Proudly Serving

Understanding Heat Stress on Trees

Heat Wave Trees

With rising temperatures, homeowners across the United States are increasingly facing the challenges of heat stress on their trees. The summer of 2023 brought record-breaking heat to several US states, with Illinois reaching over 100*F. Such extreme conditions are becoming more frequent and intense, making it crucial for homeowners to understand how heat stress affects their trees and how to mitigate its impact.

What is Heat Stress?

Heat stress occurs when trees are exposed to temperatures that exceed their normal tolerance levels. For many tree species, temperatures above 95*F to 100.4*F can lead to significant physiological stress. During these extreme heat events, trees can suffer from reduced photosynthetic function, which impacts their ability to produce energy and grow. The damage often starts at the cellular level, particularly affecting the photosynthetic machinery within the leaves.

Photosynthesis Process

How Heat Affects Tree Physiology

One of the primary sites of heat damage in trees in the photosystem II (PSII), a critical component of the photosynthetic electron transport chain. When temperatures exceed the thermal tolerance limits of PSII, the system’s efficiency drops, leading to reduced photosynthesis and energy production. This damage can be detected using chlorophyl fluorescence (CF), a non-destructive method that measures changes in leaf chlorophyll fluorescence due to altered PSII activity. A decrease in the ratio of variable to maximum fluorescence (Fv/Fm) is indicative of heat stress.

Chlorophyll Fluorescence

The Impact of Heat Waves

Extreme heat waves highlight the vulnerability of urban trees to heat stress. With heat conditions increasing year after year, such conditions underscore the need for selecting heat-tolerant tree species for urban landscapes.

Strategies for Homeowners:

  • Choose Heat-Tolerant Species
    • Select tree species that are known for their heat tolerance. For instance, certain genotypes of the Acer genus (maples) are hardy and adaptable to varied soil conditions.
Types of Maple Leaves
  • Adequate Watering
    • During heat waves, ensure your trees receive adequate water. Deep watering helps maintain soil moisture levels, which is essential for root health and overall tree vitality.
tree image winter watering
  • Mulching
    • Apply a layer of mulch around the base of your trees. Mulch helps retain soil moisture, regulate soil temperature, and reduce water evaporation.
  • Shade and Wind Protection
    • Temporary shade structures or windbreaks can protect young or sensitive trees from direct sunlight and hot winds, reducing heat stress.
  • Monitor Tree Health
    • Regularly inspect your trees for signs of heat stress, such as wilting leaves, scorched leaf edges, or premature leaf drop. Early detection allows for timely interventions.

Recovery from Heat Stress

Research indicates that some tree species can recover from heat damage if conditions improve. For example, studies on Acer species show that certain genotypes can regain their photosynthetic efficiency over a period of 72 hours following heat exposure. However, not all trees recover at the same rate. Trees that can quickly rebound to their original physiological levels are more likely to survive future heat episodes.


As temperatures continue to rise, homeowners must be proactive in managing heat stress on their trees. By selecting heat-tolerant species, ensuring proper watering and mulching, providing shade, and monitoring tree health, you can help your trees withstand extreme heat events. Investing in these practices not only protects your landscape but also contributes to the resilience and beauty of your urban environment.

For personalized advice and professional tree care services, contact our certified arborists at Homer Tree Care. Together, we can ensure your trees thrive, even in the face of rising temperatures.

Request a Quote

Request a Free Quote