The Ecology of Tidal Freshwater Marshes of the - USGS National ...

2.4 FACTORS CONT3OLLIVG PLANT DEMOGRAPHYThe distribution of plant speciespopulations in any natural situation reflectsthe response of individual speciesto soeci f ic environmental parameters. Inwetl and habitats these parameters areespecially varied, primarily due to thei nf 1 uence of water on habi tat gradients.Siologically-mediated interactions betweenplant species further coqplicate the perceptionof physical gradients. Much ofthe information concerning the causes forobserved spatial distributions of vascularflora in these marshes is anecdotal, althoughenough exists to warrant a generaldiscussion.InundationThere seems to be a general consensusamong researchers investigating plant demographyin the tidal freshwater habitatthat the frequency and duration of floodingis the primary factor governing speciesdistributions (Kiviat 1978a; Ooumleleand Silberhorn 1978; Ferren et al. 1981;McCormick and Somes 1932). Despite thefact that the vast majority of plantsoccurring in these marshes must experienceflooding on a daily basis, speciesvary greatly in their ability to withstandinundation. For some species, extensiveflooding seems to be a physiological requirement for subsistence, whereas forothers, it can be a detriment to normalgrowth and development. Scul thorpe (1967)notes that nicmerous terres tri a1 pl ants areable to survive long periods eithercompletelyor partially submerged. It isconceivable that facultative hydrophyteshave evolved in order to avoid competitionor to exploit open niches in habitats suchas these.stunted progeny in deep-wa ter experimentalplots (Yamisaki and Tange 1981). Manyother researchers working in sal t marsh,mangrove swamp, and freshwater lake environmentshave concl uded that inundationeffectively contributes to segregation ofpl ant species ~opul a ti ons a1 ong an el evationalgradient (Filandossian and Mc Intosch1950; Aifains 1963; Scul thorpe 1957; Kerwinand Pedigo 1971; Odum 1971). As yet, thisphenomenon has not been quantitativelydetermined for tidal freshwater marshes.However, evidence from other wetl and si tuationssuggests that tidal freshwaterplant communi ties segregate a1 ong inundationgradients as well.SubstrateThe soil in tidal freshwater marshescan be described as a waterlogged organicmuck with varying amounts of sand, silt,and clay (see Section 1.7). Differencesin soil stability, soil moisture retention,and soil nutrient availabil i t,y areall related to the physical characteristicsof a given substrate and may influencespecies distributions directly. Thespatial heterogeneity of substrate characteristicsis not sufficient to explaindistributions or species performance(1Jhigharn and Simpson 1975; Wetzel andPowers 1978). Wetzel and Powers (1978)concluded that substrate characteristicsaffect plant demography only in local izedzones within the marsh, and then, thesesubtl e differences are largely obscured bymajor environmental gradients acting toproduce species distributions (e.g., elevationand tidal inundation). In ouropinion, this is an area which needsconsiderably more research.Current FlowIn the progression from open water Low-gradi ent river courses of thechannels to the tnarsh-upland boundary, the Atlantic Coastal Plain tend to flow ratherspecies composition of vascular flora sluggishly except during extreme storm orchanges noticedbly, even over almost im- flood events. Aside from channel disperceptablevariation in marsh surface charge, however, the daily ebb and floodelevations (Hoover 1983). It is known of tidal water onto and off the marsh surthatcommon and narrow-leaved cattails face can produce significant currentwill segregate along a gradient of water velocities. Much of this water becomesdepth in nontidal habitats, the latter channel ized into dendri tical ly-shapedspecies found in deeper water (Grace and creeks within the marsh. These creeksGetzel 1951). Common reed and wild rice deliver ground water from high marsh toa1 SO respond to varying inundation, each low marsh to channel long after the tidespecies producing fewer and somewhat has ebbed (Hoover 1983). Concentrated27