Vegetation–environment relationship in Pinus wallichiana forests of the Swat Hindukush range of Pakistan

Abstract

Understanding the influence of environmental variables on the spatial distribution of ecological communities is essential to predict the response of vegetation to various environmental drivers. Ecological theory suggests that multiple environmental factors shape local species assemblages and should influence the various components of community structure and composition in different ways. This study aimed to classify Pinus wallichiana dominated forests in the Swat Hindukush range mountains to understand the relative influence of multiple environmental filters on its composition and structure. These forests represent the most typical of the species distribution in northern Pakistan and were not subjected to any phytosociological study. For this purpose, thirty forest stands, spanning a wide range of physical habitats were sampled using 10 × 10 m plots and the importance value index was calculated. The floristic and environmental data were subjected to Ward’s agglomerative cluster analysis for objective classification and ordinated with NMS ordination for pattern description and testing the vegetation–environmental relationships. Three floristically and ecologically distinct communities were recognized along the topographic gradient (elevation, r = 0.377; slope, r = 0.5548) coupled with soil physical (clay, r = 0.2782; silt, r = 0.3225) and chemical properties (pH, r = 0.4975; lime, r = 3982). An elevation gradient of 100 m separated the low (Pinus wallichiana–Quercus dilatata community) and middle elevations forest stands (P. wallichiana pure population) from the highland population type (Pinus wallichiana–Cedrus deodara community). The floristics and structure of these forest types respond directly or indirectly to topographic and soil variables which were evidenced from the floristic composition, species richness, and community physiognomy. These characteristics of the communities changed from heterogenous, dense stands to sparsely dispersed conifers, broadleaved-evergreen, and deciduous vegetation types along the environmental gradients. We concluded that several factors explained the spatial pattern. Thus, we predict that elevation coupled with soil physical and chemical properties could be considered if the ultimate goal is the restoration and conservation of these forests. We also recommend broad-scale phytosociological study on P. wallichiana forests including multiple sampling from diverse locations to define the floristic diversity and to better understand its ecological requirements.