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Tree densities and sex ratios in breeding populations of dioecious Central Amazonian Myristicaceae

Published online by Cambridge University Press:  10 July 2009

David D. Ackerly ,
Judy M. Rankin-De-Merona  and
William A. Rodrigues
David D. Ackerly
Affiliation:
World Wildlife Fund - US, Washington, DC, USA
Judy M. Rankin-De-Merona
Affiliation:
Departamento de Ecologia, Instituto Nacional de Pesquisas da Amazônia, Manaus, Brazil
William A. Rodrigues
Affiliation:
Departamento de Botânica, Instituto Nacional de Pesquisas da Amazônia, Manaus, Brazil
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Abstract

The densities of the breeding populations and the sex of all flowering individuals were recorded for five dioecious canopy tree species of Central Amazonian Myristicaceae, in 11 study areas of the Minimum Critical size of Ecosystems Project totalling 22.5 ha. Adult population densities were extremely low, ranging from 0.38 to 1.61 ha–1 for the five species studied. In a 10 ha study plot the mean distance to the nearest flowering conspecific ranged from 48 to 100 m, while the mean distance to the nearest opposite sex conspecific was 147 m. The two most abundant species, Iryanthera macrophylla and Virola calophylla, both showed male-biased sex ratios, of 23:9 and 20:6, respectively. The size class distribution of males, females and non-flowering individuals in V. calophylla suggests that earlier reproductive maturation of male plants may provide a partial explanation for this bias. In I. macrophylla, since 95% of the individuals were observed flowering, the observed ratio is representative of the population, and may be caused by sex shifts from male to female. The low reproductive densities, combined with the skewed sex ratios and overlapping generations of these species, create very small effective breeding populations, placing species such as these at great risk in the face of deforestation and habitat fragmentation.

RESUMO. [Densidades de árvores e razāo dos sexos em populações reprodutivas de espécies dióicas da família Myristicaceae na Amazônia Central.]

Foi realizado um Ievantamento da densidade de indivíduos adultos e do sexo de todos os indivíduos reprodutivos, para cinco espécies de árvores dióicas da família Myristicaceae. O estudo foi densenvolvido em 22,5 ha distribuídos por onze reservas biológicas na Amazônia Central dentro do Projeto de Dinámicas Biolótricas de Fragmentos Florestais. A densidade das populaçōes foi extremamente baixa.de 0,38 a 1,61 árvores ha−1, para as cinco espécies estudadas. A distância média ao coespecinco mais proximo foi de 48 a 100 m, enquanto a distância média ao coespecífico do sexo oposto mais próximo atingiu valores de até 147 m. As duas espécies mais abundantes, Iryanthera macrophylla e Virola calophylla, apresentaram razões de sexos de 23:9 e 20:6 respectivamente, com uma clara predominância de indivíduos masculinos. A distribuiçao de classes de tamanho para ambos os sexos e de indivíduos não reprodutivos em V. calophylla sugere que o amadurecimento precoce dos indivíduos masculinos seja uma das razões para a dominancia dos mesmos. Em I. macrophylla, uma vez que 95% dos indivíduos floresceram, a razãb de sexos observada é representitiva da população, e pode ser causada por mudançãs sexuais de uma fase masculina para uma fase feminina. As baixas densidades de indivíduos reprodutivos, àliadas às diferenças nas razões de sexos e à sobreposição de gerações, reduzem muito o tamanho efetivo das populações reprodutivas, colocando espécies como estas em alto risco de extinção, devido ao desmatamento e à fragmentação de seus habitats.

Keywords

AmazoniadioecyfloweringMinimum Critical Size of Ecosystems ProjectMyristicaceaesex ratiotropical trees
Type
Research Article
Information
Journal of Tropical Ecology , Volume 6 , Issue 2 , May 1990 , pp. 239 - 248
Copyright
Copyright © Cambridge University Press 1990

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