Recovery of secondary forests on swidden cultivation fallows in Laos

Species composition, structure and diversity of secondary forests recovered on abandoned swidden cultivation fields in the lowland of Laos were studied in relation to distance from the natural forest, fallow age and crop-fallow rotation cycle. All woody species with stem diameter >= 1 cm were identified, counted and their diameter at breast height (1.3 m, dbh) measured on 290 plots in 162 swidden fallows. Distance from the forest edge related non-linearly and explained 46%, 54%, 63% and 52% of the variation in species richness, stem density, basal area and Simpson's index, respectively. The basal area of secondary forests increased significantly (p<0.0001) while stem density tended to decrease (0.05 < p < 0.1) with increasing fallow age, whereas species richness and diversity measures remained insensitive. Species richness, stem density, basal area and Shannon-Wiener index were reduced by 28%. 35%, 72% and 23%, respectively while the number of bamboo clumps increased significantly by 45% as the crop-fallow rotation cycle increased from one to three. The occurrence of bamboo had a negative effect on most of the vegetation characteristics analyzed. From forest production and biodiversity conservation viewpoints, it is recommended that the natural regeneration in fallows with a history of previous severe land use intensity should be supplemented with enrichment planting irrespective of its distance from the natural forest. Although the management objectives for secondary forests on swidden fallows are not clearly set, promoting the development of bamboo forests in fallows would be one management option, given its importance in rural livelihood. (C) 2009 Published by Elsevier B.V.