Limits to Tree Height

2 November 2005PLB 203

The Soil-Plant-Atmosphere Continuum

Cohesion

Adhesion

Water flows from high pressure to low

pressure

-0.1 MPa

-1.0 MPa

-1.2 MPa

Lowest Pressure

Highest Pressure

Pa = N/m2

MPa = 106 Pa

Pressure of the Atmosphere:

135 MPa * ln (RH/100)

At 99% humidity pull of the atmosphere is

-1.36 MPa

At 30% humidity = -162 MPa

Pressure Differences

High Blood Pressure in Humans: 140 mm Hg

High Pressure in Plants: -12 MPa

= -91,200 mm Hg

OVER 600 TIMES THE PRESSURE EXPERIENCED BY HUMANS!!

Stem cavitation

PressureHigh (0 MPa)Low (-5 MPa)

% E

mbo

lism

0 %

50 %

100 %

What happens here?

Reduced stomatal conductanceReduced transpirationReduced photosynthesis

PressureHigh (0 MPa)Low (-5 MPa)

% E

mbo

lism

or %

loss

in c

ondu

ctiv

ity

0 %

50 %

100 %Ψ50 = Pressure Potential at 50 % Embolism

Sperry et al. 1995

Ψx = Ψs + Ψg + Ji* ri

Humid soil = 0 MPa

Pressure due to gravitiy = -0.0098 MPa m-1.

Transpirational pressure times path resistivity

≈ 1/3 of total pressure

-2 MPa = 0 MPa + -0.0098x + -2 MPa * (1/3)

Maximum height = 136 m

Ψ50 = -2 MPa

Some Ψ50’s for plants:

Poplar = -0.9 MPa

Button bush = -0.1 MPa

Juniper = -14 MPa

61 m

6.8 m

952 m

What heights could these plants theoretically reach on Mars where the pressure gradient of water is 0.4 times that on Earth?

Earth Mars

153 m

17 m

2381 m

Why aren’t these plants this tall?

Calculate the maximum height that these plants could obtain before reaching Ψ’s more negative than their Ψ50.

Hydrostatic gradient of water is -0.0098 MPa m-1.

Ψx = Ψs + Ψg + Ji* riHumid soil = 0 MPa

Transpirational pressure times path resistivity = 1/3 of total pressure

Dire

ctio

n of

Wat

er F

low

Path Resistivity

Pressure of stomatal closure

Cohesion

Adhesion

Cells with healthy turgor

Cells with unhealthy turgor