Flooded Forests Face Unknowns Ahead



 Trees along the Mississippi River levee that have experienced an extended time
 in flood conditions may struggle to survive.

 Photo by MSU Extension Service/Randy Rousseau







STARKVILLE, MISS.
   The stage for 2019 floods was set by heavy snowfall in the upper Midwest, followed by excessive rainfall patterns in the Plains, Midwest and South, resulting in significant flooding all along the Mississippi River.
   The spring and early summer of 2019 has been among the wettest on record for many states located along the Mississippi and Ohio rivers.
   Mississippi River remains very high, forcing the opening of various spillways to disperse water over a larger area. The effect of the flooding has impacted not only the lives of people living in these areas, but has also begun to impact forests, wildlife and the Gulf of Mexico.    One of the areas bearing the brunt of extended flooding are the 11 Mississippi counties found along the Mississippi River.
   With no end in sight to the flooding, the search has begun to determine the extent of damage to the variety of hardwood species throughout the affected area.
   Unfortunately, there are no simple or definitive answers because this natural disaster is not a stable phenomenon and continues to change. The impact will increase with time, especially as the flooding enters into the summer months.
   Uncertainty of the damage’s extent is based on several variables: rainfall patterns, floodwater movement, water temperature, oxygen levels, longevity and intensity of rainfall, and drainage time and obstructions. Topography also defines the possibility and extent of tree damage. Certainly, damage will range from very little to significant with possible mortality among species that are not tolerant to extended flooding. 
   Many factors influence a tree’s ability to survive either seasonal and/or extended flooding, such as siltation, tree age, water temperature, trapped water, species tolerance to flooding, season(s) of flooding and duration of flooding.
   Hardwood species vary greatly in their tolerance of flooding. As expected, riparian species such as black willow, eastern cottonwood, bald cypress, swamp tupelo, overcup oak and sycamore are tolerant to flooding. These species are adapted to seasonal flooding during the dormant season and early spring.
   Records taken by the U.S. Forest Service at Stoneville in the Mississippi Delta during and after the 1973 flood indicated very high mortality of newly planted seedlings. Sweetgum, sycamore, sweet pecan, water oak, Nuttall oak, Shumard oak and willow oak seedlings that had one or more full growing seasons survived much better except in areas where water became trapped. More flood-sensitive species such as black walnut and yellow poplar were shown to exhibit high mortality rates even in older trees.
   This year, the Mississippi River has remained high for a much longer period than seen even in the 1973 flood. Although we were relatively certain that the majority of the damage would have been limited to young, early-planted seedlings, the concern has shifted to older trees that will be stressed as water temperatures increase and the oxygen levels decrease.
   Early symptoms of flood stress may include leaf yellowing, small leaf size, epicormic sprouting and crown dieback. The one bright note is the resilience of healthy forests. Even if they exhibit these symptoms, they could recover over the next couple of years, if flooding is not repeated. ∆
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