Increase in the woody component of seasonal savannas under different fire regimes in Calabozo, Venezuela

Aim We tested the hypothesis that exclusion from fire and cattle is respons ible for the increase in tree cover in open savanna vegetation. Location Four plots in open savanna vegetation from the Calabozo region in central Venezuela were studied. Plot A was located in a Biological Station (EBL) that was excluded from fire and cattle between 1961 and 1991, with on ly two burning events in 1964 and 1968. The other plots (B, C, D) were loca ted within 2 km distance from A, in neighbouring farms with soils similar t o those in A but under various regimes of land use and fire frequency. Methods We measured the cover of isolated trees, small tree groups and grov es of each plot in 1960 and 1977 using geographic information system (GIS) and digitalized aerial photographs. Additionally, the plots were located in the field and the open grassland was sampled in 1995 for species compositi on and density of stems above 20 cm height. Information on land use was obt ained surveying people at the farms. Results There was an increase in the woody component of all plots during th e 17-year interval (1960-1977). Total woody cover in the four plots as a wh ole increased from 4.5% to 17.9%. All three components measured, groves, tr ee groups and isolated trees, increased despite differences in land use and fire frequency between plots. Contrary to our expectations, the field surv ey performed in 1995 showed that fire-sensitive species were abundant in th e open savanna in plots B, C and D, which were not excluded. Plot B, with t he most intense agricultural use showed the highest rate of woody increase, and plot C, under extensive cattle ranching, was second. The results also showed that woody cover increased by aggregation from single trees and smal l tree groups into groves. As a consequence of these changes, savanna physi ognomy changed from open to dense savanna parkland with a woody cover reach ing over 25% in one of the four plots. Conclusions The results agree with other reported increases in woody cover in savannas under exclusion or with annual fires during the same time perio d in Africa (Dauget & Menaut, 1992). Our results support evidence from prev ious studies showing that fire and grazing are only part of a complex syste m of interacting factors affecting the structure of savanna communities.