This paper describes the variation of the above-ground biomass, necromass, and net above-ground primary production (NAGPP) of two weed species, Panicum maximum Jacquin (Poaceae) and Pteridium arachnoideum (Kaulf.) Maxon. (Dennstaedtiaceae), at two sites in the Poço das Antas Biological Reserve, southeast Brazil. Both species form mono-dominant stands in the matrix surrounding forest fragments. The organic matter was sampled monthly from each site, separated into biomass and necromass, and net above-ground primary production (NAGPP) was calculated. There was marked intra-seasonal fluctuation for both species; Pa. maximum generally had the largest values for necromass, total mass and NAGPP (NAGPP, Pa. maximum = 3953 g.m-2.y-1, Pt. arachnoideum = 2667 g.m-2.y-1). NAGPP did not vary between the two growth periods for Pa. maximum, but marked differences were found for Pt. arachnoideum (2% compared to 44% variation around the mean). The second growth year was drier and Pa. maximum produced much greater necromass in that year; Pt. arachnoideum showed little variation in biomass but much greater productivity in the drier second season. Pteridium arachnoideum appears to be more sensitive to climate, and especially rainfall.
fire; Panicum maximum; plant invasion; Pteridium arachnoideum
