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HOW THE DROUGHT AFFECTS TREES IN OUR LANDSCAPE
Despite a few recent rains, it's been an incredibly dry summer. My rain gauge on Aug. 28 had collected just 3.75 inches in more than eight weeks! The dearth of moisture has caused me to wonder how trees and other plants manage to survive when their roots are locked in a vault of parched earth that more closely resembles concrete than soil.
Several of my mature bur oaks have clusters of brown leaves and I attribute this to the trees deciding there just isn't sufficient moisture to keep every leaf green.
I've received many phone calls about trees that have dropped leaves prematurely. Maples, especially, tend to begin showing fall colors early and dropping leaves when water is scarce. When drought occurs several summers in a row, some trees may give up the struggle and die. Most maple species prefer partial shade and a forest-type soil environment of decaying leaves and other plant material. Maples aren't well suited to the sun-baked, hard-pack soil environment common on boulevards and front lawns.
Trees can show drought stress in a variety of ways. Leaves may wilt, droop, turn yellow, show early fall color, turn brown at the tips or margins, curl or show all of these symptoms. Green leaves, stems, roots and fruits may shrink. Shrinking can cause radial cracks in tree trunks.
Through a process called transpiration, plants release considerable amounts of water vapor through tiny leaf openings called stomata. When adequate soil moisture is available, temperatures not too high, and humidity not too low, transpiration will occur during most of the day. Under normal conditions, transpiration is lowest during the hottest part of the day, greatest in the morning and late afternoon, and ceases at night. When soil water becomes limited, the plant tries to conserve water by closing the stomata in the leaves. But when the stomata are closed for extended periods of time, transpiration ceases and this causes photosynthesis to stop and the plant stops growing. If this goes on long enough, the plant will die. Plants try to protect themselves from water loss by closing stomata, slowing or stopping growth, and by prematurely dropping leaves. It is common to see leaves falling from trees in mid- summer during dry years.
Immediate effects of drought on hardwood trees are usually obvious, but delayed effects also occur. When unfavorable growth conditions are present, growth for the coming year is often affected. Plants store food reserves and prepare for the next growing season during the current growing season. For instance, buds for next year's growth will be set during the current summer. The effects of drought can carry over to the next growing season, and maybe beyond.
Lack of water also affects radial growth of trees (diameter growth). As a tree grows in diameter, each year it will produce a growth ring that consists of springwood (earlywood) and summerwood (latewood). The width of tree growth rings is greatly affected by the availability of water. During dry years, little radial growth occurs and the annual growth ring will be narrow. Because severe droughts adversely affect trees in many ways, radial growth often will be reduced for the current year and maybe even one or more subsequent years.
Scientists can study the growth rings of old trees and determine rainfall patterns for years past. This science is called dendrochronology. I've observed many times on trees that have been felled the narrow rings caused by the 1930s drought.
The general health of a tree depends a lot on where it is growing. Bottomland areas tend to be one of the most favorable sites for trees. There is usually adequate moisture coupled with deep, fertile soils. Sites that generally are not ideal for good tree health are ridges where shallow, eroded soils usually occur.
Soils with high clay content will hold water much better than sandy soils. Trees growing in clay soils tend to be shallow rooted and may be more severely impacted by prolonged drought than trees growing on loamy or sandy soils where roots will grow deeper. Most of a tree's feeder roots that absorb moisture and nutrients are located in the top 12-14 inches of the soil. When clay soil dries out, the impact on the tree can be great since the tree is not "accustomed" to sending roots deep into the soil for moisture and nutrients.
Drought-stressed trees may exhibit signs of dieback or decline. This may be the tree's way of coping with a stressful situation. If the roots are unable to supply enough moisture and nutrients to the crown of the tree, the crown will usually begin to die back to bring the tree's crown and root system into a more favorable balance. It should be mentioned that it is often difficult to determine if a tree has died from drought stress or has simply become dormant and appears to be dead.
Two simple tests can be done to help determine if a drought-stressed tree is alive. First, try snapping some 1/8- inch diameter twigs. If they snap and break easily, it is likely the tree has died. On the other hand, if the twigs bend and don't break with a snap, the tree may still be alive. Second, use your fingernail to scrape bark from a small twig or branch. If the tissue under the bark is green and moist, the tree may still be alive. To be absolutely sure the tree is not dead, wait until the next spring to see if it sprouts a new crop of leaves.
During times of drought, the best thing for trees and plants is water. Homeowners should consider watering valuable shade trees and other landscape plants to lessen the stress from drought and heat. Thoroughly water the ground area beneath the branches in the evening or early morning. About one to four inches of water should be applied; a light sprinkling is not of much value. To measure the amount of water applied, place a rain gauge or tall drinking glass in the sprinkler pattern. When there is one to four inches of water in the gauge or glass, the tree should be adequately watered. Without rainfall, watering should be done about every 10 days.
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