Foundational ecohydrology

• leaf open stomata to let C02 in and water vapour + O2 out

• this process creates high negative pressure in the leaf

• soil pressure is less negative

• water is transported via xylem

E = evaporative flux density

gL = diffusional conductance of the leaf

Xi = mole fraction of water at the surface of palisade and mesophyl cells

Xo = mole fraction of water vapour surrounding the leaf

• Solar radiation (W/m2)

• Temperature (°C)

• Specific humidity deficit (g/kg)

• Soil moisture deficit (mm)

net radiation

• way of expressing the difference between the potential maximum air moisture and the actual air moisture

• when VPD high -> large difference between actual air moisture and potential maximum air moisture -> gradient between leaf moisture and air moisture is high -> facilitating transpiration -> plant lowers conductance to decrease consumption of soil water

• deeper roots lift water to shallower depth by differences in water potential

• at shallow depth water flows from root to soil

• mainly at night (no photosynthesis)

• beneficial for plants around with shallow root system

• benefits for the plant itself:

  • assists in seedling success

  • maintain root health (smoothing the soil moisture)

  • increase water availability in daytime

  • promotes nutrient uptake

When drought:

1. hormone signals from roots to leaf (lower the gs!)

2. root morphology changes -> loss of fine roots, insulation (suberisation) of root surfaces and formation of corky layers

3. -> decrease in whole plant hydraulic conductance -> water potential more negative at lower values of E

4. in case of very severe drought: fruther lowering of conductance due to cavitation of xylem vessels (following very low water potential) -> Embolism

• soil water saturation or ponding water

• inhibits gaseous exachange with atmosphere (oxygen in soil is used up) -> worst case: root death

• flood tolerant species: changes in root morphology: root thickening and increase of porosity (increase of rate of oxygen diffusion to root tops)

• floos sensitive species: root and shoot growth rapidly reduce, root tips may be damaged

• open:

  • with increased sunlight

  • with higer temperature (up to a point -> species dependend)

  • when leaf internal CO2 concentrations fall below ambient levels in the atmosphere

• close:

  • when soil is dry (chemical signal from roots (abscisic acid, ABA)

  • as realitve humidity near to leaf gets lower (only true for around half of known species)

  • when leaf water potential drops

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• PAR: photosythetically Active Radiation (visible light)

• leaf temperature

• VPD: vapour pressure deficit

• leaf water potential

depending on:

• solute potential (NaCl molecules attract water molecules

• pressure potential

• matric potential (soil is made of small particles, providing surfaces unto which water binds (hydrogen bonding and van der Waals forces)

• gravitational potential arises from the force that gravity exerts on water

-> solute and pressure potential are of interest to us

viewing hydraulic conductance as electrical conductance

1. driving force of sap ascent is continuous decrease in Px (xylem pressure) in direction of sap flow

2. evaporative flux density from leaves is proportional to negative of pressure gradient (-dPx/dx) at any point along the transpiration stream

• the longer the water path, the longer the stem Px need to drop (species specific is drop is small or large)

• biggest resistance is in vapour transport: leaf to atmosphere (determined by stomata)