| High-fragmented landscape in the Thousand-Island Lake region was applied as a natural laboratory to examine how population sex ratio and female&male productive success of a dioecious plant species Pistacia chinensis B. was affected with landscape fragmentation. We total investigated1015individuals, distributed in30populations on the islands, and a natural population on mainland was also surveyed as comparison near the Lake area. We investigated the variation of sex ration and reproductive success of P. chinensis by comparing the50years fragmented island populations with mainland population, and detected the mechanisms under it.In the study of population sex ratio, GPS, soil nitrogen, population size, patch connectivity, sex ratio and Diameter at Breast Height (D.B.H.) of individuals were investigated for2years. Theoretical1:1sex ratio was found in mainland population, while Male-biased sex ratio was occupied more than85%island-populations. The sex ratio (F/M) was ranged from0to1.42, only4to31populations were female-biased. Both population size and soil nitrogen positive correlated with sex ration. DBH was used to calculate the age of each individuals, mainland population was shown approximate1:1sex ratio through the age class. A significant variation of sex ratio between less and older than50years individuals was found in small populations (population size<100). Moreover, sex ratio (F/M) of individuals older than50years in each population were found correlated with patch connectivity, similar result was not found in older group. Our results were in accordance with the results of many other reports on competition between male and female individuals in environment gradient, that male-biased populations preferred the environment with poor nitrogen soil, the environment stress stronger the males are more competitive and proportion of male would be higher. More over the degree of sex ratio bias was increased with population size. As patch size (population size) increased, the sex ration of P. chinensis populations also increased, the sex ratio was around1:1in large populations and mainland population. The sex ratio of P. chinensis individuals in different age classes was changed with fragmention time, The biased sex ratios were only found in younger individuals less than50-year in small populations, while a stable1:1sex ratio exhibited in the large population on the mainland. We suppose pollen limitation is another factor that influence the sex ratio, high quantity pollen has been found producing more female progeny than low quantity pollen. Therefor population with small population size and low patch connectivity will has low proportion of female.Furthermore, both male productive success (Pollen numbers on the stigma) and female productive success (100-grain weight and seed setting rate) were found correlated with male numbers in the populations. We calculated the pollen density of4populations with different size in the flowering season, a significant difference was shown between different size populations. We assumed that pollen limitation is the main factor reduces the female and male reproductive success in P.chinensis populations, and artificial pollination was used to prove it. There was a significant increase of100-grain weight and seed setting rate in artificial pollinated fruits than control in the small population, while the result of artificial pollination was not evident in big population.The conclusion was that the effects of landscape fragmentation to the dioecious population sex ratio were mainly achieved with changing the micro-environment condition in patches. Large population could hold a better environment on nutrient condition and mitigate the competition between male and female individuals for a stable sex ratio in dioecious plant populations, and also preserve high productive success for both female and male. Both population size and micro-environment condition should be paid attention for the conservation of dioecious plant species in fragmented landscape. |
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