Trees in Drought
Dennis Swartzell, Arid Plant Solutions
Board Certified Master Arborist
Hardly a week passes without some comment in the media regarding the prolonged drought in the Southwest. News of receding reservoirs, wells going dry, and the plight of farmers working to keep their crops alive are constant reminders that water remains one of our most precious resources.
Those of us in the landscape industry have been keenly aware that restrictions on water use have created some difficult challenges. In the most severely impacted regions of the west there are often debates as to whether landscapes are even necessary. Unquestionably lawns are the first to be considered expendable when it comes to rationing irrigation water. Trees located in turf often suffer as a result and may exhibit diminished health, loss of limbs, and premature death if left unchecked.
Most of the tree species in urban landscapes require a considerable amount of water to stay alive. In fact the Desert Southwest Community Tree Guide indicates that mature desert trees require up to 4,000 gallons of water per year to remain healthy. Imagine the amount of water that more mesic species such as ash and mulberry require each year.
During the hottest months, the majority of the water absorbed by tree roots is used to keep the upper canopy cool. More than 90 percent of the water taken up by a tree during the summer months is lost to transpiration. Water is also necessary to keep soft tissues firm (turgid). The moisture inside of plant cells provides the necessary support to keep leaves and soft stems from wilting. Only a small fraction of the water absorbed is utilized in photosynthesis.
From a botanical perspective trees react to water deprivation in a variety of ways. The initial symptoms are typically difficult to recognize. Broad-leaf trees may wilt during the heat of day yet recover quickly in the evening. Temporary wilting is followed by leaves turning off-color. Sunscald may occur on leaves facing southern and western exposures.
In long-term drought marginal leaf necrosis, often referred to as leaf scorch, may be observed on broad-leaf species. If the soil and/or the water sources are high in salinity the edges of leaves may burn more readily. The leaf margins are lined with microscopic pores, called hydathodes, which function as relief valves for the leaf tissue so that any excess water taken in by the leaf does not cause cells walls to rupture. In the event of high salinity the excess water exuded in a process known as guttation may cause leaf burn along the margin. Often the necrotic line will become more prominent as the summer progresses.
In advanced drought conditions the damage becomes more severe. Symptoms include stunted growth, reduced leaf size, and more severe sunscald on the foliage and exposed bark tissue. If the deprivation continues most trees begin to exhibit branch dieback and limb shedding. For susceptible species attacks by flat-headed borers may be initiated. Sunscald on bark tissue is a prime point of entry for borers.
Trees native to arid regions have evolved to deal with drought in different ways. Many of our desert trees have diminutive leaf area which helps to reduce transpirational water loss. Silvery or gray leaf color helps to reflect sunlight and keep leaves cooler. To conserve water many desert trees often have diminished canopy size producing wide, squat forms with drooping limbs that help to shade the soil surface. During severe drought most of our tree species are capable of shedding leaves to enter a brief “summer dormancy” between rain events.
Photo courtesy of Ten Eyck Landscape Architects, Inc.
Some environmentalists are now saying that clean water is the “new oil”. As we look for reliable sources of fresh water such as drilling more and deeper wells there are concerns that a water crisis will occur sooner than later. Efforts to conserve water, especially in the landscape, should become a priority for our landscape designers and managers.
Most of us will agree that trees makeup the backbone of the landscape and for that reason they should be the primary recipients of our irrigation. From a maintenance standpoint there are several things that we can do.
Water infrequently and deeply. Deep irrigation cycles will help to push excess salts, a process known as leaching, out of the root zone. It takes a considerable amount of water to flush salinity in the soil. For example, it takes six inches of good quality water to reduce the salinity level of one foot of soil by 50%.
Minimize compaction. Root depth is predicated on the availability of oxygen in the soil. Soil compaction reduces air exchange with the atmosphere and limits the ability of trees to produce deeper root systems. Heavy equipment, vehicular and pedestrian traffic should be limited near established trees.
Infrequent irrigation cycles will encourage deeper rooting, but we must provide supplemental water when necessary. During the hottest months without significant rainfall we should consider deep soaks. The use of a deep water probe will provide multiple benefits: a deep irrigation cycle, leaching of excess salts and an opening for the introduction of oxygen. In fact, the vertical holes created by the probe may be backfilled with virtually any item including gravel, mulch or compost. This “vertical mulching” is highly beneficial to root development.
Avoid the application of fertilizers during drought conditions. Contrary to popular belief fertilizer is not a cure all. In fact, fertilizers are basically salts and the addition of more salt during dry spells can cause considerable damage to the root systems of our trees and further complicate the absorption of water by roots. Besides, trees do not need to be pushed into a growth cycle when stressed.
From a design standpoint we can really be proactive. Consider the following:
We all know that choosing the right tree for the space is important. Landscape designers should utilize the best species for harsh sites. Native or regionally adaptive trees should be matched to the site.
Landscape installation specifications should address proper soil preparation. Each tree should be provided ample rooting area to develop a full-size root system. Soils should be fractured over a large area to allow for unrestricted root development.
The irrigation system should be designed to account for the mature size of the tree and proportional root system. Rarely do we see the original irrigation system expanded as trees mature which often results in stunted growth, crown decline, and increased risk of wind throw.
Yes, the southwest is in a drought. Some say the current mega-drought began in the year 2000. While opinions vary on the role of global warming, few argue that drought in the southwest is a reality. We are foolish to pretend it does not exist and ignore it at the long term peril of the quality and aesthetic of our landscape. Coupled with the prospect that the population of the “Sun Corridor” will swell by over 7 million people by 2040, clearly water conservation will remain a major issue and our landscapes will likely be subjected to greater water use restrictions. Given our current, and likely long term, challenges, the use of regionally adapted trees, landscape designs that reflect the beauty of our surrounding deserts, and the application of appropriate horticultural practices, are an attractive, practical and logical alternative.
