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

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.

Biologia reprodutiva e fenologia de Virola sebifera Aubl. (Myristicaceae) em mata mesofítica de Uberlândia, MG, Brasil

“…A razão sexual tendendo para o sexo masculino, como observada em V. sebifera, é uma característica freqüente em plantas dióicas tropicais (Opler & Bawa 1978, Thomas & La Frankie 1993, Oliveira 1996 incluindo espécies de Myristicaceae (Armstrong & Irvine 1989a, Ackerly et al 1990. Esses dois últimos autores mostraram que a maior freqüência de indivíduos masculinos florescendo se deve ao alcance precoce do estágio reprodutivo pelo sexo masculino.…”

“…Espécies da família Myristicaceae são predominantemente dióicas (Bawa et al 1985, Armstrong & Irvine 1989a, Ackerly et al 1990, Saraiva et al1996, Momose et al 1998, com flores geralmente pequenas e tubulares e polinizadas principalmente por besouros (Armstrong & Drumnond 1986, Armstrong & Irvine 1989b, Armstrong 1997, Momose et al 1998. Os visitantes florais de Myristicaceae utilizam pólen como fonte de recurso floral (Armstrong 1997, Momose et al1998, fornecido exclusivamente por flores masculinas (Armstrong & Irvine 1989b, Momose et al 1998.…”

Preliminary Results of a Large-Scale Tree Inventory of Upland Rain Forest in the Central Amazon

“…The implication for conservation is that more than one regional conservation unit of a dimension adequate to conserve viable populations will be necessary to guarantee complete species presence and persistence. A minimum estimate of the area necessary for even the most common species of the Myristicaceae examined here is greater than 200 hectares and will be much greater for rarer species (Ackerly et al, 1990). …”

Pollination of Myristica and Other Nutmegs in Natural Populations

“…Nutmegs are an ecologically important component of primarily lowland tropical forests, especially Amazonia [2][3][4][5][6][7]. Most nutmegs are dioecious trees or shrubs; some species of Iryanthera are monoecious [8][9][10]. Some species are canopy or even emergent trees, although many occupy the subcanopy and understory.…”