Drought stress and the 2003 Drought
Trees normally require an average of one inch of water per week. Many areas of the state have received less than one inch of water over the last two or three months. So trees in these areas are currently under severe drought stress. The primary effect of drought is damage and death of the roots. The feeder roots and root hairs are responsible for the uptake or water and nutrients. Because 99% of these roots occur in the top 3" of soil, they are the most at risk of damage as the soil dries. Damage to these fine roots creates a significant deficit in the entire plant, affecting a range of metabolic functions. These in turn, affect normal tree growth, maintenance and self-defense.
Between July 15th and Sept 8th, the precipitation received by most of the southern 2/3 of the state was 3" to 8" below normal, ranking the period among the worst in history (see maps). Trees most likely to show damage occur south of a line through Carlton County on the east, Wadena County in middle and Red Lake County in western Minnesota.
Effect on physiology
Plant growth occurs as a result of cell expansion, largely in response to water pressure, and cell thickening that occurs as cellulose (largely made up of water) is added to cell membranes. Available water, more than any other resource, determines the growth potential of individual trees, accounting for up to 80% of the size difference seen within tree species. Water deficits dramatically reduce both height and radial growth as well as bud production. Severe drought can limit future food production by reducing the number and vigor of leaf buds and thus the tree's ability to recover after the drought ends. Seed production may also drop for two to three years following a drought.
Water and nutrient uptake is driven by transpiration (the evaporation of water from plant leaves that pulls water through the plant from the soil, much like a soda straw). Stomates, pores in the leaves, control the amount of transpiration by opening or closing in response to heat, light and water loss. When closed, stomates reduce the amount of carbon dioxide available for photosynthesis and that reduces the amount of carbohydrates produced by the plant.
Besides plant growth and tissue maintenance, carbohydrates are used to form the basis of fats, proteins, growth regulators and many secondary metabolites. Secondary metabolites include tannins and alkaloids that are involved in defense. As the production of these compounds decrease, trees become susceptible to attack by opportunistic insects and disease organisms.
In a prolonged drought, there is a breakdown of the photosynthetic machinery itself. These tissues have to be repaired before normal processes can resume. In the meantime, carbohydrate reserves can be depleted. The result can be a downward spiral into decline and the eventual death of the tree.
The influence of secondary tree pests
For nearly all trees species, there are insect pests and disease organisms that have evolved to take advantage of weakened trees. Under normal conditions, these organisms act as recyclers, removing declining individuals, making room for others and releasing nutrients back into the ecosystem. However, during a stress event, populations of these pests can build to outbreak levels that can threaten stressed as well as healthy trees.
In pine, the two most common factors driving outbreaks of pine bark beetles are over-crowding and drought stress. Healthy pine trees produce enough pitch to push insects out or drown them in sap. Sap production in stressed pine is greatly reduced so beetles are able to successfully invade. All trees give off certain volatiles and bark beetles have evolved to pick up the change in volatiles given off by stressed pine. They flock to stressed trees in large numbers, overwhelming natural defenses, quickly killing the trees.
For oaks the two most common opportunistic pests are two-lined chestnut borer and Armillaria root rot. They often attack stressed trees in concert, increasing the stress and quickly killing those overly compromised. A combination of environmental factors and these two organisms can cause oak decline. Last year, northern Minnesota saw an increase in the incidence of two-lined chestnut as a result of the dry weather. This year some of those same areas are being hit again as well as many areas further south. While Armillaria is more difficult to survey, reports of root rot are increasing as well.
Like the two-lined chestnut borer, many other woodborers prefer stressed trees. The bronze birch borer attacks stressed birch trees and the native ash borer attacks stressed ash. The incidence of spider mites, scale insects, lace bugs and other sucking insects can also increase during a drought event. Canker organisms can advance much more rapidly in stressed trees and may girdle infected stems and branches. Armillaria and other decay organisms can influence tree health as well; so different trees may show a range of symptoms, including tree death.
Because stored food reserves may last one or several years after a stress event, related damage may not become evident that year. That can make diagnosis of plant health problems very difficult. The drought this summer followed a dry open winter that produced symptoms of stress in many, many trees. So right now, there are a lot of trees out there in a weakened condition. If the weather is favorable next year, and the trees were healthy prior to the drought, they will likely recover with little long-term damage. If the weather is not favorable next year, or if the trees were already stressed prior to the drought, the trees may begin to decline. If the trees had been predisposed to stress because of poor growing conditions, site disturbance or a history of damage, they may die this year or next with little prior warning. Foresters and plant health specialists have to shift through the numerous pest organisms that may be present and the past history of the site to determine the likely cause of any damage.
