We investigated parameters of forest diversity and structure along a fallow age gradient in secondary forests regenerating after shifting cultivation abandonment. We first measured the tree diversity and forest structure indices in regenerating secondary forests and old-growth forest. We then measured the recovery of tree diversity and forest structure parameters in relation to the old-growth forest. Finally, using linear mixed effect models (LMM), we assessed the effect of different environmental variables on the recovery of forest diversity and structure.

Species density was significantly higher in the oldest fallow sites, while Shannon’s index, species evenness, stem number, basal area and leaf area index were higher in the old-growth forest. A homogeneous species composition was found across the sites of older fallow age. Multivariate analysis revealed patch size as a strong predictor of tree diversity and forest structure recovery after shifting cultivation.

Secondary forests regenerating after shifting cultivation abandonment can recover rapidly after five years. Although recovery of forest structure was not as rapid as the tree diversity, our older fallow sites contained a similar number of species as the old-growth forest. Many of these species are also endemic to the Philippines. Novel and emerging ecosystems like tropical secondary forests are of high conservation importance and can act as a refuge for dwindling tropical forest biodiversity.

Vegetation surveys were undertaken from May to October 2013. In the secondary forests, we sampled from the sites that were at least 1 ha in size (Piotto et al., 2009). Four transects of 50 m × 5 m (parallel to each other and a minimum of 5 m distance from each other) were established at each of our sites. Altogether we had 25 sites (4 fallow age category + old-growth forest × 5 replicates) covering a total sample area of 2.5ha. We recorded diameter of each tree ≥ 5cm at diameter at breast height (dbh) using a diameter tape.

Where possible trees were identified to the species level with the help of a local expert from Visyas State University (VSU). In the case of unknown species, we used the most common Filipino name of that species. Additional information, like global and local conservation status, biogeographic origin, and successional guild were also collected.

Species diversity was described in terms of species density (S), Shannon-Wiener’s diversity index (H) and species evenness index (J). Shannon’s diversity index and species evenness index were calculated as described in Magurran (2004) while species density was the number of unique tree species per site. We used stem density (N), basal area (BA), and LAI as the measure of forest structure. Both stem density and basal area (m²) were expressed on a per site (0.1 ha) basis. Stem density or number was the number of tree individuals (≥ 5 cm dbh) per site while basal area (m²) was the total cross-sectional area of all stems (≥ 5 cm dbh) in each site. LAI was measured as the ratio between total leaf area and ground area.

We used importance value index (IVI) to compare the patterns of tree species dominance in each of the secondary forest of different fallow age categories and in our control old-growth forest. IVI was the sum of relative density, relative dominance, and relative frequency of species.

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