Heat Stress in Urban Trees: A look at how trees respond to high temperatures and reflected heat, especially in the built environment
Julianne Schieffer > Penn State Cooperative Extension Educator & Regional Urban Forester
Long, hot summer days tax trees just as it does us. Due to their fixed locations, going indoors to enjoy air conditioning never becomes an option for a tree. Although water deficit is just one dangerous effect on a plant, this article will concentrate on the impact of heat on trees.
Let’s look at how heat, a form of energy that causes a temperature difference, influences a tree. Direct energy from the sun can get reflected from surfaces surrounding the tree, radiate from local materials, or move in a circular motion as warmer air rises and cooler air drops in a process called convection. Advection, where the transfer of energy flows horizontally, typically in the form of heated air (for example, hot breezes), is the last process. Imagine at the nano-level how this heat energy transfer makes molecules move quicker, and the space between them expand.
High temperatures initiate a physical response in both humans and trees. Humans sweat while trees transpire. Fluids exit our bodies through skin pores to regulate our body temperature and remove certain compounds from our circulation. Plants transpire by moving water to evaporate from leaves, stems, and flowers. Lenticels on bark, twigs, stems, and stomates on leaves facilitate this movement.
Looking at the big tree picture, each temperature increase of 18°F, beginning at 40°F where water is densest, leads to a physical doubling of respiration and water loss in a plant. Respiration actively involves the breakdown of sugars from photosynthesis, releasing carbon dioxide and water. Those small increases in site temperature can greatly increase site water demands. Less water is available for tree life functions as a greater share of a site’s moisture is physically used to dissipate heat. Trees under heat loads need extra water.
Hot temperatures damage many old, young, and soil-limited trees. Site heat loads from parking lots, planting along streets or open squares, surrounding pavements, or convection and advection lead to several tree health symptoms. Those different sources of heat energy combine, causing tree tissue temperatures to increase, relative humidity to fall, and rising air and surface temperatures surrounding a tree. This additional heat load forces a tree, through physical processes of water loss, to lose more water in dissipating heat, not making food.
Let’s think about the soil surrounding our tree. A soil’s surface can both reflect heat and absorb it. In full sunlight, soils can reach 140°F. This heat can radiate and reflect into a landscape and onto trees, causing tremendous heat loading. Excessive heat loading causes large amounts of water to transpire, initiates metabolic problems, and can generate heat lesions just above the ground/tree contact juncture (root collar/stem base area). Look for heat lesions on the south / southwest side of stems months after the damaging event.
The approximate thermal death threshold for trees in a temperate zone is 115°F depending on their age, thermal mass, water tissue content, and the ability of the species to adjust. Optimum growing conditions for most trees range from 70°F to 85°F. Tree temperatures usually run around the air temperature (+ or – 4°F). Photosynthesis rates generally double every 18°F (10°C) until 94°F and then rapidly fall off. For this reason alone, it makes sense not to prune out the interior branches on a tree to “improve wind resistance” because the tree relies on these cooler inner branches for photosynthesis in extreme heat.
Tree tactics to dissipate heat include active transpiration and convection into the ambient air. Moist soil around the tree aids convection and evaporation. Heat radiated to tree surroundings and wind cooling keep tree temperatures near air temperatures. Without the water used for transpirational heat dissipation or “cooling,” sometimes radiated heat from surroundings adds to a tree’s heat load and increases associated water demand beyond availability. Working with this knowledge means that sometimes planting trees with companion vegetation allows healthy growth by blocking sunlight and keeping the soil moist with passive shading. A stratified vegetative canopy truly acts as an umbrella in many beneficial ways.
Now, let’s look at what leads us to first noticing the visual effects of heat on a tree. Water shortages and heat accumulate in leaves and secondarily, in the cambial and phloem area of twigs and branches. Increased temperatures escalate vapor pressure deficits between leaves and the atmosphere, as well as expand the water loss rate from other tree layers. Leaves and twigs wilt, then physically scorch, senesce or color prematurely, branches and stems sunburn, and shoot and root growth become inhibited.
On the cellular level, envision a wrapped chocolate bar melting. A double layer of lipids (fat/oils) composes tree cell membranes, keeping cellular contents intact and functioning. With rising temperatures, cells use two strategies to maintain life: increase the saturated fat proportion in membranes and increase structural proteins holding membranes together. Extreme heat loads may mean membranes become more liquid and permeable (melt), and contents leak. Enzymes and structural proteins denature as temperatures continue to climb, and the respirational by-products produce toxic materials that are difficult to transport away, destroy, compartmentalize, or excrete. Tree cell death results.
An extremely dangerous form of heat transfer is one we often overlook. Advected heat (transfer of heat or matter by the flow of a fluid), like that found over a large, paved area, heats the air above the surface and drives down relative humidity. Wind pushes this air over surrounding landscapes, heating and drying tree tissues in passing. Leaves quickly killed by heat are usually held on a tree by tough xylem tissues and lack an effective abscission zone. Realizing this, we can use structures and topographic features to modify or block advected heat flows across a site to protect perimeter trees tackled by this wind-driven heat.
Besides watering, sprinkling, or misting to reduce temperature and keep our trees water-efficient, let’s explore other measures to reduce heat loads or further damaging heat-stressed trees:
- Partial shading to reduce total incoming radiation but not filter photosynthetically active radiation (use companion vegetation or shading structures);
- Using colorants and surface treatments around landscapes and on trees to reflect and dissipate radiative heat;
- Block or channel advected heat away from trees and soils (like with berms, wooden walls, or other engineered structures to slow wind);
- Use low-density, organic surface covers, mulches, or composted materials that minimize water loss while avoiding on-site heat loading or preventing oxygen movement to roots;
- Avoid nitrogen fertilizer applications in or around trees during hot or dry periods and resume only after full leaf expansion in the following growing season;
- Prevent or minimize any soil active / osmotically active soil additions that increase the salt index or utilize soil water for dilution or activation;
- Be cautious of pesticide applications (active ingredients, carriers, wetting agents, and surface adherence) and performance under hot temperatures, low water availability, and with damaged trees;
- Minimize green-wood pruning due to the trade-offs between wounding responses, transpiration loads, and food storage reserve availability;
- Judiciously only prune dead, diseased, damaged, or crossing branches within the interior of a tree;
- Utilization of well-designed and constructed active shade structures in the landscape like arbors and trellises and
- Establish better tree-literate design and maintenance practices that deal with heat/water problems while monitoring other stresses (treat causes, not symptoms!).
Trees, hot temperatures, and water deficits are intimately bound together, and another potential way climate change may impact us and our environment. We need trees and a fit environment to keep everyone and everything healthy. Understanding the complex stresses heat places upon a tree can help guide us to avoid or mitigate potential damage to our habitat and, ultimately, ourselves.
References
Coder, Kim D. 2012. Drought, Heat & Trees. University of Georgia, Warnell School of Forestry & Natural Resources Outreach Monograph WSFNR12-10. Pp.56.
Teskey, R., Wertin, T., Baeweraerts, I., Ameye, M., McGuire, M.A., Steppe, K. 2015. Responses of tree species to heat waves and extreme heat events. Plant, Cell & Environment 38(9): 1699-1712.
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