Date of Award

1991

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Oceanography and Coastal Sciences

First Advisor

Irving A. Mendelssohn

Abstract

Mangroves are intertidal, halophytic trees limited to the tropics and subtropics because they are not cold-tolerant. In coastal Louisiana (29$\sp\circ$ N latitude), the black mangrove, Avicennia germinans, is the only mangrove found because Avicennia is the most cold-tolerant of mangrove genera. Avicennia intergrades with salt marsh cordgrass, Spartina alterniflora, with 3 observed zones: an Avicennia zone, a transition zone, containing both Avicennia and Spartina, and a Spartina zone. The purpose of this dissertation was to determine why A. germinans does not occur in the Spartina zone within a mangal/salt marsh ecotone. Three studies investigated the following zone differences in: (1) soil physicochemical variables; (2) ability of Avicennia seedlings to survive and grow and (3) propagule dispersal, establishment and predation. The transition and Spartina zones were similar across most soil physicochemical variables. However, the Avicennia zone was distinctly different, with higher elevation, soil bulk density, and redox potential, and lower sulfide levels in the interstitial water. Although the zones differed in soil physicochemical variables, these differences did not impact the survival of Avicennia seedlings, although growth was reduced in the more inundated Spartina zone. Since Avicennia seedlings can survive and grow in the Spartina zone, seedling success may be related more to the establishment capabilities and predation of Avicennia propagules. When propagules were prevented from being moved away by tides, Avicennia could establish in the Spartina zone. However, propagule predation rates were significantly higher in the Spartina zone and unaffected by propagule background density. Since Avicennia seedlings have reduced growth in the Spartina zone, the long-term survival of Avicennia in the Spartina zone is questionable. However, the most probable causes for exclusion of A. germinans from the Spartina zone are related to the fate of the propagules. Most propagules dispersed into the Spartina zone are probably washed away by tides before establishment, and the few remaining propagules are probably damaged enough by predators to render them incapable of establishment.

Pages

126

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