Summer 2000, Volume 9, Number 2
YELLOW POPLAR AND DROUGHT
by William R. Chaney Professor of Tree Physiology
Department of Forestry and Natural Resources Purdue University
Water is frequently the most limiting factor for tree growth and this was certainly the case lost year. The effects of the extended drought in 1999 are still showing up in poor growth, low vigor, and even death of trees this year. Yellow poplar appears to have been particularly susceptible to the dry hot conditions. This tree is exacting in its soil and moisture requirements. Where it occurs naturally, the sites are usually moderately moist, well drained, deep soils with room for root development. It rarely grows well in very dry or very wet conditions. Yellow poplar withstands moderate drought in humid climates, but is well known to be sensitive to drought. Indiana can certainly claim the humid climate and soils that foster yellow poplar. The pre-settlement records of large numbers of huge yellow poplars more than 100 feet fall reflect these ideal conditions. Nevertheless, the dry conditions of 1999 have really taken atoll on Indiana's state tree. It is difficult to generalize about weather effects on frees when the conditions can vary so dramatically from one end of the state to the other. Hence, I want to discuss the effects of water stress on trees in general and relate them to the situation with yellow poplar as much as possible.
Moderate Water Stress is Normal
Water stress is a daily factor of life for frees and, in fact, all land plants. Loss of water as a result of transpiration is necessary to transport essential nutrients from the soil to growing tissues. The mechanism for the uptake and movement of water up trees is strictly a physical process, relying on a water deficit gradient from the leaf surfaces to the soil. Hence, each day as stomates in leaves open and water evaporates into the surrounding air, water stress develops in the leaves. It is the loss of water and consequent mild stress that is responsible for creating the "suction" that pulls water up trees from the soil. Because transpirational loss of water is a necessary evil, trees have adopted to tolerate these modest stresses. The amount of water stress that develops in the crown of trees each day is dependent on the soil moisture content and the trees' ability to absorb it. Trees must develop even greater water deficits in leaf tissues to pull water from the soil where it is held increasingly more tightly by the soil particles as the soil dries. There is usually a lag between the loss of water from a tree crown and the movement of water up the tree from the soil. This so-called absorption lag is responsible for water stress in the leaves and twigs. The suction forces that are caused by the absorption lag can result in perceptible shrinkage in the diameter of the trunks of trees. During the night when the stomata are closed, water continues to move up trees due to the water deficit gradient until the water lost during the day is recharged. With the tension relieved, trunks expand to their original size. Moderate wilting of leaves may occur during the day without causing serious damage. These wilted leaves rehydrate at night and continue normal viable functions. The stress and recovery cycle is typical and one to which trees are adopted.
Effects of Severe Water Stress
Problems begin to arise when the amount of water available in the soil is low due to shortages in precipitation such as occurred in 1999. Leaves continue to transpire during the day, but since the water in the soil is attached more tightly as it is depleted, the leaves must develop very highwater deficits to establish the gradient for movement from the soil. Under these conditions, the recharge of water in the leaves that normally would occur at night may not be complete before the new day breaks and transpiration begins anew. Even greater water stresses in the free crown are now necessary to get movement from the soil. Eventually, the leaves and other living tissues develop such severe water stress and wilting that irreversible damage to the tissues occurs. The soil is so dry and its moisture held so firmly that recharge of tissues in tree crowns is minimal or nonexistent. Now the leaf has reached the so-called permanent wilting point. Cell membranes will have collapsed and cell organelles responsible for photosynthesis, respiration, and other essential physiological processes are literally pulled apart and irreparably damaged. Rain or irrigation now is too late to reverse the damage.
