A global view of shifting cultivation: Recent, current, and future extent

Abstract

Mosaic landscapes under shifting cultivation, with their dynamic mix of managed and natural land covers, often fall through the cracks in remote sensing-based land cover and land use classifications, as these are unable to adequately capture such landscapes' dynamic nature and complex spectral and spatial signatures. But information about such landscapes is urgently needed to improve the outcomes of global earth system modelling and large-scale carbon and greenhouse gas accounting. This study combines existing global Landsat-based deforestation data covering the years 2000 to 2014 with very high-resolution satellite imagery to visually detect the specific spatio-temporal pattern of shifting cultivation at a one-degree cell resolution worldwide. The accuracy levels of our classification were high with an overall accuracy above 87%. We estimate the current global extent of shifting cultivation and compare it to other current global mapping endeavors as well as results of literature searches. Based on an expert survey, we make a first attempt at estimating past trends as well as possible future trends in the global distribution of shifting cultivation until the end of the 21st century. With 62% of the investigated one-degree cells in the humid and sub-humid tropics currently showing signs of shifting cultivation-the majority in the Americas (41%) and Africa (37%)-this form of cultivation remains widespread, and it would be wrong to speak of its general global demise in the last decades. We estimate that shifting cultivation landscapes currently cover roughly 280 million hectares worldwide, including both cultivated fields and fallows. While only an approximation, this estimate is clearly smaller than the areas mentioned in the literature which range up to 1,000 million hectares. Based on our expert survey and historical trends we estimate a possible strong decrease in shifting cultivation over the next decades, raising issues of livelihood security and resilience among people currently depending on shifting cultivation.

Fig 1. Identification of spatio-temporal pattern based on GFC global annual deforestation data [8] and very high–resolution satellite imagery.

Fig 1A shows a one-degree square of northern Laos. The colored pixels indicate clearings in different years between 2000 and 2014 as recorded in the GFC data set [8]. Fig 1B to Fig 1E show examples of different zoom levels used to decide whether the pattern in the GFC data is indeed related to shifting cultivation Fig 1E (showing pattern of clearing for the current year of cultivation and different stages of fallow) or not Fig 1D (larger scale clearings with young rubber). The imagery used for illustrative purpose in Fig 1 is based on Copernicus Sentinel 2 data from 2016. Maps created in QGIS 2.16.

Fig 2. Creation of validation dataset.

(Fig 2A): the global distribution of the stratified sample of the 328 one-degree cells used in the validation data set. (Fig 2B): Location of the one-degree cell of Fig 2C - 2E. (Fig 2C): One-degree cell with a mesh of 1/100 degree cells as a basic unit for the validation data set, green cells having a shifting cultivation occurrence class of >1% in our global classification. The red box marks the extent of Fig 2D and Fig 2E. (Fig 2DA) and (Fig 2E): The white line grid marks the 1/100 degree cells used as basic unit for the validation data. Based on the spatio-temporal pattern of the GFC data (different colours denoted different year of clearings) and the patterns of clearing and regrowth in the very high resolution imagery (here Bing), a 1/100-degree cell is being classified as showing shifting cultivation or not. The red hatching in (B) indicates the 1/100 degree cells that were classified as having shifting cultivation. (Source of imagery in 2D and 2E: Pansharpened Landsat 8 image, acquisition date January 5 2014, available from the U.S. Geological Survey.). Maps created in QGIS 2.16.

Fig 3. Survey responses received, by country of study.

This figure was elaborated by the first author using ArcGIS 10.4.

Fig 4. Butler (1980) map of areas where land use includes “primitive subsistence agriculture,” which in the humid tropics largely consists of shifting cultivation (reproduction by first author using ArcGIS 10.4 based on Hurtt et al. [2]).

Fig 5. Estimation of landscapes showing signs of shifting cultivation around 2010 between 30°S to 30°N.

Based on visual inspection of annual global deforestation data [8] from 2000 to 2014 and very high-resolution satellite imagery. Areas in which shifting cultivation can be assumed to have never existed or disappeared decades ago have been excluded from the analysis (dark gray). This figure was elaborated by the first author using ArcGIS 10.4.

Fig 6. Comparison of the Butler map [13] (showing the status in the 1960s to 1970s) with our results (showing the status around 2010).

This figure was elaborated by the first author using ArcGIS 10.4.

Fig 7. Preliminary estimates of changes in the occurrence of shifting cultivation between today and 2030, 2060 and 2090.

This visualization is based on the estimation of landscapes showing signs of shifting cultivation around 2010 (Fig 5) as base year and estimated decreases of shifting cultivation (Table 3) based on the expert surveys and observed trend between the Butler map and our 2010. This figure was elaborated by the first author using ArcGIS 10.4.