The combination of shade, evaporation and convection air currents can lower the temperature beneath a big tree by 25 degrees. In fact, one mature tree produces as much cooling as fifteen room size air conditioners.
Trees are also pollution sinks. They collect pollutants and particulate ash from the air (by both absorption and adsorption). And as they photosynthesize, they remove carbon dioxide from the air and manufacture oxygen, which is especially useful in urban areas with poor air quality. Trees give back a great deal in exchange for a little loam and some water.
For the homeowner in a sultry climate, large trees are worth their weight in electric bills. A grove of trees will always provide a cooler spot. The higher the air temperature, the more leaf surface needed to produce enough cooling. Also, the higher the leaf canopy needs to be to provide maximum air movement.
If a tree is expected to cool efficiently, it must evaporate water vapor from its leaves, and so it is essential for it to have adequate water at all times. If the ground dries trees don’t have as much water in their leaves to give off, nor ground water in their roots to replace what they lose.
Desert plants don’t cool as much because in order to protect themselves from drought and prevent evaporation and water loss, they expose a minimum of leaf surface. Where adequate irrigation isn’t possible because of water restrictions, drought-resistant trees might have to be used. They will still provide shade, but much less evaporation of water will take place from their leaves, so they are less efficient in cooling the air.