Appendix S1. Figure S1: Sampled localities.

Appendix S2. Figure S2: Physiographic map of the North American Pacific basin and phytogeographical sketch of the study area.

Appendix S3. Strand and dune forms, main features, major vegetation types, geological and geomorphological processes.

Appendix S4. Sampled localities, bioclimates, potential natural vegetation, ecofloristic zones, zonobiomes, zonoecotones, ecoregions and phytogeographical provinces.

Appendix S5. Climate data and indices, bioclimatic classification, zonobiomes and potential natural vegetation at the association level for 226 weather stations.

Appendix S6. Distribution ranges of the phytogeographical elements and recorded taxa.

Appendix S7. Dendrograms obtained from nine ALC.

Appendix S8. Main features of the eight data sets prepared for the second ALC.

Appendix S9. Relevé groups obtained through nine successive ALC.

Appendix S10. Synoptic syntaxonomical tables.

Appendix S11. Diagnostic species and phi-values for higher syntaxa and associations.

Appendix S12. Complete phytosociological tables for new associations.

Appendix S13. Dichotomous key to associations based on floristic, phytogeographical and ecological features.

Plate S1. Beach and dune plants: 1=Abronia latifolia. 2=Abronia umbellata subsp. umbellata. 3=Mimulus guttatus. 4=Glehnia littoralis subsp. leiocarpa. 5=Astragalus fastidius. 6=Euphorbia leucophylla. 7=Rhododendron albiflorum. 8=Ambrosia chamissonis. 9=Carex macrocephala (male, left; female, right). 10=Chaenactis lacera. 11=Scaevola plumieri. 12=Stegnosperma halimifolium. 13=Lonicera involucrata var. ledebourii (inflorescence). 14=Lonicera involucrata var. ledebourii (berry). 15=Camissonia crassifolia. 16=Houstonia mucronata. 17=Lathyrus japonicus var. maritimus. 18=Senecio pseudoarnica. 19=Commelina diffusa. 20=Lotus bryantii. 21=Picea sitchensis. 22=Pseutotsuga menziesii var. menziesii. 23=Pinus muricata. 24=Pinus contorta var. contorta. 25=Portulaca pilosa. 26=Atriplex leucophylla. 27=Antigonon leptopus. 28=Polygonum paronychia. 29=Coulterella capitata. 30=Drymaria holosteoides var. crassifolia. Photographs 1–4, 7–9, 13, 14, 21–24, 26 and 28 by L. Monje, Scientific Drawing & Photography Department, Alcalá University (LMO).

Plate S2. (a) Lonicero ledebourii–Pinetum sitchensis and freshwater swamp with Nuphar luteum subsp. polysepalum on Croft Lake, Oregon. Photograph by LMO. (b) Arctostaphylo uva-ursi–Pinetum contortae on dune ridges. Bare sands colonized by Lupino littoralis–Polygonion paronychiae with Arctostaphylos uva-ursi (1) and Cytisus scoparius (2). Arctostaphylos columbiana (3) at forest edges. Umpqua River, Oregon. (c) Wind-trained Picea sitchensis and krummholzs with Gaultheria shallon, Garrya elliptica and Lonicera ledebourii. Sutton Beach, Oregon. Photo by LMO. (d) Isolated remnant of dune parabolas still bearing forest (Lonicero ledebouri–Pinetum contortae) resting upon the summit at Cape Kiwanda, Oregon. Photo by LMO. (e) Poo macranthae–Lupinetum littoralis, with Poa macrantha and Juncus breweri in the foreground, on dry deflations plains among young shore pines (Pinus contorta). Background: Arctostaphylo uva-ursi–Pinetum contortae. Nehalem Bay State Park, Oregon. (f) Leymo mollis–Caricetum macrocephalae among driftwoods. Sand Lake, Oregon. Photo by LMO. (g) Detail of Juncetum breweri–sitchensis. Dominants are Juncus falcatus subsp. sitchensis (flat leaves) and Juncus breweri (with lateral inflorescences and leafless). Bullard Beach, Oregon. (h) Foreground: (1), Mertensio maritimae–Honckenyetum majoris on debris line. Background: (2) Senecioni pseudoarnicae-Leymetum mollis. Middle Bay, Kodiak Island, Alaska.

