Slope-dependent effects of shrub encroachment on herbaceous diversity in alpine steppe of Central Asia

Despite widespread recognition of the shrub encroachment threat to grassland globally, a clear understanding of its region-specific effects on plant community diversity in Central Asian steppes remains lacking. Accordingly, field investigations in the Suusamyr Valley of Kyrgyzstan were conducted to examine the effects of Caragana aurantiaca encroachment gradients (no, light and severe encroachment) on alpine steppe herbaceous communities across different slopes. Our results revealed that 47 herbaceous species were monitored, and shrub encroachment did not alter the dominance of Festuca valesiaca and Stipa capillata in the community, but led to an increase in shade-tolerant and poisonous species. The effects of shrub encroachment on herbaceous community characteristics were modulated by slope aspect and position. Specifically, herbaceous height, cover, and aboveground biomass (AGB) varied primarily with slope position, whereas diversity was largely determined by slope aspect. On upper slope positions, severe shrub encroachment significantly increased herbaceous height (33.34%) but decreased both vegetation cover (5.74%) and AGB (17.70%). Conversely, herbaceous cover significantly increased by 22.57% on lower slope positions, while height showed no significant change under severe encroachment. Plant diversity was enhanced by shrub encroachment on lower slopes across aspects, but was significantly reduced on upper slopes. Furthermore, severe shrub encroachment significantly decreased the Shannon-Wiener index, Species richness, and herbaceous cover compared to light shrub encroachment. Overall, our study deepens the understanding of how shrub encroachment influences Central Asian grassland ecosystems, offering essential insights for developing science-based restoration and management frameworks in alpine steppe ecosystems.