Drought and Heat Damage to Trees
Dr. Mario A. Villarino, CEA-Hopkins
As the summer heat increases, several calls to the Hopkins County Extension Office were directed to heat and drought stress on trees. Many trees are showing the effect of last year’s drought and starting showing effect of this year heat wave. Dr. Coder, arborist and extension specialist who has been with the University of Georgia since 1985, mentioned that water is the most limiting ecological resource for most tree and forest sites. As soil-water content declines, trees become more stressed and begin to react to resource availability changes. A point is reached when water is so inadequately available that tree tissues and processes are damaged. Lack of water eventually leads to catastrophic biological failures and death. Growing periods with little water can lead to decreased rates of diameter and height growth, poor resistance to other stresses, disruption of food production and distribution, and changes to the timing and rate of physiological processes, like fruit production and dormancy. More than eighty percent of the variation in tree growth is because of water supply. Effects of drought can be recognized throughout a tree. The term "drought" denotes a period without precipitation, during which the water content of the soil is reduced to such an extent that trees can no longer extract sufficient water for normal life processes. Water contents in a tree under drought conditions disrupt life processes. Trees have developed a series of prioritized strategies for reacting to drought conditions. As drought continues and trees respond to decreasing water availability, various symptoms and damage occurs. Wilting is a visible effect of drought. As leaves dry, turgor pressure in leaf cells decrease causing leaf 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 reduced by morning in time for an additional water deficit to be induced the following day. During long periods of dry soil, temporary wilting grades into permanent wilting. Permanently wilted trees do not recover at night. Permanently wilted trees recover only when additional water is added to the soil. Prolonged permanent wilting kills trees. The relation between water loss from leaves and visible wilting is complicated by large differences among species in the amount of supporting tissues leaves contain. One of the earliest responses in leaves to mild water stress is stomate closure. Stomates are the small valve-like openings usually active on the underside of the leaf that allows gas exchange and water loss. Stomates often close during early stages of drought, long before leaves permanently wilt. Different species vary greatly in their stomate closing response. Gymnosperms usually undergo more leaf dehydration than angiosperms before they close their stomates. 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 water loss. Trees lose significant amounts of water directly through the leaf surface after the stomates close. Trees also lose water through lenticels on twigs, branches, roots, and stems. Trees in a dormant condition without leaves also lose water. Water loss from tree surfaces depend upon tissue temperature - the higher the temperature, the more water loss. One effect of severe drought is permanent damage that slows or prevents stomatal opening when the tree is rewatered. Additional water supplies after a sever drought period will allow leaves to recover from wilting, but stomate opening (necessary for food production) to pre-drought conditions, may not occur for a long period after rehydration. Trees resist excessive rates of water loss 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 carbon1dioxide uptake, increasing food use for maintenance, and by damaging enzyme systems. Premature senescence and shedding of leaves can be induced by drought. The loss of leaves during drought can involve either true abscission, or leaves may wither and die.
In normal abscission, an organized leaf senescence process which includes the loss of chlorophyll, precedes leaf shedding. With severe drought, leaves may be shed while still full of valuable materials. Sometimes drought caused leaf shedding may not occur until after rehydration. Abscission can be initiated by water stress but cannot be completed without adequate water to shear-off connections betwen cell walls. The oldest leaves are usually shed first. Injury to foliage and defoliation are most apparent in portions of the crown that are in full sun. These leaves show drought associated signs of leaf rolling, folding, curling, and shedding. The actual physical process of knocking1off leaves is associated with animals, wind, or rain. A major drought effect is the reduction of photosynthesis. This is caused by a decline in leaf expansion, reduction of photosynthetic 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 photosynthetic rates 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 chloroplasts, damage to stomates, and death of root tips. Often drought can damage stomates and inhibit their capacity to open despite recovery of leaf turgor. When stomatal and non-stomatal limitations to 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. Root damage also effects drought recovery. For example, photosynthesis of loblolly pine seedlings are reduced for a period of several weeks when root tips are injured by drought, even after water has been restored. Growth of vegetative and reproductive tissues are constrained by cell initiation shortages, cell enlargement problems, and inefficient food supplies. Cell enlargement depends upon hydraulic pressure for expansion and is especially sensitive to water stress. Cell division in generating new cells is also decreased by drought. Drought predisposes trees to pests 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 than in years when little water stress develops. Little water and elevated temperatures can also damage pest populations.
Supplemental watering of trees can be timed to help trees recover water and minimize pest problems on surrounding plants. Watering from dusk to dawn does not increase the normal wet period on plant surfaces 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 many pest problems. In deciduous trees, 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 affects root / soil contact (root drys and contracts) and mechanically changes tree wind-firmness. Drought also minimizes stem growth. Among the important adaptations for minimizing drought damage in tree crowns are: shedding of leaves; production of small or fewer leaves; rapid closure of stomates; thick leaf waxes; effective compartmentalization (sealing-off) of 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. For more information on drought or any other agricultural concern please contact the Hopkins County Extension Office at 903-885-3443 or e-mail me at firstname.lastname@example.org