Point pattern analysis of several psammophyte populations in the riparian ecotone in the middle reaches of Yarlung Zangbo River of Tibet, China

Abstract

Aims Artemisia wellbyi, A. younghusbandii and Sophora moorcroftiana are the major psammophyte populations on the aeolian desertified land in the riparian ecotone of the middle reaches of the Yarlung Zangbo River. Our ob-jectives were to determine how spatial scales are related to population patterns and how species spatially adapt to semi-arid conditions in the riparian ecotone, as well as to provide theoretical foundation for vegetation restoration on sand dunes in the study area. Methods We selected four representative plant communities under different habitat conditions in the riparian ecotone, measured the position, height and crown diameter of each individual in a 20 m × 30 m plot and studied the population structure, spatial distribution and association of these psammophyte populations using point pattern analysis.Important findings The population structure of A. wellbyi on semi-exposed sandy gravel land and semi-fixed sandy land is that of a growing population, while A. younghusbandii and S. moorcroftiana on fixed sandy land both have the structure of a declining population. Artemisia wellbyi and A. younghusbandii both have a clumped distri-bution at different scales, while the distribution pattern of S. moorcroftiana changed from clumped to random to clumped to random as the scale increased. Spatial distribution is mainly determined by the smaller size classes, while fluctuations or changes of population distribution are mainly determined by the larger size classes. There is a scale effect among the studied populations, and positive spatial association occurs mainly at certain scales. Spatial association is often affected by plant size and environmental heterogeneity. The spatial association of A. wellbyi and S. moorcroftiana on semi-exposed sandy gravel land or semi-fixed sandy land changes from independent to positive to independent to positive.The spatial association of A. wellbyi and Oxytropis sericopetala on semi-exposed sandy gravel land changes from positive to independent to positive to independent to negative, and that of A. younghus-bandii and S. moorcroftiana changes from positive to independent. The degree of aggregation weakens with in-creasing plant size. The smaller size class has a clumped distribution, and the larger has a random distribution or random and clumped. The positive spatial association between different size classes of the same species appears to weaken when the difference of plant size becomes greater and even changes into negative spatial association, while the spatial association of various size classes at a smaller scale often is positive or independent.