The mid-domain effect of species diversity of mountainous plants is determined by community life form and family flora in a temperate semi-arid region of China

The mid-domain effect (MDE) is a vital hypothesis to explain altitudinal patterns of species diversity of mountainous plants with different gradients, but it is bounded in terms of its application at the plant level. To verify the MDE hypothesis, we chose a typical mountain with obvious elevation gradients and considerable plant coverage as a study area in the east of the Loess Plateau and partitioned various elevation belts across this mountain. Through measuring the species diversity of arbor, shrub and herb communities in forest ecosystems, we explored altitudinal patterns of species diversity of mountainous plants with different gradients. We determined that the family numbers of the herb and shrub communities, as well as the species diversity of the arbor community, reached their maximums at intermediate elevations. The family numbers of the herb and shrub communities presented unimodal patterns across altitudinal gradients, and the highest values occurred at intermediate elevations. The family number of the arbor community showed a monotonic decreasing pattern, and the importance values of dominant families in the shrub and arbor communities presented unimodal patterns, but the lowest values occurred at intermediate elevations. The species diversity of the herb, shrub and arbor communities conformed to unimodal change patterns following altitudinal gradients, but the greatest diversity occurred at high, low and intermediate elevations, respectively. At higher elevations, weeds and grasses grew well, whereas sedges grew well at lower elevations. With respect to the importance values of different arbor life forms, their responses to altitudinal gradients indicated a certain variation pattern, which was greater for evergreen coniferous arbor species than for deciduous coniferous arbor species and deciduous broad-leaved arbor species. It is concluded that the MDE hypothesis of species diversity for mountainous plants is influenced greatly by the community life form and family flora at the plant level in a temperate semi-arid region of the Loess Plateau, China. This conclusion tests and modifies the MDE hypothesis and can be valuable for fueling prediction of biodiversity models and for the comparison with similar studies in different regions.