The role of glacial gravel in community development of vascular plants on the glacier forelands of the Third Pole

Abstract: On a deglaciated terrain, glacial gravel is the primary component of the natural habitat for vascular plant colonization and succession. Knowledge regarding the role of glacial gravel in vascular plant growth, however, remains limited. In this study, an unmanned aerial vehicle (UAV) was used to investigate plant family composition, species richness, fractional vegetation cover (FVC), and gravel cover (GC) along elevational gradients on the three glacier forelands (Kekesayi, Jiangmanjiaer, and Koxkar Baxi) of the Third Pole (including the eastern Pamir Plateau and western Tianshan Mountains) in China. We then analyzed the spatial characteristics of vascular plants followed by exploring the effect of glacial gravel on vascular plants. Findings indicated that FVC on these glacier forelands generally decreased as the elevation increased or distance from the current glacier terminus decreased. The shady slope (Kekesayi) was more vegetated in comparison to the sunny slope (Jiangmanjiaer) at the glacier basin scale, and the warm and humid deglaciated terrain (Koxkar Baxi) had the highest FVC at the regional scale. Plant family composition and species richness on the glacier forelands decreased with rising elevation, with the exception of those on the Jiangmanjiaer glacier foreland. The relationships between FVC and GC presented negative correlations; particularly, they exhibited variations in power functions on the Kekesayi and Jiangmanjiaer glacier forelands of the eastern Pamir Plateau and a linear function on the Koxkar Baxi glacier foreland of the western Tianshan Mountains. Glacial gravel was found to be conducive to vegetation colonization and development in the early succession stage up until vascular plants adapted to the cold and arid climatic condition, whereas it is unfavorable to the expansion of vascular plants in the later succession stage. These findings suggested that the spatial difference of plant characteristics had close connections with regional climatic and topographic conditions, as well as glacial gravel distribution. In addition, we concluded that aerial photographs can be an asset for studying the functions of micro-environment in vegetation colonization as well as succession on the glacier forelands.