Seasonal variation in structure, composition, phytomass, and net primary productivity in a Lasiurus scindicus henr., and Cenchrus setigerus vahl., dominated dry sandy desert site of Karachi

摘要

Variation in composition, structure, plant biomass and net primary productivity was analyzed in a grass- dominated sandy desert community of Lasiurus scindicus Henr., and Cenchrus setigerus Vahl., situated within the campus of University of Karachi. Pakistan. Life form indicated a thero-hemi-cryptophytic/geophytic flora. Phytosociological parameters of x vegetation were studied at monthly interval from June 1990 to June 1991. The density as well as live biomass-based diversity of the community was generally low and dominance high. The live-biomass-based diversity varied seasonally and was the function of precipitation and soil moisture. On the advent of rain in August with the increase in diversity, the relative abundance pattern tended to approach log-normal distribution which was otherwise geometric. The density-based diversity declined after August rains largely due to reD' high density of Tragus roxburghii (1327 plants.m~(-2)), a small graminoid, which created the condition of very high dominance concentration. The above ground live plant biomass (LB). was maximum (311 lg.m~(-2)) in September which was positively related with rainfall and soil moisture content. The effects of rainfall were better represented it in the month subsequent to its occurrence and remained noticeable for not more than 50-60 days, when major proportion of I,B entered the dead compartment in October. Standing dead (SD) biomass was maximum in November, litter was minimum in August (72.5 g.m~(-2)) and maximum in October (249 g.m(-2)). Below ground biomass was larger than the above ground biomass. Above ground net primary productivity (ANP) as evaluated by various methods varied from 368 to 587 g.m~(-2) year~(-1) (x = 448.8 +- 50.88 g.m~(-2) year~(-1); CV = 22.67%). ANP was related positively with biomass-based diversity and negatively with dominance. Below ground Net ANP was related productivity (BNP) was higher than ANP and amounted to 834.4 g.m~(-2) year~(-1). The system transfer functions also indicated that productivity was more below ground-directed. Annual efficiency of energy capture by the primary producers was estimated to be 0.47% on the basis of half of total solar radiation. Annual energy flow diagram through primary producer compartments is constructed.