Cactus  (Melocactus pachyacanthus)

Melocactus pachyacanthus is a squat, rounded cactus. Two subspecies are recognised which differ in the appearance of their stems; M. p. subsp. pachyacanthus has a waxy bluish-green sheen and woolly areoles, whilst M. p. subsp. viridis lacks wool and may have a waxy covering only when young (2). The stem is distinctively crowned by a concentration of dull pinkish-red bristles known as a 'cephalium', which may reach up to 30 centimetres high and 10 centimetres in diameter (2). This feature is characteristic of cacti from the genera Melocactus and Discocactus (4). The stem of the cactus has 9 to 11 low ribs, bearing rows of areoles from which stout reddish spines extend (2). Deep pink-red flowers are produced although they barely extend beyond the cephalium; following fertilisation, pale, cylindrical fruits develop (2).

Endemic to Bahia State in eastern Brazil, this species has a restricted range within the Rio São Francisco drainage basin in the northern part of the State (2).

Melocactus pachyacanthus inhabits the dry thorny forests region of eastern Brazil known as caatinga vegetation (2). It is usually associated with exposed limestone rocks (2).

Hummingbirds pollinate the flowers of this cactus and the fruits are probably distributed by lizards (2).

Habitat destruction is widespread in the caatinga region of eastern Brazil; over the centuries, forests have been cleared for fuel and for the conversion of the land to agriculture (2). As a result, M. p. subsp. viridis has now been lost from all but a single site within its former range, where, as of 2003, fewer than 50 individual plants survive (2).

International trade in wild-taken Melocactus pachyacanthus is restricted by its listing on Appendix II of the Convention on International Trade in Endangered Species (CITES) (3). The largest population of M. p. subsp. pachyacanthus numbers over 1000 individuals and is fully protected within the Gruta dos Brejoes protected area (2). The future of M. p. subsp. viridis is far from secure however, and urgent field research is needed in the hope of discovering new populations of this subspecies (2). A greater network of protected areas within the vast caatinga region is a conservation priority if its endemic species are going to persist.

For more information on Brazilian cacti see:

• Taylor, N.P. and Zappi, D.C. (2004) Cacti of Eastern Brazil. Royal Botanic Gardens, Kew.

Authenticated (20/3/03) by Dr Nigel Taylor, Royal Botanic Gardens, Kew.
http://www.rbgkew.org.uk