Plate S3. Baja California. (a) Coastal flat, dominated by Atriplici linearis–Frankenietum palmeri, near Laguna San Ignacio, Baja California Sur. (b) Euphorbio miserae–Lycietum californici on windy alkali sandy plains near Bahía AsunciÓn, Baja California Sur. (c) Foredune built by Suaedo taxifoliae–Allenrolfeetum occidentalis near Mulegé, Baja California Sur. Backdunes with Lycio congestiMaytenetum phyllanthoidis (2). (d) Pebble beach with Atriplicetum magdalenae bordering debris lines near Punta Abreojos, Baja California Sur. (e) Dense communities of Lycio congestiMaytenetum phyllanthoidis thriving on windward slopes of the Sanlucan backdunes. (1) Maytenus phyllanthoides; (2) Lycium congestum. Punta Conejo, Baja California Sur. (f) Sarcocaulescent thornscrubs: (1) Cyrtocarpo edulis–Maytenetum phyllanthoidis on leeward slopes of the Sanlucan backdunes; (2) Jatropho cordatae–Cyrtocarpetum edulis on inland sandy plains. Between Punta Colorada and Punta Arena, Baja California Sur. (g) Foredune hillocks of Palafoxio linearis–Abronietum maritimae in Isla Magdalena, Baja California Sur. Visually, dominants are Abronia maritima (green) and Sesuvium verrucosum (red). Dominant NW winds coming from the right side of the picture. (h) Shadow dunes and small hillocks of Euphorbio leucophyllae–Jouveetum pilosae on the upper part of the beach, and pioneer vegetation (Euphorbio leucophyllae–Drymarietum crassifoliae) along the tidal line. Near Punta Arenas, Baja California Sur. (i) Detail of Plate 4, photo E. (1),Active dunes of the Flandrian Episode II with Senecioni blochmaniae–Lupinetum chamissonis; (2) older, stabilized, parabola of the Flandrian Episode I at bottom, with Corethrogyno californicae–Ericamerietum ericoidis; (3) dominant on the foreground are Lupinus chamissonis (blue flowers), Eriophyllum staechadifolium (yellow inflorescences) and Achillea arenicola (white inflorescences). (j) Interior of the tip of a Flandrian parabola at Marina State Beach, California. Foreground: wind-exposed flank with Polygono paronychiae–Artemisietum pycnocephalae (1) and wind-sheltered depression with Dudleyo farinosae–Ericamerietum ericoidis (2). Background: Preflandrian dune sheets with irrigated crops and remnants of the climax. Photograph by LMO. (k) Wind-exposed top of the former parabola. Dune mat (Polygono paronychiae–Artemisietum pycnocephalae). Dominants are (1) Artemisia pycnocephala (canescent), (2) Polygonum paronychia and Eriogonum parvifolium (prostrate chamaephytes) and Dudleya caespitosa (yellow flowers). Photograph by LMO. l=Lee slope of the former parabola with Dudleyo farinosae–Ericamerietum ericoidis. Foreground: (1) Eriogonum latifolium (white, rounded inflorescences) and Dudleya farinosa; (2) Ericameria ericoides; Background: (3) wind-exposed crest with Polygono paronychiae–Artemisietum pycnocephalae); (4) top of a dune invaded by Carpobrotus species. Photograph by LMO.

Plate S4. (a) Dune swale at Umpqua River dune system, Oregon. (1) Pioneer clumps of Ammophila arenaria on active sands; (2) Marshy bottom with helophytes such as Eleocharis palustris and Juncus phaeocephalus, encircled by sedge and rush meadows (Argentino egedii-Caricion obnuptae); (3) sedge meadows (Argentino egedii–Caricetum obnuptae) with scattered Salix hookeriana seedlings; (4) Willow thickets (Lonicero–Salicetum hookerianae); (5) Canopies of semi-buried willows emerging from (6) hummocky areas created Leymo mollis–Ammophiletum arenariae and Lupino littoralis–Polygonion paronychiae; (7) Carici obnuptae–Pinetum contortae on wet sands; (8) Isolated remnant of dune parabolas bearing forest (Lonicero ledebouri–Pinetum contortae) resting upon the summit. Photograph by LMO. (b) Flat-topped dune and rear inland plains between Punta Colorada and Punta Arena, Baja California Sur. (1) Euphorbio leucophyllae–Drymarietum crassifoliae forming small mounds along tidal lines; (2) Euphorbio leucophyllae–Jouveetum pilosae building a first ridge on the upper part of the beach; (3) second level of the ridge (Ipomoeo imperati–Jouveetum pilosae); (4) third level of the ridge (Daleo anthonynae–Jouveetum pilosae); (5) thornscrub (Cyrtocarpo edulis–Maytenetum phyllanthoidis) thriving on leeward slopes; (6) sandy plains with sarcocaulescent thornscrubs (Jatropho cordatae–Cyrtocarpetum edulis). (c) Dry deflation plain at Nehalem Bay State Park, Oregon. Foreground and open areas: Lupino littoralis–Polygonion paronychiae with scattered Arctostaphylos uva-ursi and Pinus contorta var. contorta seedlings. Background: Arctostaphylo–Pinetum contortae. (d) Flandrian dunes perched on a steep bluff rising 100 m from the beach (BE) at Mussel Rock. Dominant NW winds coming from the left corner of the picture. Active dunes of the Flandrian Episode II (E-II) invading an older, stabilized and eroded, parabola of the Flandrian Episode I (E-I). E-II: Ambrosio chamissonis–Abronietum maritimae ambrosietosum chamissonis. E-I: Corethrogyno californicae–Ericamerietum ericoidis. Beach: Atriplex leucophylla forming shadow dunes in the upper beach zone and Cakile maritima on debris lines. Photograph by LMO. (e) Mediterranean dune complex near Guadalupe, California in a picture taken with the ocean around 2.5 km behind. Foreground: active dunes of the Flandrian Episode II (E-II) invading an older, stabilized, parabola of the Flandrian Episode I (E-I). E-II: Senecioni blochmaniae–Lupinetum chamissonis. E-I: Corethrogyno californicae–Ericamerietum ericoidis, secondarily invaded by Lupinus chamissonis and by the exotic, very aggressive, grass Ehrharta calycina. Background: stabilized Preflandrian dune sheets (PF), active Flandrian dunes of Mussel Rock (MR; see picture D), and Casmalia Hills. Photo by LMO.

Photo S5.Pholisma arenarium (Lemnoaceae), an extremely rare root parasite, endemic to the North American Southwest, always living on coastal dunes and sandy soils. In this photograph, taken near Morro Bay, California, occurs as parasitic on Ericameria ericoides, a diagnostic species of the association Corethrogyno californicae-Ericamerietum ericoidis. Photo by Luis Monje, Scientific Drawing & Photography Department, Alcalá University.

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