIER WETTER

HYDRAULIC REDISTRIBUTION NIGHT DRIER WETTER

What does HR look like? (Brooks et al. 2002; Warren et al. 2007) A 50 cm 30 cm C B HR = C – B Soil H2O use = A – B HR/Soil H2O use = (C – B)/(A – B) (Normalized redistribution) (Brooks et al. 2002; Warren et al. 2007)

Seasonal tree water loss, total evapotranspiration and hydraulic redistribution Mean daily: ET = 2.55 (± 0.08) mm d-1 (0.10in) Etree = 1.86 (± 0.06) mm d-1(0.07in) HR = 0.37 (± 0.02) mm d-1(0.02in) HR contributes more to ET and Tree E during days of low evaporative demand

Effect of hydraulic redistribution on soil moisture content: HR delays soil drying HR is initiated when soil water potential begins to decline steeply as soil water content decreases (Warren et al. 2005)

HR delays soil drying and increases as soil dries. (how easily can water be extracted by roots)

Magnitude of recharge from HR Up to c. 0.9 mm/day (0.035 in) Mean ~0.45 mm/day (0.018 in) <20% of ET in July-August 40-45% of ET in September-October HR appears to play an important role in wetland hydrological balance by maintaining high summer and early fall E and ET.

HR contributed to the water balance of the plant responsible for it, but did it also contribute to the water balance of neighboring plants ? (Dawson 1996; Brooks et al. 2006). Did HR maintain shallow root hydraulic function (Domec et al. 2004) and did it affect the availability of nutrients through the plant roots?

Where are the hydraulic resistances along the soil-plant pathway? Leaf hydraulic conductance (Kleaf) as a component in a simplified electronic circuit analog of the whole-plant system

From May to November Ktree decreased by 25 % Seasonal variations in soil-to-leaf hydraulic conductance (Ktree), in root (Kroot) and leaf (Kleaf) hydraulic conductances From May to November Ktree decreased by 25 % From May to June, Kleaf decreased by 23%, whereas Kroot increased by 12% during the same period. As soil water content declined from September to November, Kleaf was reduced by 18% and Kroot by 40%

Compared to trunk and branches, roots and leaves had the highest loss of conductivity, and contributed to more than 75% of the total tree hydraulic resistance. Proportion of leaf, trunk to branches (Trunk-Branches) and root hydraulic resistances to the overall tree hydraulic resistance (1/Ktree)

Effect of soil moisture on predawn water potential and hydraulic conductances (Ktree and Kroot) As soil moisture dropped below 50% relative extractable water (REW), Kroot declined faster than Kleaf and became the dominant regulator of Ktree.

Where are the hydraulic resistances along the soil-plant pathway? The primary control of whole-tree conductance (Ktree) and tree transpiration occurred at the level of leaves (Kleaf) and roots (Kroot). Changes in the conductance of these components explained more than 70% of Ktree, and the conductance of trunk and branches had only limited influence on tree water use. Most of the above ground resistances occurred in leaves as compared with branches and that the pathway through the needles constituted a major part (i.e. 30-40%) of the whole tree hydraulic resistance to water flow