Cycad  (Dioon edule)

Classified as Near Threatened (NT) on the IUCN Red List (1) and listed on Appendix II of CITES (4).

Dioon edule is a medium-sized cycad with rather stiff, straight, light to blue-green leaves (2) (3) (5) which, as in all cycads, are large and divided, giving the plant the appearance of a palm or tree-fern (2) (5). Each leaf bears 70 to 150 pairs of narrow leaflets, which measure around 12 centimetres in length and are sometimes slightly hairy (2) (3) (5). The trunk, like that of all cycads, is made up mainly of storage tissue, with very little true wood (5), and in this species may either stand erect or lie on the ground (2). Two varieties of Dioon edule are recognised, with Dioon edule var. angustifolium generally having narrower leaflets and a shorter stem than Dioon edule var. edule (2) (5) (6). However, a number of disjunct populations exist throughout the range, which show great variety in aspects of the stems, leaves and cones, and which may comprise distinct varieties or even distinct species (2) (5) (7).

Dioon edule is found along the eastern slopes of the Sierra Madre Oriental mountain range in Mexico, from Tamaulipas and Nuevo León in the north to Veracruz in the south. Variety angustifolium is restricted to the northern parts of the range (2) (3) (5) (6) (7).

This cycad generally occurs in rocky habitats or on steep cliffs in tropical deciduous forest or oak woodland, at elevations of between 500 and 1,500 metres (2) (3).

Although superficially resembling palms and tree-ferns, cycads differ in reproductive behaviour (5). All are dioecious, with separate male and female plants, and all produce seeds with a fleshy outer coat (sarcotesta) surrounding a hard, stony layer (sclerotesta). The fleshy coat is attractive to animals, which act as dispersal agents. Like all cycads, Dioon edule does not produce flowers, instead bearing the reproductive organs on cones, which are formed from modified leaves. Although previously believed to be wind-pollinated, cycads are now known to be mostly pollinated by insects such as beetles (2) (5) (8). The female cone of Dioon edule measures up to about 29 centimetres in length, while the male cone is slightly smaller, at up to 20 centimetres (2). The seeds are relatively large, at around 2.5 to 4.5 centimetres in length (5). All cycads are slow-growing and long-lived (5) (8), and, as in other Dioon species, the female Dioon edule only produces a single cone at each reproductive cycle (2).

In addition to normal roots, cycads possess specialised roots containing cyanobacteria, which form a symbiotic relationship with the plant, providing it with extra nutrients by converting (fixing) atmospheric nitrogen into a usable form. Cycads also posses roots which can be retracted for protection against fire and drought (2) (5).

Dioon edule is under threat from forest clearance and the removal of plants from the wild for landscaping and plant collections (2) (8) (9) (10) (11). The species is easily grown in cultivation, and many thousands have been removed as ornamentals, mainly to markets in the United States (2) (11). The Latin name of this species means ‘edible’, and, although all parts of cycads are toxic, the seeds of Dioon edule have also been extensively used as food, being ground into a flour which is then boiled or roasted to remove the toxins (2) (8). A further threat to the species is the common practice of cutting the crowns off mature plants, mainly for use as decorations. This practice has all but eliminated seed production in some areas, resulting in little or no regeneration (2) (11). Although Dioon edule still has a relatively large population and range (2), the extremely slow rates of growth and reproduction typical of cycads limit the population’s ability to recover from the ongoing decline (10).

As an ancient group, with considerable benefits to horticulture and the pharmaceutical industry, as well as to soil health and fertility through nitrogen fixing, cycads are of great conservation interest (5). Dioon edule receives some protection from international trade under its listing on Appendix II of the Convention on International Trade in Endangered Species (CITES) (4), although increasing numbers have been used in horticulture and landscaping within Mexico (2). An action plan published by the IUCN Cycad Specialist Group recommends a range of conservation actions for cycads, including setting up reserves and sustainable nurseries, regularly updating the taxonomy and status of the world’s cycads, and identifying the conservation needs and the specific actions required to preserve this unique group of plants (9).

Specific measures being undertaken for Dioon edule include the conservation of plant material in botanical gardens and seed banks, and improved protection and enforcement of legislation for wild populations (10). The first sustainable management programme for this species was initiated in Mexico in 1990, involving a nursery set up and managed by a local community to provide revenue to supplement traditional agriculture. It is hoped that initiatives such as this will reduce collecting pressure on wild populations, whilst also involving local people in the protection of both this cycad and its habitat (9) (11).

To find out more about cycads and their conservation see:

• Palm and Cycad Societies of Australia:
  http://www.pacsoa.org.au/
• Royal Botanic Gardens Sydney - The Cycad Pages:
  http://plantnet.rbgsyd.nsw.gov.au/PlantNet/cycad/index.html
• IUCN/SSC Cycad Specialist Group:
  http://www.cycadsg.org/
• The Cycad Society:
  http://www.cycad.org/