Drought Stress, Tree Health, and Management Implications
By Dr. Sharon M. Douglas
Department of Plant Pathology and Ecology
The Connecticut Agricultural Experiment Station
123 Huntington Street
P. O. Box 1106
New Haven, CT 06504-1106
Telephone: (203) 974-8601 Fax: (203) 974-8502
Connecticut has been plagued by periods of extended dry weather since 1995 and the 2001 growing season was no exception. Symptoms associated with drought and its after-effects can be seen on forest and landscape trees throughout the state. The cumulative yearly precipitation total for 2001 was 4.8 inches below the 30-year average, making it the driest year since 1995. Spring and fall were exceptionally dry. Weather records from the Experiment Station’s Lockwood Farm in Mt. Carmel also reported rainfall deficits for the growing seasons of 1999, 1998, and 1997. This trend for abnormally low levels of precipitation has been occurring since 1995, which was characterized as the worst drought in 30 years and as the driest summer since 1944. As a consequence, many woody ornamentals including drought-sensitive as well as species which are normally considered drought-tolerant exhibited symptoms of drought stress.
Drought causes primary and secondary physical damage as well as physiological changes in trees. The primary physical effect of drought or dry soil conditions is direct damage to the roots and root death. Nonwoody 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 nonfunctional, a water deficit develops since the roots cannot provide water to the top of the plant. In addition, many metabolic changes occur which 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 5-20% 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: 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 their roots, those in pockets of soil on ledges or in sandy soils, or those that have been improperly planted. Drought can exacerbate even the most subtle 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 which we have selected and planted and are therefore frequently subject to drought stress.
Symptoms of drought are manifest in many different ways depending on the tree species and the severity of the water deficit. However, it is generally agreed that symptoms are often not evident until sometime 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 OPPORTUNISTIC 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 pests. 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.
a. Armillaria Root Rot: Causal Agent: Armillaria mellea
Hosts: many species including pine, fir, oak, maple
3. Cankers: weakened trees have slowed "defenses" and wound healing is inhibited.
a. Nectria Canker: Causal Agent: Nectria spp.
Hosts: many species, maple, birch
b. Hypoxylon Canker: Causal Agent: Hypoxylon spp.
Hosts: many species, oak
c. Bleeding Canker: Causal Agent: Phytophthora spp.
Hosts: many species, esp. beech
4. Wood Rots: slowed defenses limit compartmentalization and reaction to invasion by wood-rotting fungi.
a. Fomitopsis on red spruce
b. Fomes on maple
5. Sensitivity to Pesticides: weakened trees are more sensitive to pesticides; compounds that do not cause problems for healthy trees can result in 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.
a. Diplodia Tip Blight: Causal Agent: Sphaeropsis sapinea
Hosts: pine, esp. two- and three-needled species
b. Rhizosphaera Needlecast Causal Agent: Rhizosphaera kalkhoffii
Hosts: spruce, esp. blue spruce
c. Cytospora Canker: Causal Agent: Leucostoma kunzei
Hosts: spruce, esp. blue spruce
d. Verticillium Wilt: Causal Agent: Verticillium spp.
Hosts: many tree species, esp. Japanese maple
e. Dutch Elm Disease: Causal Agent: Ophiostoma ulmi and O. novo-ulmi
f. Ash Yellows: Causal Agent: Phytoplasma
Hosts: ash, especially white and green ash
g. Elm Yellows: Causal Agent: Phytoplasma
While there is no cure for this problem, the effects of drought can be minimized by following some preventative 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 12-18 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. Frequent, light, surface watering will not help the tree and can actually cause harm by promoting growth of surface roots. A deep soaking just before the ground freezes in the fall will also help the winter-hardiness of drought-stressed plants.
2. Select the appropriate site and follow good planting practices—
Drought stress can magnify even the most subtle improper planting practices (e.g., planting too deep or too shallow, or failure to remove or cut the burlap and/or the wire basket). When planting a tree, try to anticipate the water needs of that tree at maturity.
3. Select native plants or match plant species to site conditions—
Drought-sensitive (dogwood, some oaks, ash, birch) vs. drought-tolerant (most pines, many Prunus, larch).
4. Mulch to maintain soil moisture—
Properly applied mulches can be very helpful. Mulches are usually applied 1-3 inches thick and spread evenly out to the drip line of the tree. It is also important to keep the mulch 6-12 inches away from the trunk. Mulches that are applied too thickly or too close to the base of the tree ("volcano" or "pyramid" mulches) can be harmful!
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 regeneration.
Symptoms of drought stress are manifest in different ways depending on the plant species and the severity of the water deficit. Drought or dry soil conditions result in root damage and death. In addition to direct root damage, weakened trees are subject to many secondary problems including root rots, cankers, wood rots, and increased sensitivity to pesticides and de-icing salts. This fact sheet discusses methods to minimize the impact of this important stress factor.