FLOOD STRESS ON TREES

Photo: Dave MacDonald

Moisture Stress and Trees: The Hidden Problems

July 2002, the excessive rains broke a ten and a half year drought stretching through the nineties and followed by an eight year drought in the eighties. In five short days the Texas Hill Country received excess of 45" to 50 " of rain, almost a year and a half of normal annual rain fall for the area. The past few summers, near the end of August, soil moisture conditions moved from severely dry condition to extremely dry conditions. This year 2002 most of Texas was in extremely dry conditions by the first of June. (See U.S. Moisture Conditions) When a summer has been extremely dry, it will have a long-term impact on your landscape. The cumulative effects of recurring dry conditions are not visible until future years. Water stress affects most of the physiological processes involved in plant growth. Under drought conditions, plants become more susceptible to disease and pest infestation. As is true with people, weak, stressed trees are more vulnerable to problems. For example, oak decline occurs on trees stressed by drought and improper soil conditions; bark beetles and some wood-boring beetles occur in declining trees and spider mite occurs on junipers, grown in compacted, dry soil. These pests may push trees in a marginal state of health over the edge to death.

For most trees, the majority of the small, absorbing roots are concentrated in the upper 6 inches of soil, with roots extending laterally about 1 1/2 to 4 times the distance from the trunk of the tree to the drip line. This leaves only a small percentage of deeper anchorage roots. For the most part, it is these surface roots that absorb water and nutrients needed by the tree. When the soil surface dries out, as happens during a drought, the fine surface roots become less absorbent and eventually die, but water still can be absorbed by roots at greater depths. Also, available water is affected by other factors, such as soil texture and structure. With the return of adequate moisture, a surface root system can re-establish its activity.

Drought-stressed trees and shrubs, however, may not return to their pre-drought status. Plants growing naturally on a site adapt to changing moisture levels more easily than recently transplanted materials. Also, established plantings withstand drought stress better than newer plantings. When trees are dug at a nursery, 90 percent of the root system usually is severed. Trees that have not had adequate time to re-establish strong root systems are at risk from drought. Some species, are very susceptible to drought stress. Stunted bud development and twig growth and leaf curling are symptoms of drought stress. Later signs include an overall, slowing down of growth and increased susceptibility to disease and insect attack.

During a drought, adequate moisture must be supplied to trees from irrigation. Strict recommendations are difficult since variables, like soil type and species of trees, affect the need to irrigate. Drip irrigation that saturates the top 6 inches of soil is most desirable.

Some species of trees are better able to adapt to flooded conditions. Trees that have evolved in a floodplain ecosystem have mechanisms to cope with the periodic flooding that may occur and are better able to handle flooding. However, urban areas that end up flooded are not usually forested by trees that are adapted to flooding. There are some notable urban exceptions: boxelder, hackberry, black walnut, Cedar Elm, Pecans, sycamore and bald cypress are all considered relatively tolerant to flooding stress. It should be noted that the oaks are relatively intolerant of flooded conditions. Flood severity is another factor to consider when evaluating flood-stressed trees: Even a brief flood can kill a flood-intolerant tree, and a long, severe flood with sediment accumulation can kill a flood-tolerant tree.

When examining flood damaged trees, keep in mind that urban life is stressful to many species of trees, making them particularly susceptible to flood-related damage. Flooding compounds these stresses, leaving trees highly predisposed to additional mortality due to insect and disease attacks. If death does not occur outright, then it may be due to secondary agents of plant disease. Those trees that weren't killed in the initial flood event are considered predisposed, and can die quickly due to the combination of physical injury and rapid invasion from insects or diseases. However, well-maintained and healthy trees can and do recover quickly. How well a tree copes with flooding and the secondary agents of plant disease depends upon how vigorous the tree was prior to flooding, how long the flooding occurred, and sediment accumulation around the tree base.

It is important to remember that plants respond differently to flooding and that this response depends on tree species, health and site. For this reason, trees that are flood stressed exhibit a range of symptoms that include: leaf chlorosis and subsequent defoliation, reduced leaf size, development of epicormic shoots (watersprouts or small shoots emerging from the main stem), and crown dieback. These stresses may produce early fall coloration and leaf drop. It also is not uncommon for declining trees to produce either large seed crops or no seed crops in years following a flood. Symptoms may develop over a period of several years or they may abate as the tree recovers. Finally, it is important to remember that the symptoms may progress and ultimately result in tree death or they may subside indicating the tree has recovered. . However, this tree death may occur several years after the flood. It is very difficult to link a flood to the cause of tree death several years later.

Further, certain root and collar rot diseases are favored by waterlogged, oxygen-deficient soil conditions, most notably those caused by the water mold fungi, Phytophthora spp. and Pythium spp.. Flooded soil conditions not only promote reproduction and dispersal of these fungi but also promote the susceptibility of plant roots to infection. Oxygen starvation, wounding and loss of cell permeability due to flooding provide ideal infection sites for these organisms to colonize.

Symptoms of Phytophthora collar rot of flood damaged trees include brown to reddish water-soaked lesions with abrupt margins underneath the bark. A reddish brown liquid sometimes exudes from the canker margin. Cankers may not be noticed until foliar symptoms develop, which include sparse, chlorotic (yellow) leaves, premature fall color and dieback. Pythium root rot produces less distinctive diagnostic symptoms that included root rot and dieback.

Even trees that have appeared to recover are still at risk for infection by a group of opportunistic pathogens, members of the genus Armillaria. These fungi are the causal agents of shoestring root rot. There are hundreds of species of Armillaria, some of which are virulent pathogens and others of which act only as contributing factors to tree death. Although drought is usually considered the inciting agent that allows Armillaria to establish itself and infect, flooding has been implicated as a factor of Armillaria root rot in oaks.

Flooding is but one of the many injurious factors trees face over the course of their lives. Proactive maintenance of tree health is the best way to contend with periodic flooding. As the tree recovers, it needs to replenish food reserves. To increase tree vigor, a recommended application with a nitrogen fertilizer, (See Fertilizing Trees & Shrubs) aerating the soil, mulching, and watering if soil conditions become excessively dry after the flood. Dead or cankered branches should be removed. Prune trees only when bark surfaces are dry or during the winter season or in the heat of the summer to minimize infection by opportunistic pathogens. In the absence of tree maintenance, flooding and other environmental extremes leave trees susceptible to further injury or death. How well a tree copes with these stresses is dependent upon how healthy the tree was prior to the flood.