Floristic composition and plant diversity in distribution areas of native species congeneric with Betula halophila in Xinjiang, northwest China

Betula halophila, a wild plant with extremely small populations, is endemic to Xinjiang, northwest China. Its wild populations have declined severely in the field. Understanding the patterns of floristic composition where congeneric species of B. halophila are distributed and their determinants is a necessary step to restore the wild populations. Based on literature records, specimen information, shared public data and field survey data, the patterns of floristic composition, diversity and environmental conditions of seed flora within the distribution areas of five native species (i.e. B. tianschanica, B. microphylla, B. pendula, B. rotundifolia and B. humilis), congeneric with B. halophila, were examined. The results are as follows. (1) There were 3013 species, 693 genera and 108 families of seed plants in the distribution area of these congeneric species of B. halophila, which accounted for 86.16%, 94.54% and 93.91% of the total seed plants in Xinjiang, respectively. (2) The family composition of seed flora in the distribution area of these congeneric species of B. halophila was mainly cosmopolitan; the genus composition of seed flora was dominated by temperate, mainly of northern temperate and Mediterranean components. (3) There are no significant differences existing in plant richness amongst the areas where each of the five congeneric species (B. tianshchanica, B. microphylla, B. pendula, B. rotundifolia and B. humilis) are distributed. (4) The influence of climate factors on species richness is significant across the whole distribution areas of the Betula genus, while the main environmental factors determining species richness are different amongst distribution areas of different species. Climate factors impacted significantly on species richness in distribution areas of tree species, but not in distribution areas of shrub species. This study provides a preliminary guideline for the conservation of B. halophila, a wild plant with extremely small populations in the field.