Relationships between embolism, stem water tension, and diameter changes.

A model for embolism in the sapflow process was developed, in which embolism is described as a physical process linked to real physical properties of the conduits and the thermodynamic state of water. Different mechanisms leading to embolism and their effect on water relations and especially diurnal diameter changes in a tree were examined. The mechanisms of heterogeneous nucleation, air-seeding, and bubble growth have been considered. The significance of embolism has been revealed here by examining diameter changes, which is an easily measurable quantity under field conditions. The most fundamental effects of embolism on sapflow are decrease in permeability and release of water from embolizing conduits to the transpiration stream. These can be indirectly detected by observing diameter changes. If possible changes in elasticity are not accounted for, embolism generally tends to enhance the amplitude of the diurnal diameter changes due to reduced permeability and increased tensions. In the case of reduced elasticity, embolism gives rise to smaller amplitudes of diameter changes.