Lifetime sexual dimorphism in Juniperus communis var. communis


Lena K. Ward


The sexes of dioecious Juniperus communis were differentially affected over their lifetime in response to ecological and physiological stress in populations of different ages studied over 23 years in southern England. In a young population, the female survival rate was less than the male rate, with more females dying during a severe attack by rabbits and later with fungus disease in the roots. The sex ratio (female : male) in marked individuals was predicted by age, changing from 1:1.13 in 1983 to 1:1.32 in 2005. In an old senescing population, where two-thirds of the individuals died, the sex ratio varied, but overall became more male biased (1:1.51–1:2.10). Males had a greater resistance to terminal disease, and were slightly older than females at death (110 years compared to 106). Young females grew less than males, presumably because of greater trade-offs with reproductive effort: the mean annual shoot growth was 6.7 cm compared to 8.1 cm in males. By approximately 30 years of age, heights of the sexes were significantly different. The annual growth of old females (4.8 cm) was greater than that in males (4.3 cm), possibly because males survived longer in poor health. Sexual differences in height in the old population were progressively lost. Cone abundance in females was less than that in males and cone production had greater periodicity; the young population outperformed the old. There were slightly longer time lags in inverse correlations between growth and reproductive indices in females.