2.2 Vegetation characterization and phenology
The forest in the ECNNR was divided into four habitat types: cliff face, slope, mountain peak, and valley-floor flatland. These habitats were analyzed using stratified random sampling methods. We sampled 18 plots (20 × 20 m) within the main study site, including four plots that were located in the valley-floor flatland, nine on slopes, two on cliff faces, and three on mountain peaks. These plots were located in the core area of the langurs’ home range, which included more than 4% of the langurs’ annual home range (unpublished data). Within these plots, all the trees with a ≥ 3 cm diameter at breast height (DBH) were tagged. In addition, to evaluate the dominance of each species within a plot, the re each lative density (RD), relative frequency (RF), and relative coverage (RC) were calculated for species as follows (Burton et al., 2005):
RD = number of individuals of the species i /total number of individuals in all the plots;
RF = number of plots with the species i/ total number of plots;
RC = sum of the basal area of species i /sum of the basal areas of all the species.
Based on previous studies of langurs (Zhou et al., 2006) and our two‐month pilot study, 32 plant food species were selected for phenology monitoring. Then, 10 individuals of each species were randomly selected and tagged in the main study area, and a total of 320 trees were monitored. In addition, the data from the vegetation surveys and plant phenology were combined to obtain appropriate estimates of food availability. During the monthly phenology surveys, all the tagged trees were visually inspected for the presence of young leaves, flowers, and fruit, and their abundance (% of crown cover) was scored with a five‐point scale: 0, absent; 1, 0.1–25%; 2, 25.1–50%; 3, 50.1–75%; and 4, 75.1–100%. The monthly food availability index (FAI) was calculated for the main food items (young leaves, flowers, and fruits) by integrating the density, basal area, and phenology score of the plant species. The formula was as follows:
\(\text{FAI}=\sum_{\text{i\ }=\ 1}^{n}D_{i}B_{i}P_{i}\),
where Di is the density of the tree species i (number of stems/ha), Bi is the average basal area of tree species i(m2/ha), and Pi is the mean phenology score of the particular food item in the crown of species i in a given month (Albert et al., 2013).