Last month, daytime temperatures were higher than Pacific Northwest trees would have liked. Sustained heat caused leaves’ stoma to remain shut and thus the trees were not transpiring or photosynthesizing properly. Over long hours of elevated temperature, the exposed leaves, particularly to the south and the west side, became brown or sun scorched — a tree’s equivalent of a sunburn.
Transpiration is vital for plants as about 95% of the water taken up by the plant is used for transpiration leaving only 5% for growth.
Transpiration is a process where water is being absorbed from the soil via roots, subsequently pulled upwards to the leaves. A leaf with ample water can photosynthesize and be cooled off when water evaporates through open holes called stoma. Opening or closing of the stoma is aided by two cells on its opposite ends. It is water content and electrolyte content in the leaf that dictates opening or closing of the structure.
In extreme heat without an ample source of water, a tree will choose to conserve water and keep the stoma shut, which in turn will prevent photosynthesis and thus growth. A tree may also decide to drop leaves to survive the drought spell because it cannot transport enough nutrients to them. The first sign of a tree under heat stress is browning of the leaves.
In our current drought condition, it may become necessary to deep water trees overnight and mulch the area around the base of a tree to reduce the water loss from soil. Trees can recover from the stress over time, but the recovery lengthens if the conditions persist.
It is worth noting that dry, brown canopies are more susceptible to catching fire.