Chinantec shifting cultivation and secondary vegetation : a case-study on secondary vegetation resulting from indigenous shifting cultivation in the Chinantla, Mexico / Hans van der WalChinantec shifting cultivation : InTERAcTIVE landuse : a case-study in the Chinantla, Mexico, on secondary vegetation, soils and crop performance under indigenous shifting cultivation / Hans van der WalChinantec shifting cultivation : InTERAcTIVE landuse : a case-study in the Chinantla, Mexico, on secondary vegetation, soils and crop performance under indigenous shifting cultivation / Hans van der Wal