Fire behavior of prescribed burns in grass: woody steppe on Paraná State, Brazil

Abstract

Prescribed burns are widespread in many countries, becoming a management practice in different types of vegetation. In addition to practice pasture renewal and land clearing, prescribed burns also represent a means of preventing forest fires, because through this practice, the fuel load can be reduced significantly. To be used safely and effectively, it is crucial understand the aspects of the fire behavior from the vegetation to be burned. The Grass-Woody Steppe (Natural Fields) consists of the phytogeographic regions of the state of Paraná, covering an area of approximately 20,000 km2 of east-central region of the state. The vegetation consists mainly of grasses and shrubs, with a large concentration of fine fuels (≤ 0.7 cm), representing a high danger of forest fires. For many years, the region of Grass-Woody of Paraná was used as pasture for the intensive rearing of cattle and sheep, activity that lost momentum in recent years due to the advancement of agriculture and forestry. Many farms, however, still keep areas of fields that are management by the fire. Although the burning of vegetation is still a common practice in the region, there are not however, information about the behavior of fire in this vegetation type. The objective of this research was to characterize the behavior of fire in steppe vegetation of the state of Paraná, through prescribed burns. The results aim to support studies of effects of fire on survival and regeneration of plant species in this environment, considering the hypothesis that controlled burning is a viable and safe activity for pasture management. The survey was conducted in the period preceding the winter in an area with more than eight years without human interference, located in Palmeira county, Paraná state. For the experiment, 10 plots of 3 x 20 m were prepared and burned by backfire and head fire techniques. On the day of burning, the weather conditions were: average wind speed of 2.20 km.h-1, average relative humidity of air 60.31% and average temperature of 21.83 °C. The surface fuel load was estimated at 2.53 kg.m-2, with an average moisture content of 79.77%. The results obtained in the backfire and headfire burnings were: rate of spread - 0.0536 and 0.0150 m.s-1, height of flames - 1.39 and 0.68 m, fire intensity - 119, 11 and 51.96 kcal.m-1.s-1 and heat per unit area - 2218.84 and 4259.32 kcal.m-2. It concluded with the achievement of the experiment, that the prescribed burns conducted at vegetation of Steppe within the established criteria and burning plans, are feasible and safe from the point of view of fire hazard.