Relationships between tree size, crown shape, gender segregation and sex allocation in Pinus halepensis, a Mediterranean pine tree

Background and Aims

Sex allocation has been studied mainly in small herbaceous plants but much less in monoecious wind-pollinated trees. The aim of this study was to explore changes in gender segregation and sex allocation by Pinus halepensis, a Mediterranean lowland pine tree, within tree crowns and between trees differing in their size or crown shape.

Methods

The production of new male and female cones and sex allocation of biomass, nitrogen and phosphorus were studied. The relationship between branch location, its reproductive status and proxies of branch vigour was also studied.

Key Results

Small trees produced only female cones, but, as trees grew, they produced both male and female cones. Female cones were produced mainly in the upper part of the crown, and male cones in its middle and lower parts. Lateral branch density was correlated with the number of male but not female cones; lateral branches were more dense in large than in small trees and even denser in hemispherical trees. Apical branches grew faster, were thicker and their phosphorus concentration was higher than in lateral shoots. Nitrogen concentration was higher in cone-bearing apical branches than in apical vegetative branches and in lateral branches with or without cones. Allocation to male relative to female function increased with tree size as predicted by sex allocation theory.

Conclusions

The adaptive values of sex allocation and gender segregation patterns in P. halepensis, in relation to its unique life history, are demonstrated and discussed. Small trees produce only female cones that have a higher probability of being pollinated than the probability of male cones pollinating; the female-first strategy enhances population spread. Hemispherical old trees are loaded with serotinous cones that supply enough seeds for post-fire germination; thus, allocation to males is more beneficial than to females.     

Genetic diversity and structure of the critically endangered tree Dimorphandra wilsonii and of the widespread in the Brazilian Cerrado Dimorphandra mollis: Implications for conservation

We performed a comparative analysis of the genetic diversity and structure of two congeneric tree species, one critically endangered, with only 21 known individuals in the wild, Dimorphandra wilsonii, and the other widely distributed Dimorphandra mollis. Eight populations of D. mollis and all known trees of D. wilsonii, from three areas, were screened for variability with ISSR markers. Percentage of polymorphic bands, Nei's gene diversity and Shannon's index were considerably lower in D. wilsonii (P = 40.0%, h = 0.124 and I = 0.190), as compared to D. mollis (P = 70.4%, h = 0.190 and I = 0.297). Bayesian clustering showed that D. wilsonii individuals are clustered in three populations, which had high differentiation among them. Several measures for their conservation were suggested: protection of all extant populations, ex situ conservation of seeds, production of saplings in nurseries and foundation of new populations in reserve areas.