We have all seen the devastating effects of drought in the Midwest last year. When reservoirs are depleted and crops are struggling to survive, everyone is aware of the severity of the problem.
Here in the New York area, we didn’t experience the scarcity of water that the Midwest felt. Yet 2012 was a hot, dry year, and many trees experienced a level of drought that could impact them for years to come. According to AccuWeather, 2012 was the warmest year since 1869 and precipitation was in the bottom quadrant. From late March through May, precipitation was so low that the U.S. Drought Monitor ratings ranged from ‘Abnormally Dry’ to ‘Moderate Drought’ in the New York Metro area. Only three months in the year exceeded the monthly average annual rainfall.
The long-term implications of drought can be serious. Scientists are still discovering new information about the process that allows trees to “drink.” Thanks to recent scientific research, we now have a greater insight into the mechanism of tree death from drought and a new appreciation of how vulnerable trees are.
Trees draw water through their roots and into the thread-like channels of their vascular system that distribute it to their most remote needles and leaves. In order to photosynthesize and grow, trees need to open the stomata (pores) on their leaves to take in carbon dioxide. As they do this, water evaporates. The water loss creates a suction effect that goes down all the way to the roots, where the water is replenished, similar to drinking through a straw.
When water is unavailable, this suction pressure increases, and air is drawn in. The air bubbles clog the channels and make it harder for a tree to get fluid to its leaves—like drinking from a broken straw. This “hydraulic failure” is the reason trees die from drought.
In addition to hydraulic failure, drought can impact a tree’s ability to open its stomata. Even when water is again available, some leaves are unable (or slow) to return to their work of photosynthesis, causing further dieback.
Dr. Brendan Choat from the University of Western Sydney and Dr. Steven Jansen from Ulm University in Germany lead a team of scientists that have studied hydraulic failure in trees worldwide. The results of their research show that about 70 percent of tree species have very little margin in the amount of drought they can endure before they experience hydraulic failure. Surprisingly, this was true for species that grow in marsh as well as desert habitats. The implications of this research are that a small change in the drought level of an area could have serious consequences for large numbers of trees.
Unlike perennials or even shrubs, which regenerate much of their above-ground structure every year, drought damage to trees is permanent. It can take five years to see the full effects of seasonal drought in a tree.
Watering trees in times of drought is extremely important. Even the largest tree typically has its roots concentrated within the top 12 inches of soil. They have no ability to suddenly find water when that layer is dry. For trees that are trying to survive on hilltops, rocky terrain or in clay soil, virtually no water is retained by the soil.
Trees will respond to drought initially by wilting, followed by dropping leaves or needles, and then having whole branches die. The damage can be irreversible. Signs of water insufficiency are often easier to spot in plants and shrubs. When a hydrangea starts to droop, you can assume that the tree next to it is thirsty as well.
Sophisticated systems are now available to monitor the moisture content of soil and automatically adjust watering to the appropriate level for plants, lawns or trees. These irrigation systems are now common in arid regions and have been found to reduce total water consumption. Giving trees the right amount of water at the right time can prevent hydraulic failure and also prevent the unnecessary waste of water in landscapes by up to 60 percent. I believe we’ll see this technology introduced in the Northeast over the next decade as water for our landscapes becomes a more precious commodity.