Soil seedbanks in slash-and-burn rice fields of northern Laos A DE ROUW*, M CASAGRANDE † , K PHAYNAXAY ‡ , B SOULILEUTH § & K SAITO ¶ *Institut de Recherche pour le D eveloppement (IRD), Universit e Pierre et Marie Curie, Paris Cedex, France, †Unit e SCAB, Universit e Lyon, ISARA-Lyon, Lyon Cedex, France, ‡University of Laos, Vientiane, Lao PDR, §Managing Soil Erosion Consortium, Luang Prabang, Lao PDR and ¶AfricaRice, Cotonou, Benin Received 25 December 2012 Revised version accepted 27 August 2013 Subject Editor: David Johnson, IRRI, Philippines Summary Crops in shifting cultivation fields often suffer from severe weed infestation when long fallow periods are replaced by short fallow periods. The soil seedbank as a source of weed infestation was studied in four fields that differed in their last fallow duration. The effect of burning was analysed by comparing adjacent pre-burn and post-burn samples (two sites). Surface vegetation was monitored from burning to harvest in the plots from which soil samples were taken to determine the fraction of the seedbank germinating (three sites). Seedbank size (1700–4000 seedlings m À2 ) varied depending on a single species, Mimosa diplotricha. Burning reduced emergence of most species, but stimulated emergence in others. Densities in the seed- bank were not correlated with above-ground abun- dances in the field, except for some species. Most species emerging after 50 days from the soil samples (40% of seedlings) were absent from the field after 190 days. Whilst the data from this study are derived from only four fields, the weed problems after short- term fallowing appeared to be due to a larger fraction of the seedbank emerging, possibly due to shallow bur- ial, and to a floristic shift towards adaption to burn- ing, rather than the size of the seedbank per se. Keywords: emergence, tropical, shifting cultivation, canopy shade, fallow period, invasive, plant diversity. DE ROUW A, CASAGRANDE M, PHAYNAXAY K, SOULILEUTH B&SAITO K (2013). Soil seedbanks in slash-and-burn rice fields of northern Laos. Weed Research. Introduction Slash-and-burn cultivation, commonly part of shifting cultivation, assures the subsistence of poor and rural populations, particularly in the tropics. Worldwide, approximately 190 million people practice shifting cul- tivation involving an area of about 1.5 million km 2 (Plagge et al., 2008). Natural vegetation is cut and burned, and a single crop, rarely two, is planted, followed by a period in which the field reverts to nat- ural regrowth, often secondary forest. Farmers apply no fertilisers and rely on fallow periods to restore soil fertility and suppress weeds. Harmful weeds are lar- gely absent when long fallow periods and short crop- ping intervals are maintained, but under short fallows, weeds, more than declining soil fertility, tend to limit crop yields (Roder et al., 1997; Bech Bruun et al., 2006; Saito et al., 2006). Short fallows result from increased pressure on arable land. In Laos, this is exacerbated by two government policies (Lestrelin et al., 2005): resettlement of population near roads and land allocation; the latter provides but few Correspondence: A de Rouw, BIOEMCO UPMC, case 120, 4 place Jussieu, 75252 Paris Cedex 5, France. Tel: (+33) 1 44 27 72 82; Fax: (+33) 1 48 47 55 34; E-mail: anneke.de_rouw@ird.fr © 2013 European Weed Research Society DOI: 10.1111/wre.12053