SUMMARY

Xylem water flow rate as estimated with the stem tissue heat balance (THB) method in mature trees is a very sensitive indicator of different phenomena which can occur at any part of the soil-plant-atmosphere continuum. Some situations were observed when the conductive xylem was subjected to cavitation processes under natural and experimental conditions in coniferous and broadleaved species. The response of xylem water flow rate at the stem base to rain and rarely occurring abrupt depletion of soil water was compared with its response when tree trunks or branches were excised. The experiment represents the fastest possible change in flow that could ever occur. The response of flow in the trunk was slower when branches or tree tops of spruce were removed. The fastest changes observed under natural conditions for all species were also slower. The response of flow to rain was only half that caused by removal of trunks in birch, oak and maple. The abrupt depletion of soil water moisture was followed by a change in flow rate still less than in the case of rain for oak growing in sandy soil. The process of rapid soil water depletion typical of sandy soil is usually much slower in clay or loamy soils. Nevertheless, cavitation in xylem vessels can be expected anywhere and the records of xylem water flow rate made with sufficiently high resolution show a rapid decrease under high evaporation rate, which can be easily detected with TBH method. Some examples are given which suggest cavitation even under moderate water stress. […]