A new theory of tree sap flow

Area of Science:

• Plant Physiology
• Biophysics
• Forestry Science

Background:

• The established theory of water transport in trees, driven by capillary action and root pressure, faces physical limitations regarding height.
• The capillary effect in nature is limited to approximately 1 meter, posing challenges for tall trees.
• Open-air gaps in the xylem prevent water uptake against gravity due to air aspiration.

Purpose of the Study:

• To introduce a novel theory of three-sap flow in trees.
• To validate this new theory using experimental observations and previously published data.
• To present direct measurement data supporting the proposed mechanism of water transport.

Main Methods:

• Utilized a mechanical pressure-sensing transmitter-amplifier to measure minute changes in tree diameter.
• Employed high-resolution computed tomography (CT) to visualize water distribution within oak tree cross-sections.
• Investigated the relationship between tree diameter fluctuations and water transport dynamics.

Main Results:

• Observed that tree diameter increases during the night (suction phase) and decreases during the day (pressure phase).
• CT scans revealed that the outer, living wood ring is water-rich, with pathways to internal storage.
• Experimental data aligns with literature showing similar diurnal diameter changes in trees.

Conclusions:

• Water transport in trees is influenced by diurnal pressure variations, with nocturnal suction and diurnal pressure phases.
• The findings challenge the sole reliance on physical mechanical laws for water transport, suggesting complex physiological and biophysical processes.
• The segmented nature of the xylem in tall trees accommodates high hydrostatic pressure.