Establishment and early survival of five phreatophytes of the Taklamakan Desert

The establishment and survival of young individuals of five desert phreatophytes that form dominant or codominant stands round river oases at the southern rim of the Taklamakan Desert was studied to explain the patterns observed in the established vegetation. We hypothesized to find differences between species in the rates of establishment and survival as well as in the susceptibility to grazing. We expected species that grow at largest distances to the groundwater table to establish themselves more successfully and have higher survival rates. In addition, we expected the high grazing and browsing pressure in the oasis foreland to have a negative effect on seedling establishment, with less palatable species being less affected. Exclosure experiments were carried out along the banks of an ephemeral river. Seedlings and shoot or rhizome fragments were planted in either fenced or control plots. In addition, the naturally emerging seedlings after a flooding event were monitored for survival to the subsequent vegetation period. In contrast to expectations, the species did not differ in the survival of planted seedlings and vegetative fragments. However, there were significant differences in the density of spontaneously emerging seedlings. Tamarix ramosissima had a much higher seedling density than the other species. Excluding livestock had a positive effect on the survival of planted seedlings and shoot or rhizome fragments. However, there were no species-specific exclosure effects neither effects on the survival of spontaneously emerging seedling. The exclusion of grazers and browsers might generally enhance the vegetation coverage in the oases forelands, but it does not favour different species disproportionately. In conclusion, the species’ regeneration niches explain only partly the patterns encountered in the established vegetation.