Black Mangrove (Avicennia sp.) Colony Expansion
in the Gulf of Mexico with Climate Change:
Implications for Wetland Health and Resistance
to Rising Sea Levels
By:
Rebecca Comeaux
Graduate Student
University of Texas at Austin
When: |
Friday, 7, 2010, 10:30 a.m. to 11:30 a.m. |
Where: |
Seminar Conference Room, 10100 Burnet Road, Bldg 196-ROC, Austin, Texas 78758 |
Host: |
Gail Christeson, UTIG |
Abstract
Populations of black mangroves (Avicennia sp.) are hypothesized to expand their latitudinal range with global climate change, induced by a reduction in the frequency and severity of coastal freezes, which are known to limit mangrove colony extent and individual tree size, as well as an overall warmer climate. The Gulf of Mexico is located at the northward limit of black mangrove habitat and is therefore a prime candidate for population expansion with global warming.
This expansion may come at the expense of existing Gulf coastal saline wetlands that are dominantly Spartina spp. marsh grasses. The present study was conducted to focus, not on the historical extent of this replacement, but to examine the potential implications, specifically 1) resistance to accelerating eustatic sea level rise (ESLR), 2) wetland resistance to wave attack in large storms (increased cyclonic storm frequency/intensity is predicted with future climate warming), and 3) organic carbon sequestration and estuarine productivity in adjacent water bodies.
Field sites of adjacent and intergrown Avicennia mangrove and Spartina marsh populations in similar geomorphological setting were selected in backbarrier areas near Port Aransas and Galveston, TX (two sites each) as part of a larger-scale planned study of the full latitudinal transition of the western Gulf. The reconnaissance for site surveys to date suggests that black mangrove populations are clustered near inlet areas, indicating seed transport vectors are a major control on colony establishment, and likely, the rapidity of habitat replacement. Resistance to ESLR was tested by 1) creating high-accuracy (±1 cm) elevation maps over ~5,000 m2 areas of adjacent mangrove and marsh, and 2) measuring Pb/Cs sediment accumulation rates and organic matter accumulation rates (loss on ignition) from auger cores (no compaction). Elevation surveys in Port Aransas indicate mangrove vegetated areas are 1-3 cm higher in elevation than surrounding marsh at the patch and individual mangrove scale: at the Galveston sites, any trend is complicated by the area's pre-existing geomorphology. Pb/Cs accumulation rates and loss on ignition data indicate that mineral trapping is 1.5-2 times higher in mangroves in both survey areas but organic accumulation rates show no clear trend. This additional particulate trapping in mangroves does not differ in grain size from marsh mineral accumulation. Elevation change may also be effected by root volume displacement: live root weight measurements in the rooted horizon (~0-20 cm depth) are consistently higher in mangrove cores. Porosity is lower in mangrove rooted horizons, with a corresponding increase in sediment strength (measured by shear vane in the cores), suggesting mangrove intervals may be more resistant to wave-induced erosion during storms. Analyses of organic biomarkers and pore water redox potential are ongoing and will be utilized to examine the character of organic material being buried (source, age, and diagenetic state). Initial pore water results indicate higher salinities and lower pH values are present within mangrove core rooted intervals, suggesting a distinct diagenetic environment exists relative to marsh sites. Increased salinities may be a function of the shallow rooting in mangrove areas, which focuses evapo-transpiration and salt exclusion within this zone: this may prove advantageous when competing with marsh grasses by elevating salinities to levels that are toxic for Spartina.
