Evaluation of growth rates and establishment patterns of water-elm (Planera aquatica) and baldcypress (Taxodium distichum) in response to hydrologic and climatic conditions at Catahoula Lake, Louisiana

Abstract

Hydrologic alterations frequently lead to vegetation changes in floodplain ecosystems. In Louisiana, there has been an expansion of water-elm (Planera aquatica) and to a lesser extent baldcypress (Taxodium distichum) at Catahoula Lake, a Ramsar Wetland of International Importance. Water-elm and baldcypress both are flood tolerant species; baldcypress growth is known to be influenced by hydrologic conditions more than climate. The expansion of these woody trees has reduced herbaceous vegetation valuable for waterfowl. In this study, I tested two hypotheses that establishment of water-elm trees into the lake increased after the construction of water control structures in the nearby Black River and a diversion canal and water-control structure on the lake in 1971; and that growth rates of water-elm and baldcypress are more correlated with hydrologic regimes than with local climate. Water-elm trees have been found in the lake for at least 140 years, but most (171 of 219) sampled trees were established in the period following the construction of water-control. In addition, 48 of 67 plots consisted of trees established entirely in the post-control period. Hydrologic conditions in the lake changed with the modifications, including reduced variability in June and August lake levels and reduced fall flooding. These changes coincided with apparently increased establishment of water-elm trees during the post-control period. Finally, there are several sources of uncertainty that prevent clear interpretation of the effects of water control on water-elm establishment including prevailing climatic conditions, control measures by Louisiana Department of Wildlife and Fisheries and cattle grazing. My second hypothesis that growth of water-elm and baldcypress would be more correlated with hydrologic regimes than with climate was partially supported. Growth of water-elm was more related to climatic variables than with lake levels, whereas baldcypress radial growth was more correlated with lake levels than with climatic variables. The response of water-elm radial growth to lake hydrology was limited to a negative relationship with late spring lake levels in the pre-control period; baldcypress had a consistent positive response with lake levels during several seasons in both the pre-and post-control periods.