Structure, composition and above ground standing phytomass of some grass dominated communities of Karachi: summer aspect

ABSTRACT

Some 120 species [83 dicotyledons and 37 monocotyledons] were seen growing naturally to make summer aspect of 22 grass dominated sites of Karachi, Pakistan. They have high percentage of therophytes [43.3%] followed by chamaephytes [30%]. Legumes were present invariably in all sites except that of Urochondra setulosa. The number of species in a site varied with the nature of site and its salinity status. The above ground standing biomass [AGSP] varied with species dominating the site [49.6 to 908 g/m[2]]. On phytomassbased quantitative spectrum, hemi-cryptophytes and sometimes geophytes [protected forms] were the dominant. The number of legume species varied as a function of number of grasses in a site. The litter component related positively with AGSP [r = 0.7175] and the percent proportion of legume biomass [PPLB] associated inversely with percent proportion of grass biomass [PPGB] [r = -0.6566] The more diverse communities of non-saline habitats appeared to be approaching to log-Normal distribution of biomass among the species whereas the distribution of biomass in less diverse communities of saline habitats was distinctly geometric. Twelve community types, recognized on the basis of 40% biomass-based compositional similarity, in accordance with the decreasing AGSP magnitude may be arranged as follows Cenchrus setigerus > Desmostachya bipinnata=Urochondra setulosa > Lasiurus scindicus > Dichanthium annulaturn = Sporobolus arabicus > Aeluropus lagopoides > Sporobolus halvolus > Chrysopogon aucherii > Dactyloctenium scindicum > Aristida mutabilis > Sporobolus coromendelianus. Edapho-topographic relations of these communities are described. When stability was evaluated through life-span consideration of species, diversity-stability relations appeared to be negative. Salt marsh communities were more stable than those of the salt free habitats. The results are discussed in ecological context