Genetic factors such as gene flow, genetic drift and inbreeding can change fitness in isolated small populations. We compared the fitness traits in seeds and seedlings between small and large isolated populations of Salix arbutifolia. The study showed larger mass of seeds and higher ratio of abnormal morphology and lower rates of survival and growth for current-year-seedlings in the small population than in the large one. Although the differences are probably caused by some genetic factors, what genetic factors have caused the differences are not known.

To determine the genetic factors causing the differences, we performed cross experiments within and between these populations of S. arbtifolia and examined the traits of seeds and current-year-seedlings.

The differences in the abnormal morphology ratio and growth rate of seedlings were detected among crossing treatments and associated with the differences of populations where mother trees existed. Although there were not any differences in seed mass and seedling survival rate among crossing treatments, the differences in seed mass was associated with those of mother trees. No evidence of inbreeding depressions could be detected in any traits we examined.

Therefore, maternal effects rather than inbreeding depression may be main factor to causes population differences in the fitness traits in the early life stage of S. arbutifolia.