The demographic dynamics of three populations of Dioon edule Lindl. (Zamiaceae) were studied in a fragmented landscape using projection matrix modelling. Compared with other plant species, D. edule behaves like a tree life-form species. Density and spatial distribution patterns varied among populations according to models for animal-dispersed tree species. In all scenarios, l was most sensitive to changes in abundance of adult plants. The elasticity reproductive component (F) for the three populations was zero and stasis values (L) were higher, this being a function of the permanence of non-reproductive individuals. It was detected that disturbance influences the population dynamics of D. edule as a function of adult plant persistence. This observation suggests that the conservation of adult plants is critical for D. edule and perhaps for all cycads species. Adult plant decapitation should be halted at the 'Monte Oscuro' population, subjected to sustainable management since 1990, if higher seed production is needed in rural nurseries.

The cycad Ceratozamia mirandae is endemic to Chiapas, Mexico. Demographic studies were made in two of its populations in the Sepultura Biosphere Reserve under different conservation conditions; in the nucleus zone ''Tres Picos'' (conserved) and buffer zone ''La Sombra'' (disturbed and under management). Spatial distribution of C. mirandae was aggregated, showed a clumped local distribution on shallow soils on steep slopes and male and female cones appear to be synchronous in both populations. The population structure was of type I (Bongers) for both sites. Individuals between the sites showed differences in growth pattern. The oldest plants (80-90 cm tall) were estimated to be about 490 years at ''La Sombra''. The finite growth rate (k) in the buffer zone population showed a tendency for decrease whilst in the nucleus zone this estimate remained stable. The highest elasticity values lied in the transition of the first three classes of the ''La Sombra'' population, in ''Tres Picos'' this corresponded to adult plants between 20 and 30 cm tall. Given the above, it is proposed that in the nucleus zone, reproductive adults should be of highest conservation priority, whereas in the buffer zone seedling reintroduction should be carried out regularly until the population increases. We recommend an IUCN Red List category of Vulnerable (VU C, 2a), largely due to difficult-to-control destructive annual forest fires that occur in this Reserve.

Dioon angustifolium presents high levels of genetic diversity compared with other cycad species, in spite of small population sizes. The recent bottleneck effect did not effectively reduce the genetic variation to the extent of eliminating these populations. The distribution of D. angustifolium appears to be the result of historical biogeographical effects related to the Pleistocene glaciations. It is recommended that this species be catalogued in the IUCN Red List of Threatened Species and conservation efforts be made to preserve it.

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