Flood stress on trees
Photo: Dave MacDonald
Over their long lives, trees are confronted with many of the extremes that Nature has to offer. The fact that trees live for long periods of time suggests that they have evolved elaborate mechanisms to survive these extremes, be it flooding, drought, strong winds, severe winters or late frosts. Although the impact of flooding on humans is almost immediate, how flooding affects trees is less obvious. There are several factors to take into account when considering the impact of flood stress on a particular tree. These include: 1) species tolerance to flooding, 2) length of flood, 3) overall tree health, and 4) sediment accumulation around tree roots.
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, silver maple, hackberry, green ash and downy hawthorn are all considered relatively tolerant to flooding stress. It should be noted that the pines and oaks (with the exception of pin oak) are all 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.
Symptoms of flood stress
Photo: Jeff Hahn
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. 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.
Secondary agents of plant disease
You need to be on alert for the possibility that flood-stressed trees may become invaded by insects or infected with disease. The likelihood of this occurring depends upon the severity of the flood and tree health. A tree that is already predisposed because of stressful urban conditions can have this stress compounded by flood. This tree should be considered a prime target for opportunistic insects and disease causing agents. These opportunists invade or infect only those hosts that are predisposed by stress. In this case, the insect or pathogens are actually secondary agents of tree disease. Flooding could act as the predisposing agent or an inciting agent of plant disease if the flood period is prolong or intense. Although not well understood, it is generally believed that predisposed plants emit a biochemical signal which attract secondary agents of plant diseases.
Flood stressed trees are especially susceptible to collar and root rot diseases caused by species of Phytophthora and Pythium, which are considered "water molds" and are not "true" fungi. Free-standing water aids in both the reproduction and dissemination of these fungi. 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 oak, chestnut and larch.
Symptoms of Armillaria infection include leaf chlorosis, defoliation, reduction in leaf and shoot growth, dieback, and/or death. Key diagnostic signs of this disease are white mycelial fans under the bark; black, shoestring-like rhizomorphs attached to roots, and the fall development of honey-colored mushrooms growing in clumps at the tree base. For more information about Armillaria, see the Yard & Garden brief Armillaria Root Rot.
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, the US Forest Service recommends application with a low nitrogen fertilizer, 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 dormant season 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.