Recovery of Vegetation Structure and Species Diversity after Shifting Cultivation in Northwestern Vietnam, with Special Reference to Commercially Valuable Tree Species

A fallow stand (FS) in northwestern Vietnam that was created by shifting cultivation 32 years earlier had 43% of the species number, 72% of the stem density, and 53% of the basal area when compared with nearby old-growth forest (OGF); however, the values for commercial species were lower at 35%, 67%, and 26%, respectively. In terms of species diversity, the Shannon index of OGF (3.4) was significantly higher than that of FS (2.6), while the differences were not significant in terms of Evenness and species-size class distribution. Both FS and OGF had similar patterns of stem diameter frequency distribution but the diameters were more diverse in OGF compared to FS according to the Shannon index. Fallow stand was characterized by only 2 canopy layers (lower than 10？m and 10–20？m) and was simpler in vertical structure than that of OGF which included an additional upper canopy layer higher than 20？m. Our results indicate that increasing stem density of commercial species is necessary and can be realized by artificial seeding, planting seedlings, and/or natural regeneration from remaining mother trees in the fields. 1. Introduction Agricultural encroachment by shifting cultivation (swidden or slash-and-burn agriculture) has been an important topic in the debate on tropical deforestation. Despite rapid economic development in many tropical countries, millions of people, particularly in the humid tropics, still practice shifting cultivation [1]. Shifting cultivators are often seen as the primary agents of deforestation in developing countries [2, 3]. In Vietnam, an area of 3.5 million ha is used for shifting cultivation by 50 ethnic minorities [4]. Regeneration of secondary forest is an essential part of shifting cultivation [5]. After field abandonment, the secondary forest develops naturally [6–8]. Research in tropical rain forests has produced reasonable insights into the patterns of secondary succession [9, 10], while that in tropical dry forest is lagging behind [11]. Successional studies indicate that secondary forests can serve as carbon sinks [12], and enhance regional biodiversity, environmental services, and forest-based economies [3, 13, 14]. Forests at different stages of succession differ in total biomass, net primary production, and species composition [8, 12, 15]. The rate of recovery in species diversity is higher in the humid and moist tropics, whereas regeneration to restore the structure of a mature forest is faster in the dry tropical forest [16]. However, species diversity and composition in the secondary forests usually remain distinctly