Drought Stress, Its After-Effects And Management Implications

Non-woody feeder roots, usually located in the top 15 inches of soil, are particularly sensitive and are the first ones affected. When these roots dry, shrivel and become non-functional, a water deficit develops since the roots cannot provide water to the top of the plant. In addition, many metabolic changes occur that substantially alter the physiology of drought-stressed trees. Among these are changes in hormone levels and other physiological factors (e.g. factors that influence the number of leaf initials in buds for the next year or that are responsible for the closing of stomates).

Effects of drought are particularly severe on seedlings or new transplants because their roots occupy the uppermost layers of soil where the most rapid drying occurs. In addition, recent transplants typically lose feeder roots during the transplant process. For example, balled-and-burlapped trees are estimated to contain only 5 to 20 percent of their original root mass after digging. For container-grown trees, the medium in which the transplant is growing can be a major factor - many of the soilless mixes used for container stock are highly porous, dry out very quickly and are very difficult to re-wet. This situation creates moisture stress in the rootball regardless of the availability of water in the surrounding soil. The problem often continues until the roots grow beyond the rootball.

Contrary to popular opinion, it often takes woody transplants two years to become completely established in a new site. The rule of thumb is: for each inch of tree caliper, one year of recovery from transplant shock is necessary. As a consequence, these trees should be given extra care and attention during extended periods of drought.

Established trees and shrubs are also affected by drought, especially those planted in marginal sites, such as those with pavement over roots, those in pockets of soil on ledges or in sandy soils, or those that have been improperly planted. Drought can exacerbate even the subtlest of improper planting practices. Native plants are usually adapted to regional and seasonal fluctuations in the amounts of available water. Therefore, only unusually severe drought is likely to injure plants that are growing naturally in a given site. However, most of the trees that we deal with are landscape trees that we have selected and planted and are, therefore, frequently subject to drought stress.

SYMPTOMS. Symptoms of drought are manifest in many different ways depending on the plant species and the severity of the water deficit. However, it is generally agreed that symptoms are often not evident until some time after the event has occurred - even as much as one to two years later. Unfortunately, symptoms of drought stress are usually subtle, not specific, and not diagnostic. This makes accurate diagnosis very difficult.

Symptoms are quite variable and include loss of turgor in needles and leaves, drooping, wilting, curling, yellowing, premature leaf or needle drop, bark splitting, and tip and branch dieback. Leaves on deciduous trees often develop a marginal scorch and interveinal necrosis, whereas needles on evergreens turn brown at the tips or appear off-colored. Trees can also exhibit general thinning of the canopy, poor growth and stunting. Many woody species exhibit symptoms of general decline. In extreme cases, drought can result in tree death.

SECONDARY OR OPPORTNUNISTIC PROBLEMS. In addition to direct root damage, a significant secondary effect of drought is that it weakens trees and predisposes them to secondary invaders and opportunistic pest. Among the key secondary problems are:

1. Winter Injury - Drought-stressed trees are not as winter-hardy as their healthy counterparts.



2. Root Problems - Injured or weakened root systems are more susceptible to root rots. Example:
   • Armillaria Root Rot
     Causal agent: Armillaria mellea
     Hosts: many species, including pine, fir, oak and maple



3. Cankers - Weakened trees have slowed "defenses" and wound healing is inhibited. Examples:
   • Nectria Canker
     Causal agent: Nectria spp.
     Hosts: many species, including maple and birch
   • Hypoxylon Canker
     Causal agent: Hypoxylon spp.
     Hosts: many species, including oak



4. Wood Rots - Slowed defenses limit compartmentalization and reaction to invasion by wood rotting fungi. Examples:
   • Fomitopsis on red spruce
   • Fomes on maple
   • Polyporous



5. Sensitivity to Pesticides - Weakened trees are more sensitive to pesticides; compounds that do not cause problems for healthy trees can result from phytotoxicity on drought-stressed trees. Examples:
   • Various herbicides
   • Insecticides
   • Fungicides



6. Sensitivity to De-icing Salts - Weakened trees are more sensitive to de-icing salts, especially sodium chloride.



7. Miscellaneous Diseases - An increase in the frequency and severity of several diseases has been associated with drought stress. Examples:
   • Diplodia Tip Blight
     Causal agent: Sphaeropsis sapinea
     Hosts: pine, especially two- to three-needled species
   • Rhizosphaera Needlecast
     Causal agent: Ryizosphaera kalkhoffii
     Hosts: spruce, especially blue spruce
   • Cytospora Canker
     Causal agent: Leucostoma kunzei
     Hosts: spruce, especially blue spruce
   • Verticillium Wilt
     Causal agent: Verticillium spp.
     Hosts: many tree species, especially Japanese maple

MANAGEMENT STRATEGIES. While there is no cure for this problem, the effects of drought can be minimized by following some measures:

1. Water in periods of low soil moisture - Trees and shrubs require approximately one inch of water per week. Special attention to young trees is important. Water is best applied at one time as a slow, deep soaking of the entire root zone to a depth of approximately 12 to 18 inches. The length of time required to "deep-water" will vary depending on soil type and water pressure; clay soils usually require more time than sandy soils.



2. Select the appropriate site and follow good planting practices - Drought stress can magnify even the subtlest improper planting practices (e.g. planting too deep, too shallow, failure to remove or cut burlap).



3. Select native plants or match plant species to site conditions - Drought-sensitive (e.g. dogwood, some oaks, ash, birch) vs. drought-tolerant (e.g. most pines, many Prunus, larch).



4. Mulch to maintain soil moisture - Properly applied mulches can be very helpful. Mulches are usually applied 1 to 3 inches thick and spread evenly out to the drip line of the tree. It is also important to keep the mulch 6 to 12 inches away from the trunk. Mulches that are applied too thick or too close to the base of the tree - "volcano" or "pyramid" mulches - can be harmful. Irrigate prior to mulching.



5. Prune any dead or weakened tissues to avoid secondary problems



6. Maintain plant vigor by following good cultural practices - It is generally accepted that trees under stress should not be fertilized. However, applications of biostimulants, mycorrhizae or similar compounds can be beneficial and can help to stimulate root growth and regenerations.