Visible symptoms of water stress damage usually appear gradually. First leaf tips and margins begin to brown. Then this condition spreads into areas between veins and leaves may begin to fall. Lack of moisture usually affects all the leaves on one or more branches. Leaves affected first and most severely are those exposed to afternoon sun and prevailing winds. Young leaves are the most visibly affected. Older leaves, leaves that are small and thick, and the rigid needles of conifers may not wilt visibly, but may turn brown at the tips or margins. Sometimes leaves are killed and are retained on trees. This is because the process of abscission is an active one that requires metabolic activity to soften and dissolve the cells in the abscission layer. If death comes rapidly, then the leaf remains firmly attached and considerable force from wind, rain, or ice is necessary to break it off the tree.
Cooling Effects of Transpiration
Another important function of transpiration is its cooling effect. Like perspiration evaporating from your body tends to cool you, evaporation of water from leaves requires energy and hence cools leaves. When transpiration slows down or ceases as soil moisture is depleted, the cooling effect is lost. Now the leaves of trees are exposed to the sun, absorbing its radiant energy and building up a tremendous heat load without adequate transpirational cooling to dissipate it. The elevated temperatures can have damaging effects on cells and physiological processes that are as significant as the effects due to severe water deficits.
Ability to Survive Prolonged Dry Weather
Plants generally fall into three categories relating to their capacity to acquire and conserve moisture.
1) Water spenders have extensive root systems that absorb water from a large volume of soil. As long as some of their roots are in moist soil, they can survive.
2) Drought evaders avoid water stress in several ways. They drop their leaves, sometimes even shedding twigs and branches, and become dormant in dry weather. A good example of this kind of response is yellow poplar, which is notorious for shedding many of its leaves during even moderate summer dry periods. During hot, dry weather, interior leaves may turn yellow and fall from frees. While they have leaves, however, yellow poplar usually transpire as rapidly as water spender trees.
3) Water conservers have ways of reducing water loss. Their leaves may be small, light in color to reflect more of the sun's rays, covered with hairs to reduce air movement and evaporation rates, leathery with a thick waxing coating on the leaf surface to reduce water loss, or arranged on the twigs to reduce the amount of sunlight that strikes them. Their stomata may be structured to reduce transpiration by being small, few in number, or even sunken into the leaf surface. Yellow poplar has none of these features. The leaves are large, smooth, and the stomata exposed, allowing rapid transpiration.
Long-Term Effects of Drought
The effects of drought may persist for many years after the dry conditions occurred and adequate soil moisture replenished. Repeated dry periods can have cumulative effects that are particularly persistent. Photosynthesis is reduced and carbohydrate reserves depleted. This is caused by a decline in leaf expansion, reduction of the photosynthetic machinery at the cellular level, and premature leaf senescence and fall. When water is once again available in sufficient quantities, photosynthesis of leaves that are retained on trees may or may not return to normal. Recovery will depend on the tree species, relative humidity, drought severity, and duration. It takes more time to recover photosynthetic rates after soil water is recharged than for recovery of transpiration.
If leaves are shed prematurely due to severe water or killed, the loss of photosynthesis and the reduction in accumulation of stored carbohydrates will affect growth long after the drought conditions have passed. Large quantities of carbohydrates are stored in the living cells of twigs and branches. If the drought is extended and death of these twigs and branches occurs, considerable quantities of reserve foods are lost to the free. This may be reflected in poor shoot growth and leaf development during the growing season following a drought even though environmental conditions at the time seem ideal for tree growth. Roots also may be damaged and effect the capacity of frees to absorb both nutrients and water.
Drought makes trees more susceptibility to insect and disease problems. Lower food reserves result in poorer responses to pest attack and poorer capacity to respond to pest damage. Unhealthy trees are predisposed to pest problems and drought and the consequent water stress is a major factor. Water stressed trees also show increased susceptibility to winter injury.
Frequently the year after a severe drought suppressed buds may open or adventitious shoots expand early in the growing season. However, the depleted carbohydrate reserves and damaged room system are not adequate to sustained vigorous growth and the foliage is thin and poorly developed.
Due to the sensitivity of yellow poplar to dry conditions, the effects of the 1999 drought resulted in the outright death of many yellow poplar in the state. Unfortunately, even the survivors will exhibit effects for many years.