The Caldenal region, located mainly in the Province of La Pampa in Central Argentina, occupies an area of approximately 60,000 km2. Before the European colonizers reached this area during the last decades of the past century the vegetation was characterized by an open woodland dominated by caldén (Prosopis caldenia) with an understory of bunchgrasses such as Stipa clarazii, Koleria permollis, Briza subaristata, and Poa ligularis. The Caldén is a tree with a slow growth rate which can reach a height of 8-10 m. The aspect of the landscape resembles that of a savanna. With the arrival of the European pioneers and the following generations the caldenal ecosystem was intensively exploited and changed by deforestation, agriculture and sheep and cattle grazing systems.
Fire has always been present as a natural component in this ecosystem with a fire frequency which has been estimated to be 7-10 years. These fires were low intensity due to the fact that the fine material was provided by grasses which had little or no effect upon the trees. Grasses in the Caldenal region as well as in most of Argentina have evolved under low herbivore pressure. The introduction of domestic cattle generated a disturbance which resulted in a replacement of palatable species by less preferred species first and by non-palatable species later. The germination of caldén seed depends on the fact that it needs to go first through the digestive tract of an animal. Cattle and sheep consume caldén fruits and have helped to disseminate viable seeds promoting an increase in the density of caldén and changing its spatial pattern. It also helped to increase the number of individuals due to the fact that the rate of natural fires dropped drastically because the fine material to carry the fires had been consumed by the domestic animals.
The disturbance contributed also to the invasion of several shrubs coming from the monte region which is West of the Caldenal. Due to economic considerations in the 1960s sheep were entirely replaced by cattle, and this led to an increase in shrub density and expansion of unpalatable grasses generally called pajas. In the following decade this change of vegetation structure led to more frequent large-scale wildfires which, instead of being of low intensity, became high-intensity fires because of the shrub component together with a higher density of trees (Tab.1).
The Caldenal region has an average rainfall of 450 mm in the West and 600 mm in the East. Most of the grasses grow during fall and spring, while the shrubs and grasses grow during spring and summer time. Cow-calf production is the principal activity in the region. The average ranch sizes are about 600 ha in the East and 5,000 ha in the West. Meat production has dropped because of a decrease in forage production, and ranchers try to solve this by setting fire in order to be able to have cattle grazing on the paja’s early regrowth. Unfortunately, ranchers have not been successful in improving range conditions and have caused, through fires, important damage to the tree component. Today’s scene shows a degraded ecosystem with a tree compartment which is at risk due to shrub encroachment and a grass component constituted to almost 85% of unpalatable species.
In 1988 the Estación Experimental Agropecuaria that belongs to INTA (National Institute of Agricultural and Livestock Technology) at Anguil (Province of La Pampa) began prescribed fire research. Since then three experimental fires were conducted during the month of March in the years 1991-93. In each fire 150 ha were burnt, and data were collected on the effect of prescribed fire on soil organic matter, soil total nitrogen, and cover and density of grasses and shrubs. Subsequently weather conditions and the use by cattle were monitored. Most interesting are the following observations on the burned plots:
Fire did not affect organic matter and total nitrogen in the soil
Increase in cover and density of desirable grass species
Increase in the stocking rate at least during the two first years
Decrease in undesirable grass species cover, but no effect on its density
Very low impact on old caldén trees
Very low mortality of shrubs
Although the results of prescribed fires in the Caldenal region are encouraging more research must be done before the use of fire by prescription can be applied more widely. Different cattle management techniques after fire should be tested and vegetation response to fire at different seasons still needs to be evaluated. Nevertheless, prescribed fire seems to be a very valuable tool for increasing forage productivity without affecting the tree component and, overall, improving range condition.
Tab.1. Fire statistics for the La Pampa Province, Argentina, 1976-93
(source: Ministry of Agriculture, La Pampa)
From: Andres H. Sipowicz
Instituto Nacional de Technologia Agropecuaria
ARG – 6326 Anguil, La Pampa