Detecting the driving forces underlying the divergence of spruce forests in China: evidence from phytocoenology, morphology and phylogenetics

Aims We aimed to elucidate the driving forces underlying the geographical distribution of spruce forests,as well as the morphological and phylogenetic divergence among spruce species in China.Methods One hundred and seventy two sites across the entire range of spruce forests in China(23°–53°N,75°–134°E,250–4300 m a.s.l.)were sampled for species composition,geographical coordinates,and topographic and climatic variables.Sixteen spruce taxa,which are naturally distributed in China,were respectively grouped into morphologically defined sections and phylogenetically distinct clades.Multivariate approaches,including two-way indicator species analysis,principal components analysis,detrended correspondence analysis,canonical correspondence analysis(CCA),and partial CCA,were used for data analysis.Important Findings The 172 samples grouped into 13 spruce forests,the geographical distributions of which were closely related to climate and geographical location.The variation in species composition explained by the geographical coordinates(32.01%)was significantly higher than that explained by the climatic(27.76%)and topographic variables(23.32%).Of the three morphologically defined sections,sect.Omorica occurred mainly in wetter habitats with a mean annual precipitation of ca.229 mm and 426 mm higher than the habitats of sect.Casicta and sect.Picea(P<0.01),respectively.Of the two phylogenetically distinct clades,Clade-II(an older clade)occurred in habitats with warm winters and cool summers whose mean temperature in the coldest month was ca.8–10℃ higher,yet accumulated temperature during the growing season(≥5℃)was ca.297–438℃ lower,than the habitats of Clade-III(a younger clade)(P<0.01).Our data support the hypothesis that geographical location may be a greater determinant of variation in species composition.In addition,moisture conditions tend to be the key determinants that account for the divergence among the morphologically defined sections,while the phylogenetic divergence among spruce species is mainly affected by temperature conditions.While the clades or sections of the spruce species in question carry strong climatic signals,their divergences are subject to different selective pressures.