Classification of plant communities in the Caspian Hyrcanian English yew (Taxus baccata L.) forests using environmental factors: testing the modified TWINSPAN method

Hyrcanian forests in northern Iran are one of the world's precious deciduous broadleaved forest ecosystems but are subjected to increasing disturbances mostly of anthropogenic origin. In this context, it is necessary to assess the existing vegetation and its relation to environmental factors in order to establish a scientific reference for conservation measures. The purpose of this research is to evaluate the quality of the modified and classical TWINSPAN (Two-way indicator species analysis) classification based on the environmental factors to identify the plant communities of Taxus baccata L. forests of Golestan Province (Northern Iran). Fifty 400-m2 plots were systematically selected along an elevation gradient in the Taxus baccata forests. In each plot, the soil was sampled using a composite sample of three replicates taken to a depth of 20 cm. All vascular species were recorded on each plot according to Braun-Blanquet’s method. In total, 56 plant species belonging to 48 genera and 34 families were recorded in spring 2018. The comparison of the results of modified and classical TWINSPAN showed that the modified TWINSPAN did not change the classification logic, but introduced more flexibility in the hierarchy of the division. It was also found that the compliance rate of the results provided by the classical TWINSPAN method was lower than the modified method. Using modified TWINSPAN, Detrended correspondence analysis (DCA), and Redundancy analysis (RDA), we determined four floristic groups in the Taxus baccata habitat. The first (Carpinus betulus) and second (Taxus baccata) groups were located at low altitudes and soil contents in silt and clay were higher than in the other groups. In contrast, the third (Quercus castaneifolia) and the fourth (Juniperus communis) groups were located next to each other at higher altitudes and southwestern aspects, on steeper slopes and soils that exhibited a high pH and a high sand content. We found close relationships between the distribution of vegetation groups of Taxus baccata habitat with edaphic and physiographic factors using the RDA method. To conclude, the modified TWINSPAN offers a more efficient and flexible method to identify vegetation communities. This is the first step to implementing future management and ecological restoration measures in habitats of high ecological value such as yew forests.