Effects of Summer Heat on Landscape Trees
Every summer the desert southwest reliably provides high temperature extreme that are unique to our region. The only measure that sets one summer apart from another is the magnitude and duration of the heat. In this era of urban heat islands, temperature extremes are not limited to only the daylight hours. Every year we are seeing more record-breaking high nighttime temperatures.
Locations that can benefit most from the shade and cooling effects of landscape trees, are also areas that present difficult growing conditions. Anyone who has stood in the middle of an asphalt parking lot or next to south facing walls or windows in July or August can appreciate how these surfaces seem to amplify the heat of the desert sun. Imagine the heat stress these settings present to landscape trees, 24 hours a day.
Walls (masonry or glass) and paved surfaces (concrete, brick, or asphalt) serve to both reflect and absorb (store) summer heat. During daylight hours reflected sunlight and heat can significantly increase the temperature of the environment surrounding landscape plants. The radiant energy of the sun also acts to heat these surfaces. At night, heat accumulated during the daylight hours is released back into the surrounding environment. The net effect of this re-radiated heat is to create a microclimate that does not experience a drop in temperature during nighttime hours. The combination of reflected and radiated heat creates an environment where trees are subjected to heat stress 24 hours a day during the bulk of the summer months.
The most extreme conditions are found where paved/built surfaces extend, uninterrupted, for acres and are only occasionally dotted with landscape plantings. These "heat island' conditions can be found in commercial, residential, and municipal landscape settings and are particularly common in parking lots and large commercial spaces like warehouse complexes and shopping centers. Landscape plans and maintenance practices need to appreciate the physical conditions within the landscape. Evaluate the orientation of structures and the amount and distribution of paved surfaces when determining where and what type of landscape trees are to be planted.
Studies have shown that to optimize the benefit of tree shade in cooling homes and buildings it is best to plant trees in front of the south and southwest facing walls. Leave enough space between the trunk and the wall(s) so the "mature" canopy has adequate room to expand and to provide for the maximum amount of shade. Providing space between walls and trees also allows for air circulation that can further reduce the impact of reflected heat on the trees.
It is a common misconception that all desert tree species tolerate extreme heat equally. Conditions in urban heat islands are vastly more severe than any native desert setting. In the desert, leaf canopies extend all the way to the ground, providing a dome of shade that reduces surface evaporation of precious moisture and forms a boundary layer of still air that forces drying wind up and over the leaf canopy rather than through it.
Watering schedules can be modified, by increasing the frequency and duration of irrigations, to reduce the effects of heat stress and replace water lost to uncharacteristically high evapotranspiration. This strategy may require multiple adjustments to automated irrigation time clocks, during the summer months, to respond to the unique conditions of a given summer. To avoid the risk of overwatering, these irrigation changes must be responsive to the tolerance of individual desert trees species to periodically saturated soils. Be mindful of other sources of water entering the landscape (lawn/shrub irrigation, sheet drainage from rain etc.). Well established, maturing, Mesquites, Sweet and Shoestring Acacia, Desert Willow and Palo Verdes are reasonably tolerant of mild, summer over irrigation. Recently transplanted Palo Verdes, Ironwoods, Acacia aneura (Mulga) and A. craspedocarpa (Leather Leaf Acacia) are especially vulnerable to damage from excess irrigation. Moderating temperatures, as the summer progresses, will require that irrigation time clocks be, again, readjusted in response to changing conditions and temperatures.
Tree pruning should be completed or halted during and in anticipation of periods of extreme summer heat. Removing large portions of the tree canopy (more than 20%) during anyone pruning session can lead to aggressive, unwanted re-growth, limited root development and increased vulnerability to sunburn injuries that can be colonized by wood boring insects. Such damage is, effectively, irreversible. No more than 20% of the tree foliage should be removed at any one time with 80% of this pruning concentrated on the newest growth on the outer third of the tree canopy. Pruning should be a year round practice and should be focused on tree growth and form management, encouraging optimal above and below ground growth and compliment the natural form of the tree.
Evaluating the site for potential heat problems. Careful selection of appropriate tree species and proper tree placement can reduce the detrimental effects of "heat island" microclimates within the landscape and help insure tree vigor and durability. An ounce of prevention……..