BOTANY, VOL. 4: ECOLOGY — 2007

15. VEGETATION OF THE EARTH

Five major global types of coastal vegetation

Tropical lowland sandy or coral coasts, developing under the protection of reefs and dominated by Cocos nucifera (coconut palms, col. pl. 15.16, A), along with Pandanus (screwpines, col. pl. 15.16, B) and Casuarina (col. pl. 15.16, C) in the paleotropical region extending to Australia. Ipomoea pes-caprae (Convolvulaceae) is widely distributed. The substrate is frequently eutrophied by nesting seabirds.

Tropical and subtropical muddy-clay intertidal coasts — the mangrove biome: these are salt-tolerant, dense broad-leaved thickets up to 20 m high, formed by species possessing adventitious stilt roots (col. pl. 15.16, D, Rhizophora sp., see above, Fig. 11.248) or pneumatophores (respiratory roots — Ed.) (Avicennia sp.), which help them anchor in muddy sediments (col. pl. 15.16, F, Fig. 13.29). In calm waters, mangroves can form ecosystems with extremely high species richness.

Low-lying temperate and Mediterranean sandy coasts are inhabited by clonally spreading monocots (col. pl. 15.16, J, K, Ammophila, Agropyron, Cyperus) and dicots (col. pl. 15.16, L, Convolvulaceae, Brassicaceae, Plantagina-

ceae, Asteraceae, etc.) that maintain shoreline stability (dunes, see Fig. 15.4). In the brackish waters of flat intertidal zones (H, I), salt-tolerant plants of the families Chenopodiaceae and Cyperaceae dominate (salt marshes and dunes, see 15.1.1, Fig. 15.5, M — Q).

Temperate and Mediterranean cliffed coasts are populated by a characteristic assemblage of halophytic rock species (M — O). Common representatives here include Apiaceae (Crithmum), Plumbaginaceae (Armeria, Limonium), Chenopodiaceae (Beta maritima — the ancestor of many cultivated beet relatives), Cruciferae, and various Asteraceae, Fabaceae, and Euphorbiaceae.

Cold-temperate to polar rocky coasts are characterized by an intertidal zone densely covered with Algae, particularly brown algae (col. pl. 15.16, P — R, Enteromorpha, Fucus, Laminaria). The upper tidal limit is dominated by fucoid species (R). These brown algae contain up to 40% dry weight of a highly Swelling polysaccharide (alginate; see 11.2, Phaeophyceae), which prevents them from drying out during low tide. Algae can withstand severe wave action even during heavy storms thanks to their leathery thalli and holdfasts, which anchor them firmly to the substrate.