Structure and functioning of a mountain grazed grassland and their response after exciosure

The effect of grazing and different times of exclosure on floristic composition, biomass, plant nitrogen concentration and aboveground net plant productivity were studied in a mountain grassland of Córdoba, Argentina. Plant diversity was maximum at the grazed site and no invasion of alien species was recorded at any site, but total standing biomass and aboveground net plant productivity were maximum at the site excluded from grazing for 15 years. Prostrate, graminoid and annual growth forms accounted for more than 90% of live biomass at the grazed site. Rosettes increased their biomass with grazing exclusion and tussock grasses were apparent only after 4 years of exclusion and accounted for 90% of total biomass after 15 years of exclosure. There was no site effect on seasonal dynamics of standing biomass and plant nitrogen content of different plant growth forms, but prostrates and graminoids showed a prolonged growth season under grazing, and plant nitrogen content was higher in grazed plants. Aboveground net plant productivity (ANPP) was limited to spring and summer, from October to March. Two years of exclusion from grazing slightly decreased ANPP, but significatively increased after 4 years without livestock grazing.

Key words: aboveground net primary productivity, exclosures, growth forms, mountain grassland, plant nitrogen, seasonal dynamics.