By Brandon M. Gallagher Watson
In Minnesota, where I live, we are having an incredibly wet season this year. We have had more days in June with rain than days without. We have not had two weekends in a row without rain since the first weekend of March, which is only because there was still snow coming down then. The windows where I am writing today are still dripping from last night’s thunderstorm, so it begs the question — why are we talking about drought? Clearly, it’s raining so the drought is over, right? Well, maybe, but drought isn’t a matter of it raining for a few weeks in the spring or not. Drought is a long-term climate issue, one that most of the United States is in the seventh or eighth year of, and it is taking its toll on the plants.
Last year, in fact, was one of the most stressful years for plants on record. In the northern parts of the country, folks were treated to a gloriously warm and early spring — up to six weeks ahead of schedule in many places. The trees and plants came out of dormancy to an environment where water and sunlight were in abundance and they responded accordingly. They dug into their reserves to put out flushes of growth and created large vessel elements to transport the available water quickly. All was well. Then someone turned off the tap.
By all accounts, the summer of 2012 was a historic drought. In fact, according to a recent report from the National Climatic Data Center, it was the largest drought in nearly 50 years, with 54.6 percent of the land in the lower 48 states was in drought conditions by the June. Measured by an algorithm called the “Palmer Drought Severity Index,” last year’s drought was the sixth worst drought since 1895 in terms of percent-affected land. To put this into perspective, the drought of 2012 edges the 1936 drought, and is slightly behind the 1934 drought that make up the Dust Bowl era. In the 1930s, the droughts began in the spring so the plants woke up to dry conditions and were able to plan accordingly. Thus, in terms of the drought resulting in plant stress, 2012 was perhaps the worst year ever recorded.
Even school children know that plants need water, but what are they using that water for? Like most living things, plants are mostly water; in fact, up to 90 percent of their mass is comprised of water. Not only does water provide the turgid pressure that keeps plants rigid, it is their primary means of temperature control. Nearly 95 percent of the water a plant takes up is transpired to keep the plant cool. Plants also use water for osmosis, chemical movement and photosynthesis.
Photosynthesis requires a watery matrix in which to take place, but it also requires carbon dioxide. CO2 enters the leaves through the stomates, which are little openings in the leaf that allow for gas exchange. This is where a plant is often in a quandary — it needs the stomates open to receive CO2, but if the stomates are open too long on a hot day, the plant loses too much water and become drought stressed. If the stomates stay closed, water stays in but the reduced CO2 input results in photorespiration, a much less efficient form of food production for the plant. Photorespiration can reduce a plant’s energy production by up to 25 percent, so the plants are constantly trying to avoid this. Some desert plants have evolved a means to open their stomates at night, store the CO2, and utilize it for photosynthesis when the sun is shining the next day.
The vast majority of plants, however, cannot photosynthesize this way and must open and close their stomates throughout the day, enough to let in CO2 but minimize the loss of water. Abscisic acid is a hormone produced throughout the plant that can be utilized to open and close stomates responsively as the conditions change. Commonly known as “the stress hormone,” abscisic acid is one of the plant’s first lines of defense against drought.
Trees respond a few different ways to drought. Mostly we think about wilting as a drought symptom, but usually by the time a plant is showing wilt it has been stressed for a while. Preclinical symptoms (those symptoms that occur before we can see them) include an increase in abscisic acid, and, in certain trees, a decrease in defense compounds. In order to compensate for the reduced photosynthesis, trees will often liquidate sugars stored in the roots. By doing this, they are run the risk of attracting opportunistic pathogens such as Armillaria. Trees also become more susceptible to secondary pests, such as boring insects and bark beetles. A healthy tree can fight off these invaders by pushing them out with sap and producing defense compounds to make them less tasty. When water supplies are low, the trees are unable to defend themselves and they become susceptible. In the natural forest, these insects play a key role in weeding out the weakened trees and strengthening the population. In an urban forest, an arborist may be called upon to help bolster the tree’s defenses against these pests.
Fortunately, drought is a tree stress that has several management options in the toolbox. The obvious remedy for lack of water is to simply add water. As an arborist, you will not likely be around to water your client’s tree, so you need to educate homeowners on proper watering. There are many formulas out there for calculating how much water a tree needs; but as your client is not likely going to know the bulk density of their sandy loam soil, it’s best to use simple rules-of-thumb. An easy one to pass along is five minutes of sprinkler time per inch of diameter once a week during the hot season. Thus for a 12-inch DBH tree, have the client run the sprinkler for an hour each week.
Although water alone will go a long way to keeping trees healthy when it’s not raining, adding mulch around the tree will also be a major improvement. Research has shown that replacing turf under trees with a few inches of organic mulch significantly improves tree roots by increasing soil aeration, retaining moisture, increasing nutrient availability, and, most importantly, removing the grass that is a fierce competitor for the same water resource as the tree. How much mulch should you recommend to your customers? Follow the lead of Dr. John Ball, who said, “Show me the property line and we’ll negotiate inwards.”
Stressed trees that are now susceptible to the secondary invaders, such as boring insects, can be protected by employing systemic insecticides. Systemic treatments, however, rely on transpiration, and thus water, to move within the tree. Informing the tree’s owner to keep the tree irrigated after the application is key to protecting drought-stressed plants. Utilizing highly soluble and fast-acting treatments, such dinotefuran (Transtect, Safari) and acephate (Lepitect), will have greater success in times of low moisture than slower-moving treatments, such as imidacloprid (Xytect, Merit).
A product widely used in the arborist market is palcobutrazol (Cambistat, Profile). This product is applied to base of the tree, and as it translocates to the canopy it will block the formation of gibberellic acid. This results in a reduction of vegetative growth, which has many uses in tree care from keeping trees out of power lines longer to keeping a suburban lawn tree at a reasonable size. It turns out that gibberellic acid is made from some of the same base compounds as other plant hormones like abscisic acid. So by reducing the amount of gibberellic acid that the tree is producing, we are increasing the amount of abscisic acid. This slower-growing tree will be putting more energy into root production than shoot production, and with the increased abscisic acid it can be more responsive to urban tree stress conditions. Treated trees show less scorch, and this is becoming one of the tools arborists employ to combat drought.
Drought stress is a very serious condition for urban trees, which are already dealing with hotter and drier conditions than their counterparts in the natural forest. Will 2013 return to average precipitation levels or be another drought year? Unfortunately, it is impossible to say for sure; but we do know that not many climate experts predict more rain and cooler temperatures any time soon. Fortunately, with the help of tree care professionals and better-educated homeowners, drought is a condition that can be managed, and the stress on trees can be reduced. Now we just need everyone to pray for rain this summer.
Brandon M. Gallagher Watson is director of communications at Rainbow Treecare Scientific Advancements, and is an ISA Certified Arborist (#MN-4086A).