Drought and Trees

Introduction

A tree must always be in equilibrium with its environment. Any time the tree is not in equilibrium the tree is stressed and must spend extra energy to survive. Trees can only react to their environment in genetically preset ways. People can help minimize tree stress by understanding them.

Water is the single most important substance for tree life. All the life processes of a tree take place in water. The most apparent use of water is for transporting materials from the roots to the shoots. (Leaves, twigs and branches).

As water moves from the soil through the roots and into the leaves it carries with it many essential elements that three needs to survive and grow. As water and elements move from the roots to the shoots, chemicals are added by the roots. The shoots are continuously informed by these chemicals of the water and nutrient status of the roots.

Water use in trees is primarily physical. There are a few points of biological control that override the physical process of water movement. The soil/root interactions, vascular system and leaves all provide resistance to water movement. Water movement and evaporation is a function of temperature and energy in the environment.

Trees act as conduits through which water passes. Instead of water evaporating at the soil surface, the tree provides an elevated surface for water evaporation. At the junction between tree and atmosphere is a biological control valve called a stomate. Throughout the entire water stream moving in a tree, the leaf stomate is the only portion that can be actively controlled by the tree to conserve water.

Drought has killed and will continue to kill trees. Drought leads to deceased rates of diameter and height growth, poor resistance to other stresses, disruption of food production and distribution and changes the timing and rate of physiological processes, like flower and fruit production.

Eighty to ninety percent of the variation in tree growth is because of water supply problems. Effects of drought occur throughout a tree.

The term "drought" denotes a period without precipitation, during which the wtaer content of the soil is reduced to such an extent that trees suffer from lack of water. Water deficits in a tree are formed when transpiration (the process by which tree leaves emit moisture and oxygen) exceeds the water supply available to the leaf.

Trees cope with drought generally by following these priorities:

1. sensing root zone stresses

2. osmotically adjusting to stress

3. changing stomatal conductance (opening and closing)

4. increased absorbing root production

5. using stored food

6. root suberization (development of waterproof layer around damaged areas)

7. initiating foliage, branch and/or root abscission (dropping off)

Tree decline and death is the terminal result of extensive drought

Water Use in Trees

The movement of water in a tree is governed by stomates (stomata). Stomates are tiny valve-like openings usually on the bottoms of leaves. Stomates can be closed or opened by the tree. As the stomates are opened, water begins to evaporate from the leaf. Anytime the atmosphere has less than 100% relative humidity, water will evaporate. As each water molecule evaporates, it pulls another water molecule into its place. Long chains of water molecules can be pulled by one end because this cohesive force.

Through the millions of stomates on a large tree, water is continually pulled up from the soil by the roots. All other plants in the area are also pulling water from the soil. The results can be measured by tensiometers as a landscape or tree starts to move water in the morning.

When water becomes scarce, the tree begins to have problems. With open stomates, water is still evaporating from the leaves but it is more difficult to pull water from the soil for replacement. Water is lost more quickly from the leaves than the roots can pull in it, creating a water shortage in the tree. When the water shortage becomes too great, the stomates close until water uptake in the roots catches up. When the stomates are closed, no carbon dioxide can get inside and the leaf cannot conduct photosynthesis (make food).

Water is a tree is in long columns or tubes. These long columns of water that stick together are like rubber bands. When the stomates are open, the rubber bands (water columns) are being pulled from both ends. If the stomates close, the rubber bandlike water columns are still under tension.

At night, the tension left in the water column, like a stretched rubber band, continues to pull up water even though the stomates are closed. When there is plenty of soil water available, the tension in the columns is greatly reduced by the next morning.

Water movement through a tree is controlled by the tug-of-war between the water availability and movement in the soil versus the water loss from the leaves. The normal seasonal rate of water movement in some trees can be rapid. For example, water movement can be measured in feet per hour in ring-porous trees (red oak = 92, ash = 85, hickory = 62, elm = 20), in diffuseporous trees (black walnut = 13, willow = 10, maple = 8, magnolia = 7) and in conifers (pine and cedars = 6).

