2019 Flooding: Effects on Forestry

Flooding Effects on Forests and Habitat for Wildlife

We know the Mississippi River has been at flood stage for months now but why should we be concerned, since flooding occurs on an annual basis.

You’re correct in stating that the flood stage has lasted for months and although the Mississippi River has what is termed annual flooding, this is typically limited to within the levee system or bluff areas along the river. However, the flood levels that we are experiencing while not typical seem to be getting more frequent. It is hard to compare each flood, but those noted in 1973, 1985, 1993, and 2011 resulted in significant damages. These have been termed as “100-year floods” but the changes in population growth and protection from smaller floods as dramatically affected hydrology and drainage thus exasperating movement of rainwater and resulting in more severe floods. With that said, the flood of 2019 has reached proportions and lengths that we have not seen before. In looking at the damage that might occur strictly from a forestry aspect there are some things we know but a great deal that we still will not know until this flood is completed. What is for certain is that the damage will likely range from very little to significant, with the worst being mortality among species and possible changes in forest species complex to those species better adapted to extended flooding conditions. 

What we do know is that those tree species found within the levee system, or what is termed “batture” ground includes;  eastern cottonwood, black willow, American sycamore, boxelder, sweetgum, sugarberry, various species of ash and maples, as well as water hickory. The species present depends greatly site elevation and the soil type. While the portion of ground experiences annual flooding it is usually during the dormant season or early spring where day time temperatures are mild and water temperature are cooler with high oxygen levels. Thus, the effect of this type of flooding has been noted as only minor for the more riparian species. When an area is experiencing heavy continued rainfall the drainage of the smaller rivers and streams and is either stopped or the Mississippi river forces water back (backwater) into these smaller bodies of water causing serious damage to homes, crops, and forests alike. Here a larger number of species will be apparent with light seeded riparian species as well as more heavy seeded species, such as water oak, willow oaks, Nuttall oak, overcup oak, swamp chestnut oak, swamp white oak, and sweet pecan. But if flooding is again in the winter and early spring the older trees will show little effect from this type of flooding. 

Is there anything that we know about what type of damage to expect to the forests in the flooded areas?

Unfortunately, there are no simple or definitive answers because these floods are not a stable phenomenon and can continue to change as conditions change. However, there is no doubt that the impact will only increase with time, especially as the flooding enters into the summer months. But, the uncertainty of the extent of damage is the result of not just one factor but on a large number of variables including species tolerance to flooding, rainfall patterns, longevity and intensity of rainfall, drainage time and obstructions which may trap water, tree age, season of flooding, water movement, duration, water temperature and oxygen levels as well as topography all come into play in defining the possibility and extent of tree damage. What is for certain is that damage will range from very little to significant tree damage and possible mortality among species that are not tolerant to extended flooding.  But, the most detrimental effect will be the result of backwater becoming trapped and stagnant thus causing a lowering of oxygen levels to the point where root uptake is non-existent which will in either serious dieback and/or mortality.

Even those species adapted to flooding conditions but have thinner bark, such as maples and sycamore may experience top dieback from continued inundation. This is a result of water scorch located along the stem if the height of the water remains later in the year. If fungi or bacteria enters into this area of the tree it is likely to result in mortality. It has also been found that newly planted hardwood seedlings prior to flooded conditions, are more than likely going to suffer significant mortality regardless of species, thus forcing replanting next year.

Why should we be concerned about the effects of flooding in forests?

Data from the flood of 1973 taken by the U.S. Forest Service at Stoneville, MS indicated very high mortality of newly planted seedlings, including riparian species such as eastern cottonwood. However those species including cottonwood, sweetgum, sycamore, sweet pecan, water oak, Nuttall oak, Shumard oak and willow oak seedlings that had one or more full growing season survived much better, except in areas where water became trapped.It was also shown that more flood-sensitive species such as black walnut and yellow poplar were shown to exhibit high mortality rates even in older trees.

Data from Mississippi State indicated that newly planted Shumard, Nuttall, cherrybark, and swamp chestnut oaks where the seedlings were inundated from February to April and the soil remained saturated for next two months showed mortality rates of 33% for Nuttall oak, 25% for swamp chestnut oak and nearly 90% mortality of cherrybark oak. Results from another oak study showed that the site was inundated following leaf-out for less than a month resulted in 25% mortality but a later second inundation during the same year showed an additional 30% mortality. The following stressed conditions of the trees greatly affected growth and subsequent long-term survival.

It has also been noted that effects from flooding can greatly disrupt or change the successional pattern of the environment. Trapped water will eliminate species such as oaks to more early successional species such as black willow and maples.  A very good analogy would be the effect of long-term flooding on hardwood tree species in a Green-Tree Reservoir. If the water is not removed prior to leaf-out and this is done yearly the result is first dieback and then mortality which would shift the dominant species.   Areas that have a lot of sand deposition could eliminate tree species entirely unless the trees were old enough and well rooted in soil to acquire sufficient moisture during the growing season. 

However, as the summer progresses the water temperature will continue to rise and if it stays at a flood level with little movement the root systems will soon starve from a lack of oxygen and symptoms of leaf yellowing, small leaf size, epicormics sprouting and crown dieback will be the first indication that the tree or trees are stressed. Fortunately, previously healthy trees may exhibit these symptoms but could recover over the next couple of years, if flooding is not repeated.

While forest stands are fairly resilient and even if decimated, wind-blown seed will be the first species to occupy the site. The more dominant heavy-seeded species, such as oak will take time to invade the area.