This article is part of a recurring series in collaboration with Bartlett Tree Experts on the seasonal expressions of phenology – the study of biological cycles and the timing and patterns of events in the natural world.
On warm summer days, we look to a tree for a shady spot to sit, and while it provides relief from the sun, a lot more is going on right above our heads.
Leaves have chlorophyll in their cells that absorb sunlight for energy and reflect a fraction of that light to give leaves their color. Through a process called photosynthesis, the tree uses this solar energy captured by chlorophyll to convert water and carbon dioxide into sugar as well as oxygen that is released back into the environment.
The long sunlight hours and warm temperatures of summertime allow trees the opportunity to make as much food as possible for yearly growth and storage. However, when temperatures skyrocket and soil conditions get dry, trees become stressed and slow down this production.
TREES “SWEAT” TO KEEP COOL
Urban trees experience significantly more heat stress than trees in a natural environment. They tend to be planted individually, lacking the cooling benefit of nearby trees, and have higher sun exposure to their entire canopy. These trees also experience heat radiating from buildings, roads, and other impervious ground surfaces, which increases the air temperature surrounding them. To keep cool in this hot environment, trees will increase a process called transpiration.
Transpiration is how trees “sweat” by signaling the roots to move water from the soil through the stem and up to the canopy to evaporate and cool the leaves.
When the soil becomes dry, the tree can no longer move water for transpiration and will send a signal to a structure called the stomata to close and stop releasing water. The stomata are a group of cells that open and close like doors, allowing water and air molecules to enter and exit the leaf.
This “shutdown” of normal leaf function can happen in the hottest part of the day to protect the tree, and in drought, can continue to occur, reducing photosynthesis.
SIGNS OF DROUGHT STRESS IN TREES
When a tree is experiencing this shutdown in the hot and dry summer, there are some signals of drought stress that you can recognize.
- LEAF WILTING: The first symptom of drought stress is often leaf wilting. Wilting occurs when there is a decrease of water in the leaves, resulting in drooping or curling leaves and a thin appearance.
- BROWNING/SCORCHING: If water stress continues, leaves may start to brown on the surface or around the edges and look scorched.
- LEAF SHEDDING: Later in the summer, some species, such as river birch and tulip poplar, will attempt to conserve water by shedding leaves from the canopy, which will often turn yellow and then fall.
- BARK SPLIT: When trees experience drought stress over an entire season, twigs die back, bark may split, and overall growth can be reduced.
HOW TO PREVENT DROUGHT STRESS
Trees adapt to the hot and dry conditions by modifying their physiological processes and can often be quite resilient. However, we can identify the symptoms of heat and drought stress in our trees and help prevent the negative effects of prolonged stress through proper watering and mulching.
Visit caseytrees.org/water to learn more about proper watering and mulching techniques, and be sure to keep your trees hydrated and healthy throughout the summer months.
Jessie Ward is a Research Technician with Bartlett Tree Research Laboratories and a research collaborator with Casey Trees. She is an ISA-certified Arborist and TRAQ certified. She holds a B.S. in Biology from James Madison University and M.S. in Forest Resources from Penn State, as well as field experience in production arboriculture, plant health care, and forest ecology.
