Population structure of Dioon purpusii Rose in Oaxaca, Mexico

Abstract

Cycads are currently suffering significant habitat loss from deforestation, agricultural expansion, and illegal harvesting. Dioon purpursii Rose has restricted distribution and small populations, which until now are endemic in the state of Oaxaca in Mexico, and it has been included in the category of threatened and vulnerable. Three sites with D. purpusii plants were located in the state of Oaxaca. Structural and dendrometric characteristics of the population and physiognomic, floristic and environmental characteristics of the forest community at each site were determined. Dendrometric variables were evaluated by an analysis of variance (ANOVA), and a similarity analysis between sites was applied to all characteristics, followed by a cluster analysis to express the relationships between sites. The population structure of the three sites is typical of species in which a great frequency of seedlings and juveniles suggests optimal microhabitat conditions for initial establishment. The results of the ANOVA showed that the most developed plants were found at Site 3, followed by Sites 1 and 2. The floristic composition corresponded to the type of tropical deciduous forest vegetation in the region, where D. purpusii formed part of the tree, shrub and herb layers. The phenogram resulting from the similarity and cluster analyses showed that Sites 1 and 2 are more similar (0.97), and Site 3 less similar to the other two (0.84). Although the population structure of D. purpusii at the three sites is similar to that found for other cycad species, the main threats are the small number of reproductive individuals, harvesting of leaves, and the constant threat of habitat alteration, like construction of roads, contributing to extinction.

Key words: cycadas, Dioon purpusii, population structure, physiognomy, conservation, Oaxaca, Mexico.