Colonization of Sibbaldia tetrandra Cushions on Alpine Scree in the Pamiro-Alai Mountains, Central Asia

Abstract

The population of Sibbaldia tetrandra, a cushion plant, on the scree slope at 3800 m a.s.l. in the Pamiro-Alai Mountains consists of isolated individuals with positively skewed size distribution. Direct competition among isolated individuals is improbable. The differences in cushion size are due mainly to the age of individuals and the local site quality. Linear relationship between the growth in height of a cushion and its lateral growth was found. Frost injuries are more extensive in the small cushions, but the proportion of afflicted plants is greater in the large ones. Slightly contagious spatial pattern resulting from the environmental heterogeneity was detected. Seventeen species were observed to invade the cushions. Total biomass of intruding species is highly correlated with the cushion volume as well as with biomass. Species diversity increases with cushion area. A log-linear relationship between the number of invading species and cushion area was found. Both the number and cover of the species occurring outside of the Sibbaldia tetrandra cushions are negatively correlated with the cover of Sibbaldia tetrandra. The spatial pattern of Sibbaldia populations differs considerably between the study plot on a flat site and that on the scree slope. On the flat site, Sibbaldia forms a carpet consisting of individuals which cannot be clearly distinguished from each other. There also, the number and cover of the other species increases, and the most successful species are those which are capable of colonizing Sibbaldia cushions.