Abstract:
As an integral part of the complex arid ecosystem with all its ramification of interacting biotic and antibiotic factors, vegetation comes into being as a result of the interplay of a number of environmental factors. But broadly speaking, vegetation is a reflex of two major factors i.e., the climate and the land form withy its surface deposits. How stable or fragile a particular vegetation type is depending in these two factors. (Shankar Narayan and Vinod Shankar 1983). Variation in soil moisture availability may be one of the most important factors of spatial heterogeneity in plant communities of arid land (evener et al .1971). spatial variation occurs due to redistribution of rainfall water, rainfall patchiness. The distribution of the vegetation is mainly controlled by the concentration and abundance of the water resources the ease of obtain8ing and using them, their quality and reliability. The reliable rich resources may be found in desert only in oases that constitute only a tiny part of the area. Outside the oases, resources which are rich and easy to use are usually episodic. The reliable rich resources make good use of episodic but rich resources in deserts a different set of attributes is needed, one of them is opportunistic adaptation for the use of episodes. Spatial flexibility is also common in desert plants and it is apparent in the fact that may desert perennials reduce their metabolically active surface in accordance water resources availability. Theus harmonic miniaturization and wide geographical and habit ate ranges are typical features of desert flora (shmida et al., 1986).
Complexity of desert vegetation:
In arid land ecosystems such as the Thal and cholistan (Pakistan), it is difficult to choose the correct terminology to designate the ecological status of the desert vegetation. among the features peculiar to desert vegetation are the floristic overlaps between the communities, change in the composition of plant laps between the communities, change in the composition of plant communities by mulching effect of sand, and the free admission of annuals. The other possible explanation is that the deserts are young on the evolutionary times scale and there has not been enough time to exploit these niches to evolve and evolutionary equilibrium ( Zohar, 1973)
