The geographical distribution and differentiation of Chinese beech forests and the association with Quercus

Questions: How do Chinese beech forests differ in species composition and environmental requirements? What are the environmental determinants for the distribution of Chinese beech forests? What is the interspecific relationship among the Fagus species and with Quercus in the communities? Location: The natural range of four Chinese Fagus species, including subtropical mainland China and Taiwan Island. Methods: Beech forests were sampled in 28 mountains. Community clustering and canonical correspondence analysis was applied to visualize species composition differences of the community types, and divergent environmental requirements of different Fagus species. The niche overlap among different Fagus species and with different subgroups of Quercus was measured. Based on generalized linear models, hierarchical variation partitioning was applied to estimate the contributions of climatic and topographic variables to the spatial variation in the abundance of different Fagus and Quercus species. Results: (1) Among the Chinese beech forests dominated by four Fagus species, F.engleriana communities and F.hayatae communities are closer in species composition, as are the F.lucida communities and F.longipetiolata communities. (2) F.longipetiolata has the weakest and F.engleriana the strongest tolerance to freezing. The mainland F.hayatae, F.lucida and the island F.hayatae communities differ prominently along a humidity gradient from dry to wet. The spatial variation of abundance of all Fagus species is primarily related to precipitation seasonality, while the distributions of Quercus species are more sensitive to topographic feature at local scale. (3) The niche overlaps among the Fagus species are low at community level. F.engleriana and F.lucida have higher dominances in the communities than F.hayatae and F.longipetiolata. Quercus species are common components of beech forests in mainland China, but only Quercus subgenus Cyclobalanopsis species occur in F.hayatae communities in Taiwan Island. Different Fagus species have different associations with Quercus species in subgenus Quercus or Cyclobalanopsis. Conclusions: The distribution of beech forests in China is primarily related to the precipitation seasonality of the monsoon climate, and differentiated on both temperature and moisture gradients. Within species ranges, the presence of oak in the beech forest is significantly regulated by topography.