By Robbie Hutchins – Area Forestry / Wildlife Extension Agent
LSU AgCenter Central Region
During the last several years Louisiana has definitely lived up to the old often quoted axiom that states “half of the state is under water and the other half is under indictment”. Well at least Louisiana has done its best to live up to the first part. Although Louisiana is no stranger to rain and floods, during the years of 2014-2017 Louisiana has seen a record number of extreme rain events and flooding. Unfortunately, no area of the state has been immune from these flooding events.
After a flood, the severity of the damage to homes, schools, businesses, and row crops can be fairly obvious. However, what about flood damage to trees growing in our neighborhoods and forests? The good news is that Louisiana’s native trees are very resilient. Most of the time our native trees are able to tolerate infrequent standing water and saturated soils caused by extreme rain events and most can even withstand sporadic catastrophic flooding. However, trees can be stressed, damaged, or even killed by excessive standing water and flooding or saturated anaerobic soil under the right conditions. Unfortunately, it may take several years before trees show the effects of flooding.
There are five primary factors that determine whether or not a tree will experience damage from extreme rain events and flooding. These five factors are tree species, tree size and age, flood timing, flood duration, and flood severity. In reality, it is usually a combination of two or more of these factors that actually determine how a tree is affected by a flood. Let’s take a look at each of the five factors and attempt to understand how they make a difference.
The first factor that determines whether or not a tree will experience damage from flooding is tree species. Fortunately, Louisiana’s native trees have a certain degree of tolerance to saturated soils and flooding. The actual degree of tolerance and resiliency changes with each tree species and can even be significantly different within a family or even a genus. For example, loblolly pine (Pinus taeda) is highly tolerant of saturated soils and flooding to the point that the species is used as a wetland indicator species by the Corps of Engineers in wetland delineation. However, the closely related longleaf pine (Pinus palustris) is intolerant of flooding and prefers sandy upland soils. The same is true for hardwoods where water hickory (Carya aquatica) flourishes in low areas of floodplains while the closely related species mockernut hickory (Carya tomentosa) is intolerant of flooding and prefers dry upland sites.
The second factor that determines whether or not a tree will experience damage from flooding is tree size and age. Younger smaller trees are much more susceptible to damage than older trees. Tree height is the most important aspect of size that determines whether or not a tree is damaged from flooding. Smaller trees are more easily overtopped by flood waters and tree injury increases in direct proportion to the percent of the crown covered by water. Young trees and seedlings often die as a result of flooding because they can be pushed over, buried in mud, or uprooted by floodwaters. The outer bark on smaller younger trees is thinner than the bark on larger adult trees and offers less protection to the cambium layer during warm weather flooding. Excessive soil saturation and flooding limits the proper development of the root systems of young trees which can lead to stunting, stressing, or mortality of the tree over time. Mature trees in the dominant crown survive flooding much better than trees in the lower crown classes.
The third factor that determines if a tree will be damaged is flood timing. A popular misconception is that Louisiana experiences most of its extreme rain events and flooding in conjunction with tropical storms and hurricanes. However, according to the climatological records, normally our extreme rain events and floods occur between November and May when trees are dormant. This flooding cycle is a natural part of life for our native trees and they can withstand this “normal” flooding with little or no ill effects. But, when the flooding occurs during summer months when the water is warmer and the trees are actively growing trees are often damaged or killed. This is especially true if all or part of a tree is under water or if the root system is completely inundated for more than a short time during an untimely flood. Research has indicated that bottomland hardwoods can be especially vulnerable to mortality if the tree is inundated by flood waters within two weeks after the first flush of growth.
The fourth factor that determines if a tree will be stressed or damaged as a result of an extreme rain event or flood is the duration of the flood. When it comes to flood duration the shorter amount of time a tree is inundated the better. This is especially important if the flood happens during the growing season. Although mature trees can tolerate short periods of flooding during the growing season, there are very few species that can survive a month or more of inundation during the growing season. Mature trees can withstand up to four months of flooding if the trees are dormant. However, repeated long term flooding events leads to the death and decay of large portions of a tree’s root system. Flood duration is directly related to the hydrology of the site, the hydrology downstream, and the type of land use in the drainage area. Over the years, Louisiana’s hydrology has been significantly altered by a wide variety of manmade and natural factors. More recently, Louisiana has seen the widespread development of many areas that were traditionally in forests and fields. Taking these changes into account, it is no wonder that the most recent evidence shows that the average duration of our flooding events is increasing.
The final factor that determines if a tree will be damaged as a result of an extreme rain event or flood is the severity of the flood. Normally, the main ingredient severity is the depth of the flood waters. Generally speaking the deeper the water the greater the severity of the flood. However, flood severity can also include other aspects. One aspect of flood severity could be the presence of herbicides or other chemicals in the water that could harm trees. Another aspect of flood severity could be the presence of debris in the water that could do mechanical injury to the tree. Yet another aspect of flood severity could be the amount of current or destructive flow associated with the flooding which could uproot of break off trees. When taken into account all of these aspects give a complete picture of the actual severity of the flooding event.
Some of the damage done to trees by extreme rain events and/or flooding can be seen shortly after flood waters recede. Trees that have been uprooted or pushed over during floods or small trees that were overtopped during a growing season flood generally die quickly. Unfortunately most of the damage done may not be evident for several years. Root dieback and root rot are often hidden damages associated with flooding but usually result in death of the tree when the soil eventually dries out. Flooding moves massive amounts of soil and that soil is often deposited in the root zones of large trees effectively smothering the roots and eventually killing the tree. Stress caused by flooding and saturated soils makes a tree more vulnerable to diseases such as various root rots and attack by variety of opportunistic insects. The presence of an excessive amount of lichens on the limbs and trunk can be a good indicator the tree is under stress as a result of too much moisture.
Unfortunately, there is little that a homeowner or landowner can do to completely protect their trees from stress or damage resulting from extreme rain and/or flooding, but there are three things that can be done to help lessen the severity of the damage. First, always select the right tree for the soil type and site and make sure the tree is planted correctly. Second, make sure that the site has adequate drainage. Third, as much is practical, make sure downstream drainage is open and unimpeded by natural factors such as beavers and manmade factors such as stopped up culverts.