Beach vegetation and plant distribution patterns along the northern Gulf of Mexico
Barbour, M.G.; Rejmanek, M.; Johnson, A. F.; Pavlik, B. M.
We sampled the leading 50 ± m of beach vegetation along the U. S. portion of the Gulf of Mexico with 63 strip transects, made up of contiguous 1 x 1 m quadrats. Only stable or prograding beaches were sampled. Cluster analysis and ordination of samples based on 73 taxa revealed three vegetation types: southern Florida, Florida panhandle to Mississippi, and Louisiana to Texas. These differed in dominants, species richness, and array of growth forms. In choosing growth form traits, we emphasized leaf characteristics, including the presence/absence of Kranz anatomy. Environmental factors which were correlated with regional vegetation included sand texture, sand chemistry, topographic roughness of the transects, and climatic or historical factors linked with latitude and longitude. Uniola paniculata was a leading dominant everywhere except in Louisiana, where it was replaced by Spartina patens. The most arid sites exhibited relatively high cover by taxa with narrow, small, or pubescent leaves, but were low in cover by succulent, sclerophyllous, or C4 taxa. The coldest and wettest sites exhibited the lowest cover by succulents, but were high in relative cover by C4 taxa (76 %). Only sites relatively undisturbed by hurricanes in the past 16 yr were sampled, but even with this constraint, there was a negative correlation between species richness and severity of past disturbance. Very few taxa exhibited increasing cover with increasing disturbance. At the local scale, most taxa were not randomly or uniformly distributed, but rather appeared to peak in cover at particular distances inland from the leading edge of vegetation. However, only 37 taxa were widely enough distributed to permit a statistical evaluation of local pattern. Six had significantly more cover in the leading 20 m of transects: all were prostate forbs, either succulents or C4 plants, and with pubescent leaves. Twelve taxa were significantly more frequent on the most exposed/stressed parts of transects and 14 were more frequent on the most protected parts. The former most often were forbs with large, pubescent, or succulent leaves (Atriplex, Cakile, Cassia, Croton, Ipomoea, Oenothera, Panicum, Paspalum, Scaevola, Sesuvium); the latter most often were erect or rigid or forbs with small, non-pubescent. leaves (Cenchrus, Chamaescyce, Conyza, Heterotheca, Hydrocotyle, Iva, Oenothera, Opuntia, Paronychia. Polygonella, Sporobolus). Several dominant C4 grasses exhibited no pattern (Schizachyrium. Spartina, Uniola). In general, C4 plants exhibited no significant spatial or topographic patterns. Leaf anatomy of 62 taxa examined fell into five subjective categories or types. including some which appear to be novel to the literature. The largest fraction (25 taxa) exhibited relatively undifferentiated, isolateral mesophylls of total thickness averaging 660 My.