Explain the process by which water ascends xylem vessels in the trunks of trees.
Cavitation is the creation of air-filled spaces within the columns of water in xylem vessels in trees. These spaces develop more frequently when water is moving through xylem vessels at a high rate. When such an air-filled space appears in a xylem vessel, a sound is produced that can be detected as a ‘click’ by a sensor positioned close to the xylem vessels in the trunk of a tree. Students studied how two environmental factors were related to cavitation rate in a Scots pine tree, Pinus sylvestris, across a 50-hour period. The two factors they investigated were: photosynthetically active radiation (PAR), which is the light energy available to plants that they absorb and use in photosynthesis, and wind speed. Cavitation rate was estimated by counting the clicks detected by the sensor. The results are shown in Fig. 3.1. Suggest the conclusions that can be drawn from the data in Fig. 3.1.