Tidal Wetlands Primer: An Introduction to Their Ecology, Natural History, Status, and Conservation

4. Plant Response to the Tidal Environment

77 Tidal wetlands are subject to natural forces and human actions. From an organism’s standpoint, the intertidal environment is rigorous, harsh, and characterized by fluctuating conditions. Tides ebb and flow exposing and then rewetting substrates; salinities vary with tides, precipitation, and river discharges; temperatures change seasonally , daily, and hourly; and the vagaries of weather all create unique conditions and challenges for both plant and animal life. Although many animals can move in and out of wetlands, occupying them at favorable times, plants do not have such freedom. Once established, plants must cope with the physical stresses posed by the intertidal environment, disturbances (e.g., storms, herbivory, and human activities), and competition. The interaction of physical , chemical, and biological factors affects the germination, growth, and reproduction of plants as well as productivity and­ species richness (e.g., McFalls et al. 2010). Species colonizing coastal wetlands must develop morphological and/or physiological adaptations and other strategies to cope with these challenging conditions. In this chapter I address plant response to various stressors—the physiochemical factors and biotic interactions that largely determine the presence or absence of vegetation and species composition of tidal wetlands. Each factor is discussed separately; however, it must be emphasized that multiple stressors characterize different environments and they operate both individually and collectively to affect plant life. Since the frequency and duration of tidal inundation are largely responsible for site wetness and salinity, separating out the influence of tidal hydrology from salinity on vegetation is understandably difficult in estuarine ecosystems. They, in large part, form the foundation of what is the intertidal estuarine environment and are strongly interrelated. Hydrology Plant distribution in the intertidal zone is in part attributed to differences in flooding and soil saturation. Salt marsh vegetation tends to grow above mean sea level, with aquatic vascular species and macroalgae colonizing the lower intertidal zone. Some plants appear to range widely within the marshes, while others may have predictable elevation ranges, so small changes in marsh topography are often reflected in plant distribution (e.g., Adams 1963; Bartoldus 1984; Chmura et al. 1997; Figure 4.1). Flooding Changes in the frequency and duration of flooding have significant effects on­ vegetation patterns and productivity in estuaries and tidal freshwater environments. Conditions range from permanently flooded (subtidal) to infrequent flooding by storm tides with periodic changes in the amplitude of all tides during the metonic cycle. 4 Plant Response to the Tidal Environment 78  Chapter 4 In estuaries, differences in tidal hydrology also influence soil salinity. Since marsh herbs and mangroves grow to only a certain level below mean high tide, the effect of longduration flooding on these vascular plants is evident. Their tolerance of submergence then sets the lower limit for these vascular plants, with lower areas being characterized by tidal flats and nonvascular plants. Vascular plants become abundant only when exposure to air is limited in subtidal waters, with eel-grass (Zostera marina) in temperate marine waters and a wider variety of species in the tropics (e.g., Thalassia testudinum , Cymodocea filiformis, Halodule, and Halophila). Smooth cordgrass (Spartina alterniflora), the dominant salt marsh emergent species along the water’s edge in eastern North America, typically grows seaward to the midpoint of the mean tide range (the half-tide level), but can colonize lower levels about halfway between this mark and mean low water in some places (Figure 4.2; Lagna 1975; McKee and Patrick 1988). Its lower limit may be less in macrotidal regions as marshes in the Bay of Fundy are reportedly restricted to the upper quarter of the tidal range (Wells and Hirvonen 1988). In the simplest terms, tidal wetlands can be separated into two zones: 1) regularly flooded zone (flooded at least once daily), and 2) the irregularly flooded zone (flooded less often). Within the latter, permanently flooded water bodies may be found (e.g., shallow ponds in northern marshes and even lakes in Louisiana’s marshes). From April to September 1985, the average number of days that Rhode Island salt marsh species experienced tidal flooding varied by ­ species: tall form of smooth cordgrass, 30.5 days per month; short form of smooth cordgrass, 23.5; salt hay grass (Spartina patens), 17.3; and black grass (Juncus­gerardii), 8.7 days (Bertness and Ellison Figure 4.1. Species occurrence along an elevation gradient in a Long Island salt marsh relative to mean low water. Mean tide level (MTL) is 3.73 feet (1.14 m), while the mean tide range for this area...