Date of Award

1986

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biological Sciences

Abstract

Native vegetation has recently been planted to control erosion along the Louisiana Gulf coast. The literature has characterized the dune habitat as xeric and its vegetation as adapted to low moisture availability. To analyze the drought ecology of Louisiana's coastal dune vegetation, a one-year field investigation of plant water relations and subsequent greenhouse drought-stress experiments were performed on Spartina patens (Aiton) Muhl., Panicum amarum Ell., Hydrocotyle bonariensis Lam., and Solidago sempervirens L. In addition, the sources of soil moisture and edaphic parameters related to soil-moisture availability were investigated. The field investigation showed dune and swale soils to be mainly fine sand with a low water-holding capacity. Precipitation was identified as the main source of soil moisture for plant growth. Soil moisture averaged 3% in the dune area and 17% in the swale. Monthly diurnal leaf xylem pressure and leaf conductance of each species were measured. Dune populations of S. patens and H. bonariensis has lower leaf xylem pressure than the swale populations when soil moisture dropped below 2.5%. Seasonal patterns of leaf xylem pressure and leaf conductance for dune plants coincided with a summer decrease in soil moisture. Roots, concentrated in the top 20 cm of the dune soil profile, may partially account for differences between species in leaf xylem pressure and leaf conductance. In the greenhouse, plant responses to drought, medium-watered, and flooded conditions were measured to quantify responses described in the field. Leaves were sampled for changes in leaf xylem pressure, leaf conductance, and leaf elongation rate. Each species acclimated to drought by increasing the length of each progressive drought cycle. The ability of the dune population of S. patens to withstand low leaf xylem pressure and continue to grow supports evidence reported in the literature of ecotypic differences between dune and swale populations. Drought-stress experiments verified that H. bonariensis was the most sensitive to water stress, and employed stomatal closure as a drought-avoidance mechanism. Panicum amarum displayed drought avoidance by maintaining low leaf conductance and relatively high leaf xylem pressure when soil moisture decreased. The lack of stomatal control as leaf xylem pressure decreased in the field was verified in the greenhouse for S. sempervirens.

Pages

151

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