Measuring Xylem Water Potential

Measurements of xylem water potentials can be made during the growing season in an effort to understand the seasonal pattern of soil moisture and tree water status under natural or experimentally manipulated conditions.

Relevant links:


  • To measure the predawn and midday xylem water potential of trees during the growing season.
  • To identify the timing and magnitude of any periods of water stress.
  • To measure the differences in growing season plant and soil water status between trees growing under different conditions.


Xylem water potentials of are measured using a pressure the pressure bomb method using a PMS instrument. All measurements are made on Abies lasiocarpa twigs from trees in a particular size class (large trees in 2010, probably saplings in the future). Twigs are excised from south-facing (downhill) branches at a consistent height above the ground. These twigs are stripped of phloem at the cut end and measured in the chamber within 10 minutes of being cut. Midday measurements are made between 1 and 5pm, and predawn measurements are made between 4 and 7am on consecutive days. Days immediately following rain events are avoided.

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