Species composition, community structure, and floristic characteristics of desert riparian forest community along the mainstream of Tarim River

Desert riparian forest is a typical temperate deciduous broad-leaved secondary forest and is mainly distributed in the Tarim Basin. To better understand the mechanisms of community dynamics and species coexistence of this forest type, six 1-hm² plots were established and divided into 150 subplots. We analyzed species composition (e.g., importance value, abundance), community structure (e.g., vertical structure, size-class structure), spatial distribution of woody plants, and floristic composition. Results indicated that community structure of the desert riparian forest was simple and species diversity was low, with 16 species, belonging to 15 genera and 12 families identified. Salicaceae had the highest importance value (28.97%) and was the dominant family in the community. When the sampling area was larger than 0.04 hm², 87.5% of species were counted. Rare and occasional species accounted for 12.5% of all species, respectively. Floristic composition was poor, with single species, genus, and temperate components being dominant, although ancient Mediterranean components also occupied an important position, with typical temperate flora. We counted 2497 living free-standing trees in the plots. Species abundance, basal area, and importance values showed that Populus pruinosa was the obviously dominant species. The number of families, genera, and species decreased with increasing height class, and the vertical structure was composed of canopy, shrub, and herb layers, indicating apparent vertical stratification in this forest. The average DBH of all trees was 15.57 cm, and DBH size and height classes showed skewed normal distributions, with few individuals with a height lower than 2 m, indicating weak regeneration across the community. The DBH structures of P. pruinosa and P. euphratica generally were unimodal; in particular, P. euphratica lacked small DBH trees and P. pruinosa had extremely high mortality during development from seedling to young tree, thus suggesting declining populations for these two species. The spatial distribution patterns of the two dominant species were clustered and varied with diameter in different habitats. In addition, the two dominant species were spatially mutually exclusive and occupied different habitats, suggesting that interspecific competition and habitat heterogeneity may be responsible for the species coexistence and spatial distribution of dominant species.