Drought Effects on Trees

Wilting. Wilting is a visible effect of drought. As leaves dry, turgor decreases, which causes petiole drooping and leaf blade wilting. The amount of water lost before visible leaf wilting varies by species.

Temporary wilting is the visible drooping of leaves during the day followed by rehydration and recovery during the night. Internal water deficits are eliminated by morning in time for a new deficit to be induced the following day.

During long period of dry soil, temporary wilting grades into permanent wilting. Permanently wilted trees do not recover at night; they recover only when water is added to the soil. Prolonged permanent wilting will kill trees.

The relation between water loss from leaves and visible wilting is complicated by large differences among species in the amount of supporting tissues their leaves contain. Leaves of black cherry, dogwood, and delicate-leaved shrubs wilt readily. By comparison, the leaves of live oak and pine are permeated with abundant sclerenchyma (i.e. tough, strong tissue) and do not droop readily even after they lose considerable water.

Stomatal Control. One of the earliest responses in leaves to mild water stress is stomatal (stomates) closure. Stomates are the small valves or holes usually on the underside of the leaf that allow gas exchange and water loss. Stomates often close during early stages of drought, long before leaves wilt.

Many trees normally close stomates temporarily in the middle of the day, in response to rapid water loss. Midday stomatal closure is generally followed by reopening and increased transpiration in the late afternoon. Final daily closure occurs as light intensity decreases just before sundown.

The extent of midday stomatal closure depends upon air humidity and soil moisture availability. As soil dries, the daily duration of stomatal opening is reduced. When the soil is very dry, the stomates may not open at all.

Stomatal closure will not prevent all water loss and tree death. Trees lose significant amounts of water directly through the leaf surface after the stomates close. Trees also lose water through lenticles on twigs, branches and stems. Trees in a dormant condition without leaves can also lose water.

One effect of severe drought is permanent damage that slows or prevent stomatal opening when three tree is rewatered. Under these conditions, leaves may recover from wilting, but stomate opening (necessary for food production), may not occur after watering.

Trees resist excessive rates of water through stomatal regulation. Stomates can be controlled by growth regulators transported from the roots during droughts. Drought effects on roots, stomates and other leaf cells can limit photosynthesis by decreasing carbon dioxide uptake.

Leaf Shedding. Premature senescence (aging) and shedding of leaves can be induced by drought. The loss of leaves during drought can involve either true abscission or leaves merely withering and dying. In normal abscission, leaf senescence, including the loss of chlorophyll, precedes leaf shedding. With severe drought, leaves may be shed while still green and full of valuable material.

Sycamore sheds some leaves and sassafras may shed all of its leaves as drought continues. On the other hand, leaves of dogwood or azalea usually wilt and die rather than abscise (fall off). If water becomes available later in the growing season, some trees defoliated by drought may produce a second crop of small leaves from previously dormant buds.

Sometimes drought-caused leaf shedding may not occur until after rehydration. This shows that abscission is initiated by water stress injury but cannot be completed without adequate water. The oldest leaves usually shed first. This is evidenced by leaves dying "from the bottom of the tree upward" into the crown, and "from the inside (of the crown) outward" to the edge of the canopy.

Injury to foliage and defoliation are most apparent in portions of the crown that are in full sun. The leaves show drought associated signs of leaf rolling, folding, curling, and shedding. The actual physical process of knocking-off leaves is associated with animals, wind, or rain.

Growth Inhibition. Growth of vegetative and reproductive tissues are constrained by cell enlargement problems and inefficient food supply. Cell enlargement depends upon hydraulic pressure for expansion and is especially sensitive to water stress. Cell division is also decreased by drought.

Shoot Growth. Internal water deficits in tree constrain growth of shoots by influencing development of new shoots. A period of drought has a carryover effect in many species from the year of bud formation to the year of expansion of that bud into a shoot. Drought also has a short-term effect inhibiting expansion of shoots within any one year.

Shoots of some trees elongate in only a few weeks in late spring. This growth form is called fixed or determinant growth. Other species elongate shoots over a period of several months which is called multiple flushing or continuous growth.

