Measuring sap flow in plants not only informs us of total plant water use but can also offer insights into drought responses.
Sap flow patterns in response to drought is not complicated – simply, as plants experience moisture deficit peak sap flow declines. This can be seen in this seven day data set for 2m tall Eucalyptus crenulata saplings growing in Armidale, NSW, Australia. The blue line are well watered saplings whereas the red line are saplings where water was withheld.
E. crenulata appears to be very drought sensitive. There were only 5 days where water was withheld and peak sap flow declined dramatically. Depending on a species sensitivity to drought, this decline in peak sap flow will be either quick, as seen above, or can take many more days. In all plants though, no matter the species, peak sap flow will decline when water is withheld or during drought conditions. The primary reason for this decline is partial stomatal closure which, in turn, is caused by the plant trying to maintain hydration, or plant water potential.
The relationship between sap flow and plant water potential, as measured by a PSY1 Stem Psychrometer, is seen in this graph:
These are data from the same eucalypt saplings but during a different time period. In this graph it can be clearly seen that as peak sap flow declines, plant water potential becomes more negative. That is, the measured plant is becoming less hydrated and more water stressed. On day 6 water was applied to the saplings and almost immediately plant water potential recovers. Although this species is drought sensitive, it appears it can also rebound very quickly once water is available. Note that peak sap flow did not recover to the amount prior to the drought until 2 days after watering. On the 3rd day, or day 9 of this dataset, peak sap flow is even higher than previously.
Plants can respond rapidly to irrigation and with sap flow instruments, such as the SFM1 Sap Flow Meter, we can measure and monitor this response.
For example, here is a dataset from a 3.5m tall Acer (maple) tree which has also experience moisture deficit. On the sixth day, irrigation is applied and almost instantly there is a spike in sap flow. Over subsequent days sap flow returns to what it was earlier in the week.
Another interesting feature of this graph is the values on the y-axis. Note that sap flow has a peak between 100 and 120 cm3 per hour when there is plenty of soil moisture. Compare these values with the eucalypt seedling discussed above. The eucalypt seedling has peak values around 25 cm3 per hour. So the maple tree has four times the amount of sap flow than the eucalypt. The maple was 3.5m tall whereas the eucalypt was 2m tall so this definitely explains some of the difference. But it will be interesting to know if there were also differences in stomatal conductance, leaf area of total sapwood area.