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ALEXANDRIA — Although no stranger to rain and floods, Louisiana has seen a record number of extreme weather occurrences since 2014, and no area of the state has been immune, said LSU AgCenter forestry agent Robbie Hutchins.
After a flood, the severity of the damage to homes, schools, businesses and agricultural crops can be fairly obvious, Hutchins said. But what about flood damage to trees growing in our neighborhoods and forests?
“The good news is that Louisiana native trees are resilient,” he said. “Most of the time, native trees can tolerate infrequent standing water and saturated soils caused by extreme rain, and most can even withstand sporadic catastrophic flooding.”
Trees can be stressed, damaged or even killed by excessive standing water or saturated soil under the right conditions, and it may take several years before trees show the effects of flooding.
Hutchins said five primary factors determine whether a tree will experience damage from extreme rain and flooding: tree species, tree size and age, flood timing, flood duration and flood severity.
“It is usually a combination of two or more of these factors that actually determine how a tree is affected by a flood,” he said.
—Tree species. Tolerance and resiliency can be significantly different among various trees. For example, loblolly pine is highly tolerant of saturated soils and flooding, and is used as a wetland indicator species by the Army Corps of Engineers. However, the closely related longleaf pine is intolerant of flooding and prefers sandy upland soils. The same is true for hardwoods, where water hickory flourishes in low areas of floodplains while the closely related mockernut hickory is intolerant of flooding and prefers dry upland sites.
—Tree size and age: Younger, smaller trees are much more susceptible to damage than older trees. Tree height is the most important measure that determines whether a tree may be damaged by flooding.
Young trees and seedlings often die in a flood because they can be pushed over, buried in mud or uprooted. Excessive soil saturation and flooding can lead to stunting, stressing or mortality of a young tree over time while mature trees survive flooding much better, Hutchins said.
—Flood timing: A popular misconception is that Louisiana experiences most of its extreme rains and floods in conjunction with tropical storms and hurricanes. Climatological records, however, show these events normally occur between November and May when trees are dormant. This flooding cycle is a natural part of life for native trees, and they can withstand this “normal” flooding with little or no ill effects. But trees are often damaged or killed by summer floods when the water is warmer and the trees are actively growing.
—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, few species can survive a month or more.
How long an area stays flooded is directly related to the hydrology of the site, the hydrology downstream and the type of land use in the drainage area, Hutchins said. Over the years, Louisiana hydrology has been significantly altered by a wide variety of manmade and natural factors, including the development of many areas that were traditionally in forests and fields.
“Considering these changes, it is no wonder that the most recent evidence shows that the average duration of our flooding is increasing,” Hutchins said.
—Flood severity: The main indictor of severity is the depth of the floodwaters; the deeper the water, the greater the severity of the flood. Other aspects include the possible presence of herbicides or other chemicals that could harm trees or debris that could do physical injury. In addition, the current associated with the flowing water could uproot or break off trees.
“When taken into account, all of these aspects provide a picture of the potential severity of a flood,” Hutchins said.
Adding one stress on top of another has a greater and greater effect on tree survival, he said.
“A tree is a tree, no matter where it is growing,” Hutchins said. However, homeowners have a habit of planting species in the wrong location in their yards. Many times this leads to the tree or shrub being really stressed before the flood occurs.
After the major floods in southeast Louisiana in August 2016, the overwhelming majority of trees in the flooded areas survived, said AgCenter consumer horticulturist Dan Gill. “Some Leyland cypresses were observed to die within a month after the floods, and that species is well-known to be especially prone to root rot,” he said
During the catastrophic flooding of New Orleans in 2005 following Hurricane Katrina, the urban forest came through the weeks of several feet of brackish water in remarkably good shape, Gill said. Species of magnolia were the only trees that consistently did not survive the flooding.
Some of the damage done to trees by extreme rains or floods can be seen shortly after floodwaters recede. Trees that have been uprooted or pushed over 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,” Hutchins said.
Root dieback and root rot are often hidden and may result in death of the tree when the soil eventually dries out.
An excessive amount of lichens on the limbs and trunk can be a good indicator a tree is under stress because of too much moisture, he said.
“A homeowner or landowner can do little to completely protect their trees from stress or damage resulting from extreme rain or flooding,” Hutchins said. “But three things 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 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 or catch basins.