A late July drought may not affect current-year shoot elongation in species with fixed growth, like oaks. Oak shoots expand only during the early part of the growing season. A late July drought can inhibit expansion of shoots from multiple flushing species, like sycamore, which elongate its shoots during much of the summer. The most recent droughts have occurred over spring and summer, which damages both types of trees.

In the southern pines, late summer droughts will influence expansion of shoots in the upper crown to a greater extent than those in the lower crown. This is because the number of seasonal growth flushes varies with shoot location in the crown. Shoots in the upper crown normally exhibit more seasonal growth flushes than those in the lower crown. Buds of some lower branches may not open at all.

The timing of leaf expansion is obviously later than that of shoot expansion. If shoot expansion finishes early, a summer drought may affect leaf expansion but not shoot expansion.

Cambial Growth. Drought will effect the width of the annual ring, the distribution of the annual ring along the trunk and branches, duration of cambial growth, proportion of xylem to phoelm, and timing and duration of late wood production. Cambial growth slows or accelerates with rainfall.

Cambial growth is constrained by water supply of both the current and previous year. Last year's annual ring sets growth material supply limits on this years growth. This year's drought will affect next year's cambial growth. Such a delayed effect is the result of the drought year(s) influence upon crown development, food production and tree health. Drought will produce both rapid and delayed responses along the cambium.

Lag Effects. In fixed growth species, environmental conditions during the year of bud formation can control next year's shoots lengths to a greater degree than the environmental conditions during the ear of shoot expansion. Shoot formation in fixed growth species is a two-year process involving bud development the first year and extension of parts within the bud during the second year.

Drought during the year of bud formation in fixed growth trees decreases the number of new leaves formed in the bud and the new stem segments (internodes) present. Drought then influences the number of leaves, leaf surface area and twig extension the following year when those buds expand.

Summer droughts can greatly reduce shoot elongation in species that exhibit continuous growth or multiple flushing. Drought may not inhibit the first growth flush, but may decrease the number of stem units formed in the new bud that will expand during the second (or third, etc.) flush of growth.

Tree Responses. In deciduous trees (those losing leaves in the fall and winter), curling, bending, rolling, mottling, marginal browning (scorching), chlorosis, shedding and early autumn coloration of leaves are well-known responses to drought In conifers, drought may cause yellowing and browning of needle tips.

As drought intensifies, its harmful effects may be expressed in dieback of twigs and branches in tree crowns. Leaves in the top-most branch ends generate the lowest water potentials and decline and die. Drought effects on roots cause inhibition of elongation, branching, and cambial growth. Drought will also minimize stem growth.

Among the important adaptations for minimizing drought damage by tree crowns are the shedding of leaves, production of small or fewer leaves, rapid chlorosis, thick leaf waxes, effective compartmentalization (sealing-off twigs and branches) and greater development of food producing leaf cells.

The most important drought-minimizing adaptations of tree roots are production of an extensive root system (high root-shoot ratio), high root regeneration potential, production of adventitious roots near the soil surface, and effective suberization and compartmentalization of root areas.

Root Growth. Water in soil penetrated by tree roots is largely unavailable. Trees with widely penetrating and branching root systems absorb water effectively. This type of root system acts to prevent or postpone drought injury.

When first exposed to drought, the allocation of food to root growth may increase. This provides more root absorptive area per unit area of foliage and increases the volume of soil explored. Extended drought leads to roots being suberized to prevent water loss to the soil.

A high root-shoot ratio reflects water-absorbing capacity. Good watering absorbing ability coupled with a low transpiration rate for the amount of food produced (high water-use efficiency), allow trees to survive drought conditions.

The annual root system (absorbing roots) take up a majority of the water in a tree. Annual roots are not the woody rods seen when a tree is dug. The large woody roots have bark. Any bark crack or damage is quickly sealed-off so little water flows through these areas. It is the young roots, the roots easily damaged by drought, that are the major absorbers of water and essential elements for a tree.

A major drought effect is the reduction of photosynthesis. This is caused by a decline in leaf expansion, reduction of photosynthesis machinery, premature leaf senescence and associated reduction in food production.

