Hydraulic redistribution of soil water in two old-growth

coniferous forests: quantifying patterns and controls

Principles underlying the hydraulic redistribution of water by large

trees: A review of Warren, et al. 2007

Biologically-mediated hydraulic redistribution of water: How?

Plants photosynthesize during daylight

Plants transpire water through stomata to fix carbon for growth; occurs along water potential gradients

Plants cease transpiration during darkness

Roots still drawing water to saturate tissues, alter soil water potentials

Biologically-mediated hydraulic redistribution of water

• Hydraulic redistribution of soil water (HR), in which root systems passively transfer water from moist to drier soil along a water potential gradient () is a widespread phenomenon…”

– Warren, et al 2007.

• Upward or downward through soil• Also referred to as hydraulic lift

Biologically-mediated hydraulic redistribution of water

• Can be explained by water potential gradients– At night, the plant is at equilibrium with root

water potential

• Soil properties and hydraulic conductivity explained by Darcy’s Law

Darcy’s Law

• Formulated by Henry Darcy in 1856• Describes the rate of flow of water

through porous media• “The rate of flow of water in porous

media is proportional to, and in the direction of, the hydraulic gradient and inversely proportional to the thickness of the bed”

• Ward & Trimble Environ. Hydrology, P. 434

Darcy’s Law

Can be modified to find K/ instead of a velocity (V):

K = qL / Ah

K: Hydraulic conductivity

q: rate of outflow

L: length of core sample

A: Core cross-sectional area

h: depth of the constant head applied

-Ward & Trimble Environ. Hydrology, P. 79

The Research…

Warren, J.M., Meinzer, F.C., Brooks, J.R., Domec, J.C. & R. Coulombe (2007). Hydraulic redistribution of soil water in two old-growth coniferous forests: quantifying patterns and controls. New Phytologist, 173: 753-765.

Look to explain hydraulic lift through the natural variability in water potential gradients and seasonal courses of root conductivity (soil/plant biophysical properties)…

An experiment in the WA/OR Cascades is born…

Two old-growth forests

• Deschutes Nat. Forest (OR)– Metolius Research Natural Area

• Gifford Pinchot Nat. Forest (WA)– Wind River Canopy Crane Facility

• Both have pronounced dry seasons (50-150 days) and deeply-rooted (2-3m) conifers (Pseudotsuga menziesii & Pinus ponderosa)

Measuring water potential

• Soil measured using thermocopule psychrometers at depths 20-100cm

• Plant measured using pre-dawn removed shoots and a Scholander pressure chamber– Establish values for both P. ponderosa &

P. menziesii

Soil Volumetric Water Content ()

• Quantified with multi-sensor probes every 10-30 minutes at various locales

• Averaged for each depth

• Daily water-depletion rates were calculated between daily max and min

Darcy’s Law Revisited

Can be modified to find K/ instead of a velocity (V):

K = qL / Ah

K: Hydraulic conductivity

q: rate of outflow

L: length of core sample

A: Core cross-sectional area

h: depth of the constant head applied

-Ward & Trimble Environ. Hydrology, P. 79

Modification of Darcy’s Law by Warren, et al

• Qp= KA / L ≈ (100-PLC)A / L

• Qp: Potential water flux (max value= 1)

• A: Cross-sectional area

• K: Root hydraulic conductivity: Change in soil between two

points separated by distance L

• PLC: % loss of root conductivity

Putting it all together…

• They found a positive linear relationship between overnight recovery of and

• R2 > .7• For a given value of soil , soil

was .01-.05 Mpa greater during nocturnal periods than during daytime periods

What does it mean?

• Soil moisture affects understory plant performance, distribution

• Hydraulic lift may change with global climate– Reduced precipitation >> Larger role hydraulically

lifted water plays in ecosystems?

• Increased ambient CO2 means more closed/fewer stomata– Does this mean less transpiration (less negative

H20 potential in leaves?)

Future Work

• Global and continental runoff balance?• Global and continental streamflow?• Transpiration’s role in the global

hydrologic cycle with changing hydraulic lift?

• How do belowground biotic processes affect/ how are they affected by abiotic (water table, etc) processes?