Enhanced herbaceous encroachment due to niche overlap in alpine tundra of Northeast China

The directional ecological filtering hypothesis posits that species in high-altitude environments typically exhibit broad environmental tolerance, influencing the colonization success of nonnative species through niche overlap with natives. This study examines this hypothesis on Changbai Mountain, focusing on the encroachment of the herb Deyeuxia angustifolia from mountain birch forests into alpine tundra, where it competes with the endemic Rhododendron aureum. We established a permanent plot containing 10,000 continuous quadrats (2 x 2 m) in the summer of 2014, covering from treeline to the upper limit of alpine tundra. Each quadrat was divided into 40 x 40 cm resolution cells, totaling 250,000 cells. In 2014 we photographed and manually interpreted tundra plant species for all cells, assessing topographic and edaphic niche characteristics (the range of the factors) for both species. Findings reveal: (1) Over 50 % niche overlap between D. angustifolia and R. aureum, suggesting intense competition; (2) Broader niche width for R. aureum compared to D. angustifolia; and (3) The expansion of D. angustifolia is facilitated by available nitrogen, while R. aureum is uniquely adapted to low phosphorus levels at higher altitudes. These results demonstrate that ecological and environmental filtering jointly shape distribution patterns of R. aureum and D. angustifolia, supporting the idea that environmental filtering is critical in the colonization of invading plant species. Our research sheds light on the pivotal roles played by microenvironmental factors, notably topography and soil, in determining the composition and distribution of plant communities, and contributes valuable insights for understanding and forecasting future shifts in plant composition of alpine tundra.