When trees under drought are watered, photosynthesis may or may not return to normal. Recovery will depend upon species, relative humidity, drought severity and duration. It takes more time to recover photosynthesis after watering than for recovery of transpiration. Considerable time is required for leaf cells to rebuild full photosynthetic machinery.

Failure of water-stressed trees to recover photosynthetic capacity after rewatering may indicate permanent damage, including injury to chloroplast, damage to stomata, and death of root tips. Often drought can damage stomates and inhibit their capacity to open despite recovery of leaf turgor (water pressure).

Root damage is also affected by drought. For example, photosynthesis are compared, the stomatal limitations can be quite small. This means that other processes besides carbon-dioxide uptake through open stomates are being damaged by drought effects.

Site Evaporation. Little can be done to reduce the evapotranspiration by trees because it is controlled primarily by the energy (heat) available to evaporate water and soil-water availability. Antitranspiration can block water movement but lead to heat-kill of tissue. In forestry-urban forestry, the efficiency of water can be improved by increases in the density of tree cover and mulches so that little heat reaches the soil surface and evaporation is kept to a minimum. Under these conditions, the largest possible portion of the incoming energy can be used in photosynthesis and the most food produced per unit of water evaporated.

Soil-Water Movement. When a soil is wet, the rate of water absorption by tree roots can be great. Resistance of wet soil to water movement is low because only small forces are necessary to move water through water-filled pores. As soil dries, resistance to water movement increases in the soil and in the tree. Water movement becomes a problem because soil-root contact is lost from root shrinkage and resistance in increased because of root suberization and compartmentalization.

Capillary movement of soil water from wet to dry regions in soil with a moisture content at or below field capacity is slow.

The soil immediately surrounding the absorbing roots dries rapidly. Continuous root extension into zones of moist soil is critical for sufficient absorption of water to replace the water lost by transpiration. All areas where tree roots grow must have plenty of oxygen, as well, or roots will die. Soil drainage is critical to root growth and health.

Trees are always undergoing dehydration-hydration cycles because the rate of absorption of soil water lags behind the rate of transpiration from tree crown. Trees dehydrate during the day, particularly on hot, sunny days. Trees will refill with water during the night. The rate of absorption of water by roots may be impeded by:

1. low soil moisture content

2. small or slow-growing root system

3. poor soil aeration low soil temperature

4. poor soil-tree root contact high concentration of soil solution

(and interaction of the above)

Trees obtain all of their water from the soil. Under some conditions, other sources of atmospheric moisture in the form of dew or fog may prevent or postpone dehydration of tree crowns.

The water in the soil consists of:

1. Gravitational water occupies large soil pores and drains away under the influence of gravity. It is available to trees but usually drains away too fast to be important.

2. Capillary water the most important source of water for trees, is held in films around soil particles and in capillary pores.

3. Hygroscopic water the water in air-dry soils held so tightly by soil particles that it moves only as vapor and is generally unavailable to trees.

4. Water vapor which occurs in the soil atmosphere and is not used directly by trees.

After a rain, the rate of water drainage through a soil decreases rapidly with time until it stops. When water drainage out of a saturated soil stops, the soil is at "field capacity." At field capacity, which is considered the upper limit of useable soil water, capillary movement of water is slow.

The lower limit of useable soil water is the "wilting point". The wilting point is the soil water content below which trees cannot extract enough water for survival.

As soils dry, there is less and less water being held by the surfaces of the soil particles. Sandy soils dry rapidly because the spaces between the particles are large. Little water sticks to the surface of the sand particles. In clay soils, the water is held tightly on and between the clay surfaces. But some water is held so tightly it is difficult for the tree to pull this (hygroscopic) water out of the soil.

Leaves must develop large water shortages to pull up the last bits of water from a soil. And water shortages in the leaf may be large enough that it damages leaf tissues. With tissue damage, water loss in the tree continues. Even tree death does not stop water movement. Standing, dead trees continue to be a pathway of water loss from the soil.

Trees and Soils. As soil dries, the availability of water begins to be limited by decreasing water potentials and hydraulic conductivity. Soil aeration, soil temperature, and the concentration and composition of the soil solution can also affect the absorption of water by trees.

