IPCC models predict that future climates will be characterised by more frequent and severe heat waves. Over the summer of 2013-14, Australia experienced a severe heat wave which is expected to become the norm for summer later this century. For scientists, how trees, and plants in general, respond to heat waves has become an area of intense research interest.
A recent study by Doronila, published in the International Journal of Phytoremediation, highlighted how three Eucalyptus species responded to a heat wave event. The response variable was sap flow, which can also be viewed as transpiration, and was measured using the SFM1 Sap Flow Meter. The study was located in Stawell, western Victoria, Australia, as part of a phytoremediation project at a gold mine:
On a mild day, where maximum temperature reaches 24°C and there is no cloud cover, tree sap flow (transpiration) shows this type of pattern:
Where temperatures are extreme, and there is still no cloud cover, tree sap flow (transpiration) shows this type of pattern:
Note that on a hot day tree sap flow increases in the morning and then suddenly drops off. It then levels off at some point to show a “shoulder effect”. Sap flow, or transpiration, does not completely shut down and go to zero. If it did, the tree is at risk of overheating and suffering from heat stress. Plants not only transpire in order to uptake CO2 for photosynthesis, but also to remain cool – similar to how humans sweat or dogs pant when we are active.
The study by Doronila demonstrated that different species show this “shoulder effect” at different temperatures. Some species show the effect around 30°C whereas the species Eucalyptus cladocalyx (Sugar Gum) did not show the effect until temperatures reached 40°C. Here is Figure 2 showing the heat wave event and how the three eucalypt species fared:
The dashed vertical line indicates a rainfall event and the cessation of the heat wave. The day following the rainfall event shows that sap flow in all three trees recovered. Therefore, in this heat wave event, the trees were able to tolerate the extreme temperatures by undergoing partial stomatal closure and keeping canopy temperatures relatively cool. Canopy temperature could have been measured with an infrared thermometer although this was not done in this study. It would seem the heat wave did not have any short-term detrimental effects on these trees however with longer term monitoring, and with longer and more severe heat wave, the outcome could possibly be different.