Episodic Flooding of The Ouachita River: Levee-mediated Mortality of Trees and Saplings in a Bottomland Hardwood Restoration Area

Abstract

The Mollicy Farms Unit of Upper Ouachita National Wildlife Refuge, LA, consists of former agricultural land replanted with traditional bottomland hardwood species. Much of it is surrounded by a containment levee built to hold back the annual floodwaters of the Ouachita River. In 2009, two extreme floods, with water levels over 4 m above the flood stage, breached the levee, leaving the area inside the levee inundated for an extended period of time. We investigated the mortality of trees and saplings following these floods. During the initial reforestation efforts, which began in 1998, trees were planted both inside and outside the levee, allowing us to compare tree and sapling mortality based on location, inside or outside the levee. The average mortality of all trees was 40.59%, and the average mortality of all saplings was 48.23%. Both tree and sapling mortality resulted from a significant interaction between elevation and location inside or outside the levee. Overall, results indicated increased mortality at lower elevations for the area inside the levee. Outside the levee, mortality was unaffected by elevation because floodwaters were able to recede naturally. Levee removal would restore a more traditional flooding regime, likely reducing tree and sapling mortality during future floods.