| When a plant increases resource allocation to reproduction from its limited reserves, the allocation to the other functions is reduced. The trade-off between reproduction, vegetative growth and maintenance is a major issue in the life history of an organism. Dioecious plants provide an excellent opportunity for detecting such possible trade-offs in resource allocation. The reproductive success of a female plant in a dioecious species may be affected by pollen limitation and resource limitation. This study presents the dimorphism in reproductive allometry, studies the trade-offs between reproduction and vegetative growth, reveals the limited factors of reproductive success, and detects compensating mechanism for reproduction costs. This paper aims to deepen the understanding for plant reproductive strategy in mixed broadleaf-conifer forest in Northeastern China. The brief results are as follows:1. The dimorphism in vegetative and reproductive allometriesRhamnus davurica shows highly significant correlations for all selected combinations of different vegetative components. These correlations are much less pronounced, and sometimes even non-significant in Rhamnus schneideri. An power function and a linear model were found suitable for describing different components of the vegetative allometry. In addition, a linear regression was used to estimate the number of flowers and/or fruits based on observed stem size. However, the slope of the linear relations differs between the sexes again indicating gender-related differences in reproductive size. Both females and males show consistent patterns of reproductive size dependency. According to theory, females allocate more biomass to reproduction than males, which negatively affects their vegetative growth. However, in this study, flowering females did not show less vegetative growth than males suggesting that the two Rhamnus species behave contrary to expectation.2. The trade-offs between reproduction and vegetative growth The self-organization of plant organ is different at different modular levels. The trade-offs between reproduction and vegetative growth was detected at different modular levels. There were no intra-annual trade-offs between reproduction and foliage biomass in either sex at shoot/sub-branch level, branch level and shrub level. Inter-annual trade-offs were detected in females. Inter-annual trade-offs existed at branch level in R. davurica females. At the population level, the sex ratio was female-biased in2010, and it did not significantly deviate from1:1in2011in R. davurica. There were no intra-annual trade-offs between reproduction and foliage biomass at shoot/sub-branch level, branch level and shrub level. Inter-annual trade-offs existed at all three different modular levels in R. schneideri females. At the population level, the sex ratios were significantly female-biased in both2010and2011in R. schneideri. This study has shown that the degree of autonomy of the different plant organs influenced the trade-offs between reproduction and growth, which suggests a species-and sex-dependent modular autonomy.These trade-offs were examined at three different modular levels of Acer barbinerve shrubs, i.e., shoot, branch and shrub levels. An intra-annual trade-off was detected at the shoot level for both genders in2011and2012. Both males and females showed a negative correlation between reproduction and vegetative growth, but this was more prominent in males. For the females of the species, inter-annual trade-offs were only found at branch and shrub levels. Slightly negative correlations in females were detected between reproduction in2012and reproduction in the two previous years. The gender ratio was significantly male-biased during the three successive years of our investigation. Females had higher mortality rates in the larger DBH classes, both in2011and2012.3. Limitations to reproductive success in the dioecious plantThe mortality rate of females was higher than that of males indicating a trade-off between reproduction and survival. In the three years of the study (2010-2012), elevation, competition and female size had no significant effect on the fruit set. The distance to the nearest male, however, had a significant effect on fruit set. Number of fruits and fruit set were decreased with increasing distance to the nearest male. It was possible to estimate maximum fruit set, based on the comparatively large dataset. The number of fruits and the fruit set are exponentially related to the distance to the nearest male and the relationships are described by an exponential model. The results of this study support the importance of pollen limitation on the reproductive success in R. davurica.The trade-offs between reproduction and vegetative growth was negatively affected by the slope for R. schneideri. The fruitset, the number of fruit and the trade-offs between reproduction and vegetative growth increased with the increase of slope. Reproduction and resource allocation was negatively affected by the soil nutrient. Such as, the fruitset was negatively affected by available nitrogen, total potassium, available potassium and available magnesium. The number of fruit was negatively affected by available nitrogen, total phosphorus, available potassium and available magnesium. The number of flower was negatively affected by available nitrogen, total phosphorus, available phosphorus, available potassium and available magnesium. The trade-offs between reproduction and vegetative growth was negatively affected by total phosphorus, total potassium, available potassium and available magnesium. Thus, the increase of soil nutrient will decrease the resource allocation in reproduction, but increase in vegetative growth. The fruitset, the number of flower, the number of fruit and the trade-offs between reproduction and vegetative growth positively affected by soil total magnesium. In addition, reproductive allocation is significantly size-dependent.Reproductive biomass related significantly with vegetative biomass for both male and female in A. barbinerve, and vegetative biomass increased with the increase of reproductive biomass. Topography, soil moisture, leaf area index and female dbh affected significantly the number of flower, but did not for the number of fruit, the fruitset and the trade-offs between reproduction and vegetative growth. The number of flower was negatively affected by the slope, but positively affected by soil moisture and female dbh. The pollen supply quantity was positively affected by leaf area index, while the number of flower in female trees was negatively affected by the pollen supply quantity. Thus, leaf area index can affect indirectly the number of flower in female trees. But topography, soil moisture, leaf area index and male dbh size did not significantly associate with the number of flower and the trade-offs between reproduction and vegetative growth.4. Compensation mechanism for reproductive costs in dioecious plantsBased on the theory of life history, female tree will pay higher cost in reproduction because of flowering and fruting. So female may have a lower vegetative growth than male. However, that trad-off between vegetation and reproduction does not exist in female tree of R. davarica. Although female trees of R. davarica have higher reproductive biomass, meanwhile females also have higher vegetative biomass. It means that female may have compensatory mechanism for their higher reproductive costs. Acorrding to the result of light response curve, female has higher net photosynthesis rate when the light intensity is higher. It indicates that females have higher photosynthesis ability than males to compensate their higher reproductive cost. |
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