A phytosociological and phytogeographical survey of the coastal vegetation of western North America: beach and dune vegetation from Baja California to Alaska

Abstract Questions: What is the floristic composition of the plant communities that inhabit the beaches and dunes of the Pacific coast of North America? What are their ecological relationships in the zonal and successional gradients typical of coastal dune systems? Does climate affect the latitudinal distribution of the azonal vegetation? What other environmental factors influence their distribution on the regional or local scale? Location: Pacific coast of North America, from Baja California (23°02′ N) to Cook Inlet, Alaska (61°30′ N). Methods: A total of 1730 phytosociological relevés were obtained by sampling 279 coastal localities. In all localities, zonation was interpreted by considering transects from the shoreline inland. Through traditional phytosociological tabular classification and average linkage clustering, relevés were syntaxonomically classified. Syntaxa, supported by fidelity calculations, are described and interpreted according to their phytogeographical distribution, their relationships with macrobioclimate (MB) and bioclimates (BIO), and to the topographic and ecological gradients typical of coastal sandy areas. Results: Our bioclimate analysis served to define four MB and 11 BIO, each characterized by a particular type of potential natural vegetation. By floristically analysing the 522 vascular taxa detected, these were related to 16 phytogeographical elements. Syntaxonomically, the communities observed comprise 74 vegetation types, 70 of which are considered associations characterized by their diagnostic combination of species. These associations were classified into 24 alliances, 16 orders and 12 phytosociological classes. Two new classes, five new orders, 10 new alliances and 34 new associations are presented. Some syntaxa have been revised or validated. Conclusions: The coastal vegetation of the northern Pacific shows a distribution that is mainly linked to the four MB of the latitudinal gradient examined. Higher syntaxa are preferentially linked to a macrobioclimate, although some edaphic factors determine that some occupy two different but geographically close bioclimate zones. Regional factors, such as alkaline sands, induce the appearance of specialized vegetation types rich in endemic plants.