Symptoms of drought stress
Leaves may curl, roll, wilt, yellow, or scorch (leaf browning, usually around the margins) depending on the level of stress and the tree species. Leaf loss may result when an abscission layer is formed and may thus resemble normal fall color change, or when the leaves lose turgor pressure, wilt and die. Different tree species have different mechanisms for coping with drought stress, so the initial symptoms may vary. But they are usually more pronounced in the more exposed areas of the tree (for instance the top or south-facing branches) and the more exposed trees within a planting (for instance those along the street or facing the sun). Growth lose is common among trees demonstrating leaf scorch. But if the stress event is temporary or if the drought event occurs late in the season, when most trees are shutting down for the winter anyway, trees may recover the following year with minimal long-term impact.
Severe scorch (greater than 60%) early in the season may force trees to put on a second flush of growth. That may further deplete food reserves and diminish natural defenses. Secondary pests become may become a problem. Under extreme drought conditions, existing buds may shrivel and fail to leaf out the following spring. Branches may dieback and the tree may begin to decline.
Effect on fall color
Obviously, if the leaves are already brown, there's little chance of seeing the pretty reds and yellows typical of the fall season. Although they may be still green, curled, folded or shriveled leaves are likely to drop this fall without turning color. In either case, areas under severe drought stress are unlikely to see many fall colors.
Under normal conditions, fall colors depend on a number of factors, only one of which is moisture. Other factors include light exposure, temperature, nutrient content of the leaves, tree vigor and timing of the first frost. The pigments that create typical fall colors are produced and stored in the chloroplasts. Production of these compounds is influenced by nutrient and carbohydrate availability. Production is also influenced by chloroplast integrity. Damaged chloroplasts and/or nutrient deficiencies induced by limited root uptake can lower the concentration of these compounds and thus color intensity. Temperature and the timing of first frost also influences fall color.
All of these factors are very site specific. So while fall colors in general may not be as intense this fall, there will be areas of the state with normal, if not, spectacular color. Check with local authorities before planning fall color tours.
The future of your trees will depend on their health and vigor prior to the drought and the severity of the drought at your particular location. Growing conditions vary by individual site and micro-climatic. Variations in available resources can mean one tree thrives while another 50' away rapidly declines. So predicting future tree health on a broad scale is nearly impossible. However, we are likely to see a slight increase in the incidence of tree mortality across the entire area affected by the drought with isolated pockets of high mortality. Urban trees, recently disturbed or damaged trees, over-mature trees and those growing in sandy drought-prone soils are at more risk of mortality as a result of the current drought. In those cases, extra water is critical to tree survival.
What can be done?
Water your trees!!! Apply water, preferably with drip irrigation, deep-root waterer or soaker hose in a single application, once a week. Frequent, light waterings typically associated with turf grasses, are not only ineffective for trees, but may be detrimental because they promote shallow drought-prone root growth. A long, slow soaking allows water to percolate down to through the roots encouraging them to grow deeper into the soil.
Mulch the 'critical root zone'. This zone should be at least as wide as the drip line of the tree and preferably 1.5 times the tree's diameter in feet. For instance, a 4" diameter tree would receive a 6' ring of mulch. Mulch should be 4-6" deep, but kept away from the trunk itself. Mulch placed up against the trunk can promote decay fungi and stem girdling root development.
Minimize other stresses. The tree must divide existing food reserves among the demands of repair, maintenance, growth and defense. Any additional demands on its reserves will delay if not inhibit recovery.
Be sure of your growing conditions and plant trees suited to your site. Even the most drought tolerant tree species will suffer if planted in a site (or in a way) that limits the resources needed for vigorous growth. These include soil moisture, drainage and nutrient levels, light (or shade), rooting depth and volume, and freedom from salts and other toxic compounds. Protect your soil resources when building a new home. Yard trees don't do well when planted in compacted subsoils, whether or not your dress the site up with a few inches of topsoil. Avoid placing your septic tank or draining your pool near high-value trees. And protect them from injury.
Douglas, S. 2002. Minimizing the Long-term Effects of Drought on Trees and Shrubs. Obtained on 9/10/03 from The Connecticut Agricultural Experiment Station