Disrupting woody steady states in the Chaco region (Argentina): Responses to combined disturbance treatments

Vegetation states with low suitability for cattle raising (‘woody states’) are widespread in the Chaco region (NW Argentina). We assessed the success of roller-chopper (RR); roller-chopper with seeding of Panicum maximum cv green panic (RS), and roller chopper followed by prescribed fire (RF) in disrupting a woody state in two separate experiments, monitored from 1997 to 2002. We evaluated amount and temporal dynamics of woody plant volume, standing herbaceous biomass and livestock accessibility. We also monitored soil moisture at two soil depths, sunlight availability and the dynamics of germination of grass, forbs and woody species. Spatial variation was included as ecological sites, and all treatments were grazed. The longevity for each treatment was estimated by monitoring the woody volume through time. Data were analyzed using variance and regression analysis. MRPP techniques were used to study the effect on species diversity. Results were compared with information in literature to define thresholds and success of restoration. RF showed the lowest mean woody canopy (p>F=0.0001), the largest mean accessibility (>70%), and longest return interval (>6 years) among treatments. RS showed the largest amount of herbaceous standing biomass (>5000kg dry matterha⁻¹, p>F=0.0001), and a longevity >5 years. Prescribed fire was successful in controlling the increase of woody volume only at lowland and midland ecosites. At upland ecosites in RS and RR, mean woody volume and accessibility were higher than in the other two sites. Plant diversity was significantly affected by ecosite rather than by treatment and time. Soil moisture was influenced by site (p>F=0.001), while sunlight availability was affected by treatment (p>F=0.001). Plant germination was affected by treatment and site. The roller chopper is an adequate tool for disrupting woody states in the Chaco and could be successfully combined with prescribed fire.