Limits to Tree Height 2 November 2005 PLB 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/m 2 MPa = 10 6 Pa Pressure of the Atmosphere: 135 MPa * ln (RH/100) At 99% humidity pull of the atmosphere is 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

Pressure High (0 MPa) Low (-5 MPa) % Embolism 0 % 50 % 100 % What happens here? Reduced stomatal conductance Reduced transpiration Reduced photosynthesis

Pressure High (0 MPa) Low (-5 MPa) % Embolism or % loss in conductivity 0 % 50 % 100 % Ψ 50 = Pressure Potential at 50 % Embolism

Sperry et al. 1995

Ψ x = Ψ s + Ψ g + J i * r i Humid soil = 0 MPa Pressure due to gravitiy = MPa m -1. Transpirational pressure times path resistivity ≈ 1/3 of total pressure -2 MPa = 0 MPa x + -2 MPa * (1/3) Maximum height = 136 m Ψ 50 = -2 MPa

Some Ψ 50 ’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 Ψ 50. Hydrostatic gradient of water is MPa m -1. Ψ x = Ψ s + Ψ g + J i * r i Humid soil = 0 MPa Transpirational pressure times path resistivity = 1/3 of total pressure

Direction of Water Flow Path Resistivity Pressure of stomatal closure Cohesion Adhesion

Cells with healthy turgor Cells with unhealthy turgor