ABOVEGROUND NET PRIMARY PRODUCTIVITY AND COVER OF VEGETATION EXPOSED TO VARIOUS DISTURBANCES IN ARID ARGENTINA

We contrasted total herbaceous ANPP and cover between various disturbance types (Control, Burning, Shrub Control, Old Field 1, Old Field 2, Overgrazing) at a functional group (CSPG= coolseason perennial grasses; WSPG= warm-season perennial grasses; CSAG + D= cool-season annual grasses + dicots) and a species scale during 1984-1992. Results showed that (1) precipitation appeared to be more important than disturbance type in determining total ANPP (CSPG + WSPG + CSAG + Dicots); (2) CSPG made the greatest (P<0.05) contribution to ANPP and plant cover compared with that made by the other two functional groups in all disturbance types and years; (3) the preferred, late-seral Nassella clarazii and Poa ligularis showed a greater (P<0.05) ANPP than the earlier-seral, intermediate Pappostipa speciosa and non-preferred Amelichloa ambigua and N. trichotoma in 23 out of 27 comparisons in the Control, Burning and Shrub Control disturbance types. In the Old Field 1, Old Field 2 and Overgrazing disturbance types, however, N. clarazii, P. ligularis, P. speciosa, A. ambigua and N. trichotoma showed a similar (P>0.05) ANPP most of the times during 1984-1992; (4) Total plant cover of CSAG+D was most often greater (P<0.05) than that of WSPG at the Old Field 1 and Overgrazing disturbances.