How are Trees Affected with Flood

Natural forests in floodplains have evolved to handle periodic flooding and, therefore, many trees in the flooded areas will indeed survive. However, the 1993 flooding has been prolonged and has occurred during the growing season, adding severe stress. The flooding has included not only areas forested by well adapted swamp hardwoods, but has also flooded many urban areas forested by trees that are not well adapted to flooding. In addition, many of the forests now flooded are located behind broken dikes. These forests developed after the dikes were installed in the 1950's and have not previously been flooded, did not evolve to handle periodic flooding and have a different species composition than typical bottomland floodplain forests. These forests, along with urban trees, could be highly susceptible to additional tree mortality from insect and disease attacks due to their weakened state. Trees that survive flooding are in varying stages of health or vigor. Vigor is influenced by stress. Prolonged flooding is a major stress, especially during the growing season. Low vigor trees often die quickly from a combination of physical injury and rapid invasion from insects or diseases. High vigor trees may recover very quickly and could be healthy as early as the next growing season. Many trees are somewhere in the middle, struggling to survive and slowly trying to regain a higher level of vigor. Until trees regain an adequate level of vigor, they are susceptible to attack by insects and/or diseases. If an insect or disease is successful in invading a tree, the survival of that tree becomes less likely. The battle to regain vigor and ward off attacks from insects and diseases may continue for several years, causing tree death over a period as long as 3-5 years. Which trees become and continue to be stressed will depend on many factors including a species tolerance to flooding, length of inundation, sediment levels, etc. (see Flood Tolerance of Trees section). Stress Symptoms Flood stressed trees exhibit a range of symptoms including leaf chlorosis (yellowing), defoliation, reduced leaf size and shoot growth, sprouting, and crown dieback. Early fall coloration and leaf drop often occur. It also is common for stressed trees to produce large seed crops in years following a stressing event such as flooding. Again, it may not be unusual for symptoms to occur over several years. The symptoms may progress and eventually lead to tree death or they may subside indicating the tree has recovered. A critical factor in determining the survival of flood stressed trees is whether they become invaded by insects and/or diseases. Flood stressed trees are prime targets for attack by "secondary organisms." Secondary organisms include a wide variety of opportunistic fungi and insects that selectively invade hosts only after they are weakened or predisposed by stress. It is believed that predisposing stresses such as flooding, drought, and defoliation impair host resistance mechanisms, and trigger biochemical responses which release carbohydrates, glucose, and other nutrients which stimulate secondary insects and diseases. 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. Finally, the wood of trees that have died as a result of the flooding will also be quickly attacked and utilized by wood boring insects and blue staining and decay fungi. Where landowners wish to salvage and sell dead or severely declining trees, they will need to be aware of the decline in wood quality that can occur quickly from insect and disease attack. This can significantly reduce the value of wood products. (See discussion of salvage considerations in Management Implications section). More of this topic in the following links: flood restoration south auckland