The resistance of water flow through soil increases rapidly as the soil dries. As the soil dries, shrinkage of both the soil and roots occurs. The close contact between soil and root is lost in many places. This increases resistance of water movement from soil to roots. The presence of mycorrhizae (fungal infected roots) can act to increase drought resistance in trees.

Soil texture will affect the water available in any soil. The finer soils "hold" water tighter and, during drought, a tree might not be able to "pull" hard enough to remove water from the soil.

Vascular System. Trees have a vascular system with a large water transport capacity. The transport system can deliver water rapidly and preferentially to those parts of the canopy which are most actively transpiring. The transport system is also resistant to environmental stresses, especially temperature extremes and pest attack.

Vertical water movement is restricted to the outermost one or two annual rings in trees like oaks. The pattern of water movement is more complex in conifers and trees since a larger number of annual rings can be involved (2-10 annual rings). Horizontal water movement, or storage, occurs throughout the sapwood rings.

Because of water storage in the wood, as transpiration increases in the mornings, absorption does not begin to increase until later. The decreasing leaf water potentials must produce sufficient tension in the xylem water columns to overcome the resistances to water flow through the xylem and from the soil into the root. As water is removed from leaf cells, when the soil is dry, a water deficit can be developed severe enough to cause temporary wilting of leaves.

The lag period between leaves transpiring and strong absorption shows there are significant resistances to water movement in soil, root, stems, branches, and leaves. In the evening, as the temperature decreases and stomates close, transpiration is rapidly reduced. Water absorption continues until the water potential in the tree comes into equilibrium with the soil. This absorption process may require all night. As the soil dries, there is less recovery on succeeding nights until permanent wilting occurs. A prolonged, severe water deficit will cause death.

The path of water flow between the roots and foliage is not a simple, single path. At each junction (i.e. branch to stem, branch to branch or leaf to branch), there is a reduction in water conduction. Because of the greater resistance to flow associated with junctions, a priority of water and associated essential element distribution is established.

Water Content Differences. Because of differences in shading and concentration of cell solutions, various parts of the shoot lose water at different rates. Different parts of the tree will have different water deficits. Since water moves from highest concentration to lowest concentration, tree parts that develop the lowest water potentials, like young shoots, obtain water at the expense of older tissue.

Top-most branch ends must generate the lowest water potentials and can be damaged quickest and die. Lower, shaded branches also suffer because they produce less food than upper, better exposed leaves. They are less able to compete for water but still must be self-sufficient in food production. Severe drought can also lead to death of lower, shaded branches.

Pest Problems. Drought predispose trees to pest attacks because of lower food reserves, poorer response to pest attack, and poorer adjustment to pest damage. Unhealthy trees are more prone to pest problems. Drought creates unhealthy trees.

Attacks on trees by boring insects that live in the inner bark and outer wood can be more severe in dry years when little water stress develops. Little water and elevated temperatures may also damage pest populations.

Supplemental watering of trees can be timed to help trees recover water and minimize pest problems on. Watering from dusk to dawn does not increase the normal wet period of ground covers since dew usually forms around dusk. Watering during the normal wet period will not change pest/host dynamics. Watering that extends the wet period into the morning or begins the wet period earlier in the evening can initiate pest problems.

Drought Hardening. Trees that have previously been subjected to water stress suffer less injury from drought than trees not previously stressed. Trees watered daily have higher rates of stomatal and surface water losses than trees watered less frequently.

Tree Selection

Drought resistant tree selection is along-term solution to low-maintenance landscapes. Drought resistance requires that tree leaves use water efficiently and continue to grow and make food at relatively low water potentials. Drought resistance involves characteristics like extensive root systems, thick leaf waxes and bark, good stomatal control, and the capacity for leaf cells to function at low water contents. Plant drought resistant tree species. Once established, these trees can better survive drought periods during the growing season.

A good method of determining which native tree species are drought resistant in an area is to check for ones withstanding the current drought best, while others around them are dying. Some of the more resistant species tend to be live oak, elms, persimmon, bur oak, yaupon, post oak, sweetgum, blackgum and cherry laurel.