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

of sorne form (e.g . , major community t-ypes)are apparent in most tidal freshwaterhabitats, ascribing such a phenomenon tocompetition per se is difficult.The mechanisms involved in competitiveplant interactions are varied. Onlya few studies have experimental 1 v detnonstratedthe importance of competitive displacementin maintaining wet1 and y1 antdistributions. Grace and idetzel (1981)showed that populations of common andnarrow-leaved cattail s segregate accordingto water depth, the former competitivelysuperior in shallow water due to itsgreater 1 eaf surface area. However,narrow-1 eaved cattail has the potenti a1 togrow in deeper water than comqon cattail,a capacity facil i tated by phenotypictraits such as taller, narrower leaves andgreater rhizome storage. Cahoon (1982)also noted phenotypic responses in twotidal freshwater species with over1 appingdistributions. Pose-ma1 1 ow was found torespond to the presence of narrow-leavedcattail by increasing its leaf size. However,the consequence of such a strategywas a concomitant reduction in reproductiveoutput. Buttery and La~nbert (1965)found that manna-grass dominated a particularportion of a habitat gradientstrictly through its ability to opoortunistical ly outcompete another speci es , thecommon reed. Without further studies, itis difficult to accurately ascertain theimportance of competition on species distributionpatterns. The evidence avail -able thus far, however, suggests that competiti ve pressures act in con junction withpi~ysical factors to produce speciesniches.rnents with leaf and petiole extracts, aswell as soil extracts, showed that thesespecies vary in their abil ity to affectthe germination of bioassay species, suggestingthat similar interactions may occurbetween marshland species.2.5 SEASONAL SUCCESS1 ONA unique aspect of tidal freshwatermarshes is the continually changingappearance of the vegetation over thecourse of the growing season (Figure 15)A1 lelopathyChemicals derived from one plantwhich have inhibitory effects on thegrowth and development of another plantare termed a1 lelochewics. The concentrationin the soil of alleleochemics from adominant plant [nay exclude many otherpl ant species from the community (Whittaker1975). Mcriaughton (1968) suggestedthat cattails have a1 lelopathic effects onother aquatic sgecies. Bonasera et al.(1979) cornpared the a1 lelopaihic potential Figure 15. Winter and early summer scenesof four species common to tidal freshwater at the same location on tidal freshwaterhabitats --- giant ragweed, arrow-arum, Poto~nac River. Photographs by !lj chaelburinari gold, and comnlon cattai 1. Experi- Dunn.