Ecogeomorphology of Mangroves

The environmental settings of mangroves are a complex behavior of regional climate, tides, river discharge, wind, and oceanographic currents (Figure 1). There are about

240 x 103 km2 of mangroves that dominate tropical continental margins from river deltas, lagoons, and estuarine settings to islands in oceanic formations (noncontinental). The landform characteristics of a coastal region together with geophysical processes control the basic patterns in forest structure and growth. These coastal geomorphic settings can be found in a variety of life zones that depend on regional climate and oceanographic processes. Hydroperiod of mangroves resulting from gradients in microtopography and tidal hydrology (Figure 1) can influence the zonation of mangroves from shoreline to more inland locations forming ecological types of mangrove wetlands. Lugo and Snedaker identified ecological types of mangroves based on topographic location and patterns of inundation at local scales (riverine, fringe, basin, and dwarf; Figure 1) that Woodroffe summarized into basically three geomorphic types (riverine, fringe, and inland). A combination of ecological types of mangroves can occur within any one of the geomorphic settings occurring at a hierarchy of spatial scales that can be used to classify mangrove wetlands.

Various combinations of geophysical processes and geomorphologic landscapes produce gradients of regulators, resources, and hydroperiod that control mangrove growth (Figure 2). Regulator gradients include salinity, sulfide, pH, and redox that are nonresource variables that influence mangrove growth. Resource gradients include nutrients, light, space, and other variables that are consumed and contribute to mangrove productivity. The third gradient, hydroperiod, is one of the critical characteristics of wetland landscapes that controls wetland productivity. The interactions of these three gradients have been proposed as a constraint envelope for defining the structure and productivity of mangrove wetlands based on the relative degree of stress conditions (Figure 2). At low levels of stress for all three environmental gradients (such as low salinity, high nutrients, and intermediate flooding), mangrove wetlands reach

Figure 1 Hierarchical classification system to describe patterns of mangrove structure and function based on global, geomorphic (regional), and ecological (local) factors that control the concentration of nutrient resources and regulators in soil along gradients from fringe to more interior locations from shore. Modified from Twilley RR, Gottfried RR, Rivera-Monroy VH, Armijos MM, and Bodero A (1998) An approach and preliminary model of integrating ecological and economic constraints of environmental quality in the Guayas River estuary, Ecuador. Environmental Science and Policy 1: 271-288 and Twilley RR and Rivera-Monroy VH (2005) Developing performance measures of mangrove wetlands using simulation models of hydrology, nutrient biogeochemistry, and community dynamics. Journal of Coastal Research 40: 79-93.

Figure 1 Hierarchical classification system to describe patterns of mangrove structure and function based on global, geomorphic (regional), and ecological (local) factors that control the concentration of nutrient resources and regulators in soil along gradients from fringe to more interior locations from shore. Modified from Twilley RR, Gottfried RR, Rivera-Monroy VH, Armijos MM, and Bodero A (1998) An approach and preliminary model of integrating ecological and economic constraints of environmental quality in the Guayas River estuary, Ecuador. Environmental Science and Policy 1: 271-288 and Twilley RR and Rivera-Monroy VH (2005) Developing performance measures of mangrove wetlands using simulation models of hydrology, nutrient biogeochemistry, and community dynamics. Journal of Coastal Research 40: 79-93.

Regulator gradient Resource gradient Hydroperiod

Figure 2 Interaction of three factors controlling the productivity of coastal wetlands, including regulator gradients, resource gradients, and hydroperiod. The bottom panel defines stress conditions associated with how gradients in each factor control growth of wetland vegetation. From Twilley RR and Rivera-Monroy VH (2005) Developing performance measures of mangrove wetlands using simulation models of hydrology, nutrient biogeochemistry, and community dynamics. Journal of Coastal Research 40: 79-93.

their maximum levels of biomass and net ecosystem productivity.

Soil nutrients are not uniformly distributed within mangrove ecosystems, resulting in multiple patterns of nutrient limitation. Along a microtidal gradient in carbonate reef islands, trees were generally N-limited in the fringe zone and P-limited in the interior or scrub zone. Fertilization studies demonstrated that not all ecological processes respond similarly to or are limited by the same nutrient. It is also apparent that mangrove forests growing in other ecogeomorphic settings are also prone to P-limitation associated with different geophysical processes. One of the most critical regulator gradients (Figure 2) controlling mangrove establishment, seedling survival, growth, height, and zonation is salinity, depending on their ability to balance water and salt. Interspecific differential response of mangrove propagules to salinity occurs at salinities from 45 to 60gkg~\ The 613C and

6 N signatures of mangrove leaf tissue can indicate stress conditions such as drought, limited nutrients, and hyper-salinity across a variety of environmental settings.

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