Reproductive allocation in dioecious shrub, Rhamnus davurica
Life-history theory in plants has an important role in the development of many fields in evolutionary biology. The theory deals specifically with assumptions relating to the trade-off between reproductive and vegetative processes. Research on trade-offs has largely focused on dioecious species and the differences between sexes regarding patterns of resource allocation. In this research, the reproductive allocation was studied and specific assumptions were tested regarding the trade-off between reproductive and vegetative processes in the dioecious species Rhamnus davurica. Assuming automomy of different modular levels of plant physiology, we observed the reproductive allocation at branch level, shrub level and population level, respectively. The main results are; (1) at branch level, reproductive biomass shows significant difference between males and females ( P<0.001). The flower biomass is also significantly different between the sexes ( P<0. 001 ). However, there is no significant difference in foliage biomass between the sexes (P>0. 05). The flower, foliage and reproduction biomass (the sum of flower and fruit biomass) also showed significant differences among individuals ( P < 0.001). The mean biomass value of the female flowers is 1. 84 times greater than that of the male flowers. The mean biomass value of female reproduction (the sum of flower and fruit biomass) is 16 times greater than that of the male reproduction ( male flower biomass). The mean biomass value of the male foliage is greater than that of the females, though this difference is not significant. (2 ) at shrub level, the flower biomass, reproductive biomass (including flower and fruit biomass) and the diameter increment was significantly different between sexes (P<0. 05) , but there was no significant difference between sexes regarding the foliage biomass (P>0.05). An analysis of variance shows that DBH size is positively correlated with flower biomass, reproductive biomass, foliage biomass and diameter increment (P<0. 05). The average value of flower biomass of females is 1.6 times greater than that of the males. The average reproductive biomass of females (the sum of flower biomass and fruit biomass) is 1.93 times greater than that of the males and their average foliage biomass is 1.1 times greater than that of the males. The average female diameter increment is 3.04 greater than that of the males. (3) at population level, male and female trees show differences in phenology. Male trees bloom earlier than female trees. The first male blooming was observed on 25 May, the first female blooming on 28 May. Female trees began to fruit on 8 June. In our 360mx660m research plot, there were 310 female and 53 male trees and 225 non-reproductive trees in 2010. The population sex ratio of 0.17 deviated significantly from 1:1, based on a Chi-square test ( P<0.001). The individuals in 0-2 cm, 2-4 cm, 4-6 cm and 6-8 cm DBH classes also exhibited female-biased sex ratios. The average annual ring widths of males and females were 0. 78 mm and 0. 83mm, respectively. Based on these results, we conclude that the relationship between vegetative growth and reproductive growth does not agree with trade-off theory at different levels of plant physiology (branch, shrub and population level) in Rhamnus davurica. It may be assumed that higher reproductive and vegetative growth of females might be due to their more effective photosynthetic rates.
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