Effects of slash-and-burn fallow periods on landscape structure

Slash-and-burn is a widespread agricultural system practised by more than 250 million people in tropical regions. It is sustainable when fallow periods allow for the restoration of organic matter and nutrient losses that occur during the cropping phase. However, fallow periods have been reduced in most slash-and-burn systems as population density has increased. The consequent landscape structure modification occurring throughout this process can affect several ecological processes. As a first step in understanding these modifications, the present study aimed to test the effects of fallow period reduction on landscape structure by examining a total of 34 lots of 25-250 ha each in the north-east of the Brazilian Amazon. Sixteen lots with long fallow periods (about 10 years) were compared with 18 lots where fallow periods were reduced to c. four years. The reduction in fallow period created a more homogeneous landscape, largely dominated by agricultural and young secondary vegetation. The main mechanism leading to this homogenization was a more intensive use of initial secondary vegetation for agriculture. On average, the percentage of land used more intensively for agriculture was four times higher in short fallow period areas than in long-period areas. The increase in agricultural area is related to both an increase in the number of agricultural fields and also to an increase in the area of these fields. The proportions of forest and old secondary vegetation in the landscape were reduced as a consequence of a reduction in patch size, without significant fragmentation or patch losses. These structural changes appeared to be more usual in large properties belonging to several families. Further studies are needed to understand the consequences of these modifications for landscape functioning, but it is possible to speculate, based on previous studies, that these structural changes can lead to a reduction in the speed of regeneration processes.