We are not always fully aware of the impact that heat has on our trees. The effects of heat damage are more subtle and appear in many different parts of the plant. Cold can kill instantly, but plant death from heat is slow and lingering. The heat damage is always linked to the lack of water available to the trees. Leaves droop, chlorophyll production quits, leaves turn white or brown or fall off, stress moves in and the tree and roots stops growing, insects and pathogens attack. With depletion of water and intense heat desiccation elevates and the growth enzymes deactivate and mortality is eminent. Plant tissue must contain sufficient amounts of water to keep their cells turgid and sustain the plants processes of chemical and energy transport. Other factors also come into play -- depletion of soil gases, nitrogen and oxygen, soil pH, lack of nutrients, increase soil toxins and soil based microbes go into a dormant mode. All this can screw up a tree's metabolism. Call it heat/drought stress or summer scorch, it all comes down to tree mortality.
Watering directly at the roots with a soaker hose helps conserves water and reduces evaporation and run off, and the tree becomes more efficient in water uptake. The lack of water in the root zone controls the availability of oxygen which is required for respiration and lack of water can lead to soil toxin build up and almost total oxygen depletion and availability.
Hot, dry air movement will have a devastating effect and cause rapid dehydration. The tree loses water faster than it can replenish it's water needs. Man-made structures and asphalt landscapes also increase the heat and air movement factors.
Light is essential for photosynthesis-providing energy for growth and development. Light also provides heat, which affects plant temperature. Day length is also critical in regulating growth and development. The long hot, dry days of summer add substantial to heat and stress and have a profound effect on the tree's survival.
Our soils are high pH alkaline and this greatly affects the tree's ability to absorb water and nutrients. With the lack of soil moisture, soil based microbes go dormant and toxins increase, creating more problems for trees.
Trees vary greatly in the ratio and form of nutrients they need for consistent healthy plant growth. Some trees tolerate heat and drought better than others and some adjust to these conditions by simply dropping leaves, thus reducing photosynthesis and lowering energy growth requirements, basically shutting down into a semi-dormant state for preservation of life. Like any factory, if trees cannot receive the required raw material to create a finished product, then production has to slow down, halt or come to total shut down. Death.
(See article "Leaf Scorch")