Fire severity in slash-and-burn agriculture in southern Brazil: an overview

ABSTRACT Slash-and-burn is a traditional agricultural system still widely used in Brazil. The variation in temperature during fires results in different levels of physical, chemical, and biological changes in the soil, which makes it necessarily better to understand the dynamics of fire severity in this system. The aim of this study was to make an analytical comparison of the variation in the temperature reached in the soil during fires in slash-and-burn agriculture. Temperature data were measured in burnt areas with similar fallow times at the following soil depths 0; 2.5 and 5 cm and compared with secondary data from other studies in the same slash-and-burn system in southern Brazil. The peak temperature showed more significant variations in the surface (0 cm) of different soils and different types of regeneration vegetation in each area. Overall, the peak temperatures ranged from 32 to 673 ºC across the three depths. The peak temperature reduction in the 5 cm layer was 88 % compared to the surface and 30 % compared to the 2.5 cm layer. The sandier soils showed greater thermal conductivity at depth. The surface litter seems to exert a more significant influence on the burning severity than the biomass load cut for burning, suggesting that the characteristics of the litter layer on the surface should be better characterized in future studies on fire severity.摘要 刀耕火种（slash-and-burn）是目前巴西仍广泛使用的传统农业耕作模式。火灾过程中的温度波动会使土壤产生不同程度的物理、化学与生物学变化，因此有必要深入理解该体系下火灾烈度的动态变化规律。本研究旨在对刀耕火种农业过程中火灾发生时土壤所达温度的变化特征开展分析对比。研究人员在休耕期相近的火烧区域，于0 cm、2.5 cm及5 cm三个土壤深度处采集温度数据，并与巴西南部同一刀耕火种体系下其他研究的二手数据进行对比。结果显示，不同土壤类型及各区域不同类型的更新植被的地表（0 cm）峰值温度存在更为显著的差异。整体而言，三个深度下的峰值温度区间为32 ℃至673 ℃。相较于地表层，5 cm土层的峰值温度降幅达88%，相较于2.5 cm土层则降幅为30%。砂质土壤在深层的导热率更高。地表枯落物层相较于用于燃烧的生物量负荷，对燃烧烈度的影响更为显著，这提示未来有关火灾烈度的研究中，应进一步细化地表枯落物层的特征表征。