Abiotic and Biotic Influences on Black Mangrove (Avicennia germinans) Growth and Survival Along Their Northern Range Limit in the Gulf of Mexico

Abstract

In response to changing temperature and rainfall regimes, mangroves are expanding their range edges poleward and increasing areal coverage at the range edge. The coast of Texas encompasses a large portion of the black mangrove (Avicennia germinans) range edge in the Gulf of Mexico. A gradient of black mangrove coverage ranges from populations with 100% areal canopy cover in the southern Coastal Bend region, to relatively small stands of mangroves interspersed among salt marsh plants on the Upper Coast. My research investigated the importance of herbivory and temperature stress on black mangroves at the range edge. Herbivore pressure was determined by quantifying leaf loss due to herbivory over short and long timescales over a gradient of mangrove cover and was compared between sites along the range edge. Herbivore pressure along the range edge varied and sites on the leading edge of the range edge had less herbivore pressure than sites closer to the core range. This work indicates that herbivory, which is an important dynamic within the black mangrove core range, is not an important factor limiting black mangrove populations at the range edge in the northern Gulf of Mexico. Temperature stress was studied by monitoring the growth rate and reproductive output of black mangroves after two cold events of varying intensity. Even the milder cold events had growth and reproduction consequences for several growing seasons following the cold event. By the third growing season mangrove canopies had not recovered their full pre-cold event volume and propagule production had not returned to normal. After a severe cold event black mangrove recovery through regrowth and seedling recruitment occurred primarily at higher elevations; there was widespread mangrove death and little evidence of recovery at lower elevations, which may result in eventual subsidence and wetland loss. These results highlight the vulnerability of black mangroves to even mild cold events and the vulnerabilities of areas with aggressive black mangrove encroachment to severe cold events.