Abstract
The variation in composition, structure, phytomass and net primary productivity was analyzed in a Cenchrus setigerus Vahl. dominated community in a moist-saline site of Karachi, Pakistan. Phyto-ecological parameters of vegetation were studied at monthly interval for a year. Only 41 species were recorded from this site. On the basis of phytomass, C. setigerus was the leading dominant throughout the year. Initial density of this site, in the month of May, was 16 live plants.m-2. The Maximum density occurred in August when it reached to 95.5.plants.m-2 . Around 34.3% of the density at this point of time was contributed by Dactyloctenium. scindicum Boiss. (32.8 plants.m-2 ). This species exhibited two peaks of density – one in August and other in February – coinciding with rains. The density of Cenchrus setigerus was quite low initially (around 5 plants.m-2 ). It increased to around 12 plants.m-2 after rains in August, then, with the rise of salinity, declined slowly to reach to 7 in February and 3 plants.m -2 in post-winter rains period. The changes in the aboveground live plant biomass (LB), standing dead (SD), litter (L) and belowground biomass were studied. LB proportion of grasses was high round the year (85.5 to 99.2% of the total LB). The live biomass of legumes was negligible except in November when it was 9.85% of the total LB due to the growth of Rhynchosia schimperi Hochst Ex. Boiss. LB was maximum (507.1 ± 90.9 g.m -2 ) in September. The effects of rainfall remained noticeable for not more than up to 50- 60 days. SD was maximum in March (593.03 ± 94.8 g.m -2 ). Litter was maximum in October (193.18 ± 10.69 g.m -2 ) and minimum in May (95.84 ± 9.07 g.m -2 ). Above- ground net primary productivity (ANP) as evaluated by various methods. It varied from 415.7 to 748.9 g.m-2 .year-1 (average = 603.88 ± 71.10g.m -2 .year -1 ; CV = 23.5%). Belowground net productivity (BNP) was 1137.12 g.m -2 .year -1 (up to 30 cm depth). System transfer function data indicated that some 63.08% of Total Net Primary Productivity (TNP) was directed towards belowground productivity. Annual efficiency of energy capture by the primary producers was estimated to be 0.72% on the basis of half total solar radiation. Annual energy flow diagram through primary producer compartments is presented.
