Conocarpus erectus L.
Buttonwood (Conocarpus erectus) is a widespread species of terrestrial mangrove along tropical and subtropical coasts of the Americas and West Africa. It grows as a shrub or small tree on on the coastal mainland and on islands of a variety of sizes, down to the smallest vegetated islands (it is sometimes the only species of terrestrial vascular plant on an island). There are both "silver" and "green" morphs (with some intermediates), having highly pubescent (downy) and nearly glabrous (smooth) leaves, respectively. (Schoener 1988; Nettel 2008)
Nettel et al. (2008) have developed microsatellite markers for Buttonwood.
Buttonwood is often shrubby in form (although it can grow to 6 meters or more in height) and usually grows on the landward side of tidal mangrove swamp forests. In Florida and the Caribbean, it is the only mangrove species with alternately arranged leaves. The lance-shaped or elliptic yellow-green leaves are long-pointed at both ends, leathery and slightly fleshy, 3 to 8 cm long and 1 to 3 cm wide. Leaves have slightly winged petioles, 0.5 to 1 cm long, with 2 dot-like glands. Leaf blade edges are untoothed and usually have several dot-like glands near vein angles on the lower surface. The fragrant greenish flowers are tiny, less than 2 mm across, and crowded in balls less than 6 mm in diameter in terminal and lateral clusters. Flowers are mostly bisexual, but some trees bear heads of male flowers. Purplish-brown rounded cone-like fruits are 1 cm in diameter and are composed of many scale-like 1-seeded fruits about 3 mm long. (Little and Wadsworth 1964)
Buttonwood is found in mangrove swamp forests on silty shores near salt and brackish water, usually growing on the landward side. It may also occur on rocky and sandy shores (as an ornamental, it grows in much broader conditions). (Little and Wadsworth 1964)
A study of patterns of herbivore damage to Buttonwood leaves on 74 small islands in the central Bahamas found that leaves on islands inhabited by lizards were less damaged than leaves on lizard-free islands and that pubescent (silver) leaves showed less damage than others (Schoener 1988). Furthermore, the silver pubescent form was relatively more common (as measured by percent cover) on lizard-free islands than on islands with lizards (Schoener 1987). There is evidence that at least some insect herbivores prefer green (smooth) leaves to silver (pubescent) ones (Agrawal and Spiller 2004) and much evidence from herbivory studies on other plants that leaf "hairiness" can effectively deter herbivory. Experimental introductions of lizards have reduced leaf damage (Schoener and Spiller 1999). These findings suggest that the presence of insect-eating lizards may reduce damage to Buttonwood leaves caused by insect herbivores, and that in the absence of lizards pubescent-leaved Buttonwoods may be favored over smooth-leaved ones by natural selection as a result of their greater resistance to herbivory. The full story, however, is likely more complex than this. For example, Agrawal and Spiller (2004) note that the silver and green leaves differ in ways other than hairiness (for example, in nitrogen content and leaf toughness). They also report that silver morphs are more common on protected islands, while green morphs are more common on exposed islands, suggesting that the green morph may be favored in the disturbed environment on exposed islands. Thus, some environmental conditions may favor the green form, while other conditions favor the silver form, which could explain the persistence of this polymorphism, but more experimental work is needed to clarify these interactions. Spiller and Schoener (2007) found that after hurricanes, herbivory increased dramatically, as did the disparity between herbivory levels on islands with and without lizards. The authors attributed the increase in herbivory to a reduction in numbers of both lizards and predatory arthropods.
The geographic range of Buttonwood includes the shores of central and southern Florida, including the Florida Keys; Bermuda; most of the West Indies; both coasts of continental tropical America from Mexico south through Central America and northern South America to Ecuador and the Galapagos Islands and Brazil; and tropical West Africa. (Little and Wadsworth 1964; Nettel et al. 2008)
Semple (1970) gives the distribution as "along the coasts of west tropical Africa, the Atlantic and Pacific coasts of tropical and subtropical North and South America and throughout the West Indies". Semple notes that pubescent-leaved individuals (as well as the typical glabrous, or smooth-leaved ones) are restricted to the northern West Indies, southern Florida, and northern Central America.
Buttonwood has been introduced to Hawaii at least twice and the silver-leaved variety sericeus is still commonly planted as an ornamental. The green-leaved variety was introduced to Oahu before 1910, possibly from Florida, and the variety sericeus was introduced to Oahu from the Bahamas in 1946. Both forms of Buttonwood have escaped cultivation and established small wild populations on some islands. In contrast to Red Mangrove, which was introduced to Hawaii and is now very common and widespread there, Buttonwood has not shown much tendency to spread beyong the initial introduction sites. In Hawaii it is cultivated and sparingly naturalized in coastal areas of Kauai, Oahu, Lanai, and Molokai. (Allen 1998 and references therein)
In a study of Buttonwood in Mexico, flowering and fruiting were found to be continuous, but seed viability was less than 12%. During the dry season, fewer than 5% of seeds germinated and only 1.6% of seedlings survived to flower after 25 to 28 months (Hernandez and Espino 1999).
Evolution and Systematics
Systematics and Taxonomy
In the northern portion of its range, silvery pubescent-leaved Buttonwood may occur along with the usual green smooth-leaved ones. These have historically been viewed as a distinct species, but today are treated either as a variety (Conocarpus erectus var. sericeus) or as examples of natural variation that do not warrant any distinct taxonomic status (Semple 1970).
- Conocarpus erectus var. sericeus Fors ex DC. (synonym)