The physiological integration of physically connected ramets may modify phenotypic plasticity and result in differences in plastic responses among hierarchical levels of clonal plants. In order to demonstrate the hierarchical responses to natural selection in clonal plants, we conducted a study to investigate the variances in the life history traits among grazing treatments and among hierarchical levels of clonal plant Kobresia humilis in an alpine meadow from 1999 to 2001. Our study site was located at the Haibei Alpine Meadow Research Station in Menyuan County, Qinghai Province, China. A fenced Tibetan sheep pasture, which was divided into four grazing pressures of ungrazed control, light grazing, moderate grazing and heavy grazing, was established in 1998. Three levels of modular hierarchy are genet level, ramet fragment level and ramet level in a clone. At the genet hierarchical level, the number of ramets and leaves per genet differed among grazing treatments for three years (F_(3,56)>F_0.05，p<0.05) but the dead leaf and rament percentages, genet size and reproductive allocation remained constant. Traits differed among grazing treatments for 42.9% of all measured traits in the level in whole study period. At the ramet fragment and ramet levels, over 50% of measured traits differed among grazing treatments in first year and then all of them differed in last year. Differences in reproductive allocation among grazing treatments at the ramet fragment and ramet levels occurred in the second and last years respectively. The number of traits that showed differences among grazing treatments for genet, ramet fragment and ramet hierarchical level in last year account for 42%, 100% and 100% of all measured traits, respectively. These results indicated that there are different variances of measured traits among grazing treatments for three levels of the modular hierarchy in the clonal plant K. humilis. The ratio of traits showing differences among grazing treatments is greater at the ramet fragment and ramet levels than at the genet level, and trait differences among grazing treatments appeared earlier at the ramet fragment and ramet levels than at the genet level.
Coefficients of variance of measured traits were highest at the ramet level, intermediate at the ramet fragment level and lowest at the genet level but showed no differences among grazing treatments. Traits showing significant differences in coefficient of variance among hierarchical levels comprised 42%, 71% and 85% of all measured traits in 1999, 2000 and 2001, respectively. These results indicate that different grazing intensities did not affect the variances of traits among hierarchies but lengthening time of grazing disturbances will gradually enlarge trait variances. Our results showed that the extent of phenotypic variation in the modular hierarchy of K. humilis is greatest at the ramet level and lowest at the genet level. The model of hierarchical responses to grazing defoliation may be “ramet level>ramet fragment level>genet level". In this model, the ramet is a fundamental unit of functional response and natural selection in the K. humilis modular hierarchy and has greatest phenotypic variation in response to disturbance, whereas the genet is a relative stable unit in modular hierarchy. Thus we demonstrated the existence of hierarchical selection in this clonal plant species.