Of Cacti & Trees: A Diatribe of Drought!
As you walk outside your home or office on the hot days of Summer and Fall of 2019, allow that blast of heat that hits you to transport you 1,500 miles to the west, to a very different type of “woodland”. You see, if you find yourself walking among the towering plants of Arizona’s Tonto National Forest, you would find them not to be trees at all, but Saguaro Cacti. The Saguaro (pronounced: “sa-WAH-row”) forests occupy over 100,000 square miles of the Sonoran Desert in Arizona, California, and Sonora, Mexico. There may be up to 50,000 cacti in each square mile, many towering 30 feet or more in height. They are amazing survivors, perfectly adapted to a land that only receives 8-12 inches of rain per year, as compared with the 48-52 inches that we receive here in the Southeast. It is ironic, then, to learn that a Saguaro cactus is 94% water! A 20-foot plant without branches contains an estimated volume of 960 quarts of water, and, at 2.2 pounds per quart, that plant will weigh over a ton! The outside of a Saguaro is fluted – ribs composed of fleshy water storage tissue are alternated with furrows that allow the plant to expand when water is plentiful and contract when scarce. A well hydrated cactus may look like a smooth-walled cylinder, while that same plant will take on the appearance of a fluted column of a building during drier periods. Another key to the survival of the Saguaro is slow growth: it may take over 30 years for a plant to grow to 1 foot in height from a seed! That 20- foot tall plant mentioned earlier is likely 70 to 90 years old. Branches, which mostly grow to provide more fruits and seeds, begin to form when the plant is about 15 feet tall. Saguaros are generally spaced fairly far apart in the forest to compete for the scarce and sporadic water that generally comes in the form of thunderstorms in summer.
What brings on this tirade and tribute to the Saguaro cactus? Honestly, it is the result of struggle to find yet another new way to approach a topic discussed off and on for millennia – DROUGHT!
We are most certainly in the middle of one right now, with rainfall over 4 inches below normal at this writing and 3 months of daily high temperatures at 90 degrees F or above. Trees in the Eastern U.S. are no strangers to drought and we are certainly very familiar with the effects they have had on our woodlands. Surprisingly, the trees we are familiar with have many of the same types of adaptations for water conservation that cacti do. For example, the early leaf coloration and drop that we have seen this year is actually a water conservation measure: the leaves, in addition to being the photosynthetic food factories of the trees, are also the critical link in water movement from the roots and up through the plant. Shedding leaves reduces the surface area from which water vapor might escape, allowing trees to conserve water in the tissues of trunks and branches. This is immensely helpful as a drought-survival mechanism, as illustrated by a mature Beech tree, which can send 130 gallons of water a day into the atmosphere through its leaves. Like the cactus that can store immense quantities of water in its stems, trees, too are able to save water during times of the year when rainfall is plentiful, as in the winter when they are utilizing less for photosynthesis and transpiration (water released into the atmosphere for cooling and as a by-product of photosynthesis).
That inch per week of natural rainfall that I alluded to earlier, falling gently on the forests of Eastern North America, would be plenty for optimal growth and health of the trees if it would only fall evenly! If it did, there would a greatly diminished need for irrigation systems and supplemental watering of any kind. Such is not the case, however, and periods of drought can have many detrimental effects on trees, among them susceptibility to boring insects, increased flowering and fruiting in an effort to produce seed for the purposes of survival, root loss, and accelerated decline and death spirals, especially in trees that are older and /or are under stress from physical root damage. We have certainly seen these effects of the drought on our trees in recent weeks. The recent photos here show a drought-stressed Maple tree, as well as two in which the current drought has led to the end of their death spiral. What, if anything, can be done? While watering every tree in the Eastern forest during a drought is neither practical nor possible, it does make sense to take certain measures to protect the important trees that grace our landscapes. Here are a few tips:
- Get yourself a rain gauge – This inexpensive device will easily tell you how much water was provided by Mother Nature in a given week, and by knowing this, the amount you may have to supplement for your important trees. Remember, optimal rainfall for established trees is 1 inch of rainfall per week.
- Trees need approximately 5 gallons of water per inch per week – Diameter at Breast Height (DBH) if rainfall is lacking. If you have a Dogwood tree, for example, that is 6” DBH, it needs approximately 30 gallons of water per week to approximate 1 inch of rainfall. If the tree received ½” of rain in a hot, dry week, then 15 gallons of supplemental water would be optimal.
- If you have a large, mature tree that is important, it can be watered with an inexpensive sprinkler just as easily as a smaller tree, utilizing one or more rain gauges to determine how much water is being applied.
- Calibrate your irrigation system – Systems that are set to water a certain number of times per week for a certain amount of time are probably wasting a lot of water! Systems should be calibrated using one or more rain gauges toward that rate of 1 inch per week, and the system should only be used to supplement natural rainfall. The best systems have separate zones for trees and shrubs, turf, and flower beds, as the water requirements for these three types of plantings vary greatly.
Before a final word about cacti, let’s consider a few of the more drought-tolerant trees that we might consider planting. In the Native Category:
- Pin Oak
- Willow Oak
- Shumard Oak
- Overcup Oak
- Georgia Oak
- Bald Cypress
- Yaupon Holly
In the Exotic Category:
- Lusterleaf Holly
- Chinese Pistache
While the Saguaro Cactus is a tough competitor in an environment fraught with drought, it struggles on and may live 150 years or more. The longer-lived trees in our forest may exceed that lifespan by at least a century, enduring periods of drought such as we are seeing now. The distance and conditions that separate these 2 forms of flora have created plant life that may be very different on the outside, but have some striking similarities within.