The University of Tennessee - Forest Resources Research and Education Center

The effects of the freeze were still evident. Although the hillsides and landscapes were much greener than two weeks before, a closer inspection revealed that brown leaves and dead blossoms were still clinging to the tree branches. The yellow poplars proved to be much slower at re-leafing than the red oaks and hickories. The lower branches of the poplars showed encouraging signs of re-leafing at the one month mark, but the other native trees had out-paced them.

In the article below, Dr. Wayne Clatterbuck, Associate Professor, Forest Management and Silviculture, UT Department of Forestry, Wildlife & Fisheries, describes the technical details of the freeze damage to the native trees and explains why we were still in a "wait and see" mode:

Freeze Damage to Trees: Impacts and Recovery

The below-freezing temperatures in early April have affected many trees and other plants in Tennessee. The stage of leaf development or emergence at the time of the freeze made a difference as to which trees were impacted. The buds with young tissues where leaves and fruiting structures were emerging were most affected, while those that had already been growing for a few weeks with older tissue were not as affected. Most of the damage from freezing temperatures occurred at elevations below 1,200 feet in Tennessee; at the higher elevations, leaves on most trees had not emerged. The freeze was widespread, extending to Athens, GA and Birmingham, AL, into the Carolinas and northward through Kentucky and Illinois. Red and white oaks, hickories, yellow-poplar, hackberry and sycamore were affected most by these freezing temperatures; maples were not. The flowers of most fruit trees were destroyed by the freeze. The combination of the above-average temperatures in March, allowing bud break to proceed earlier than normal, and the severe cold temperatures that followed took a toll on many trees. The stage of leaf emergence and flowering was probably the contributing factor that resulted in freeze damage rather than the species involved.

Late-season freezes are not common, but they do occur. The literature is rich with examples of freeze damage to trees. One early reference (1926) from the Forest Service Bent Creek Research Station described the effects of a late May freeze on white oaks near Wytheville, VA in 1925. Late freezes with extended droughts from Pennsylvania to northern Georgia in the 1950s are often mentioned in forestry literature.

What impact do these late-season freezes have on the tree? Trees use a great amount of stored carbohydrate (energy) to produce new leaves. The plant tissues produced from these energy reserves were destroyed during the freeze. Trees must then re-mobilize additional carbohydrates to releaf, a demand that creates unexpected stress. Most trees progressively burst buds, such that there are some buds remaining to leaf out after the freeze. Dormant and other suppressed buds are activated during the second leaf emergence. The tree’s carbohydrate and energy reserves are lower during the second refoliation. The crowns of refoliated trees are sparse with fewer, smaller and less dense leaves than normal.

The freeze affects plant tissue in at least two ways. First, ice crystals form in the cells of young, emerging leaves, rupturing the cell membranes so the cells can not rehydrate. These leaves rapidly die after a freeze. Secondly, the continuity of the water column is blocked and disrupted. These obstructions occur when dissolved gases in the water produce air bubbles when frozen. When the ice thaws, the air bubbles are released, and they obstruct the flow of water through the vessels to the leaves.

Most ring-porous species (oaks and hickories) have large earlywood vessels. These vessels are produced in the spring before the leaves expand. These species conduct water primarily in the youngest growth ring. Thus, trees produce a very efficient water-conducting system within a short time before it is needed. However, this efficiency takes place at the expense of safety. The slightest injury to the latest growth ring can cause disruption of the tree’s water-conducting system. With the immobilization of the earlywood vessels, the latewood vessels from the previous year remain functional, but the efficiency of water conductance is comparatively less. Alternatively, most diffuse-porous species (maple) have very small vessels that survive low temperatures more easily. Gas bubbles are smaller, more confined and can redissolve in solution. The vessels are not as likely to become blocked. Thus, refoliation and the resumption of efficient water flow in the vessels takes place much sooner in diffuse-porous species when compared to ring-porous species. Oaks and hickories will take much longer to refoliate because of the inefficiencies of water transport in the vessels.

Most trees will recover from a single defoliation associated with a freeze. However, any other stress encountered by the tree may cause irreversible damage and loss. Defoliation is a primary, initial cause of tree decline and the role of climatic and other environmental factors influence the severity of the defoliation. Both prolonged droughts and the secondary impacts of opportunistic insects (defoliators and wood borers) and disease (fungi and cankers) that are attracted to stressed trees can have severe detrimental impacts on tree health. To alleviate additional tree stress, keep landscape trees watered during droughty periods. Slowly, trees should regain lost carbohydrates and continue to strengthen energy reserves. In forested areas, there is little that can be done on a large scale. If trees have declined severely and the trees have an economic value, a harvest should be considered before the trees further deteriorate. For more information on managing tree decline or why trees die, refer to the following UT Extension publications: SP 615 and SP675.

What should you do to care for damaged trees now? For most trees, doing nothing is best. Wait for several months for the tree to refoliate. For trees like yellow-poplar and sweetgum, the refoliation is already occurring. For others, particularly white oaks, red oaks and hickories, it may take a month or more. Do not fertilize. Fertilization encourages rapid shoot growth causing stressed trees to further deplete energy reserves. Water trees if possible, especially during dry periods, to alleviate further stress to the tree. Restrain from pruning until you can definitively determine what is dead and what is still living. Pruning should occur late in the growing season or during the dormant season (September through February).

Trees that were already stressed before the freeze could succumb because of limited carbohydrate reserves. These would be trees that were damaged beforehand or older trees that were already declining or senescing. It may take two or three years before such trees completely die. However, most healthy trees that were not stressed before the freeze should be able to compensate and remain viable.

A future consequence of the freeze will be limited flower and subsequent fruit and seed production for both the current and the upcoming year. Not only are the current years’ flowers affected, but reduced energy reserves will impact next year’s flowers too. Most all commercial fruit orchards have substantial losses. The yield of both hard and soft mast (acorns, nuts and fruits) will be low. The flowers of white oaks were affected such that acorn production will be limited. The limited acorn crop will impact wildlife populations. Because it takes two years from pollination to produce red oak acorns, red oak acorn production will be affected in 2008. The acornets of red oaks from the flowers pollinated in spring 2006 will probably produce some acorns for this year, if they are not aborted due to stress.

Recently, the National Arbor Day Foundation created a hardiness map from data collected by the National Climatic Data Center. The Arborday.org map shows much of the country in warmer zones for the last 16 years than those indicated on the 1990 USDA Hardiness Zone Map. Most areas of Tennessee increased by one zone or about five to ten degrees in temperature.

This new map should be considered carefully. Trees planted north of their adapted range may grow satisfactorily through a series of milder than normal winters, but when an especially severe winter occurs, they could be killed. Many of the trees on the northern range of the zones for Tennessee, such as crape myrtle and Japanese maple, were especially hard hit by the late-season freeze.

Trees affected by the late-season freeze are stressed. If these trees were relatively healthy before the freeze and no other stressors such as severe drought or secondary pest attacks occur, the trees should recover. However, if the trees were already in a weakened condition prior to the freeze, many will not have the carbohydrates or energy to refoliate and will likely succumb. The freeze is only one contributing cause in the decline of trees. However, considering that much of Tennessee is presently several inches below normal rainfall levels, the increased stress from extended water deficits, coupled with defoliation from the late-season freeze, can have detrimental impacts on the health of trees. If possible, water trees during moisture deficits (droughts) to alleviate further tree stress and encourage more rapid recovery.

The Spring Freeze: Wait and See

"What we saw... six weeks after the freeze"