3. Results

3.1 Sampling effort

Total 545 vascular plants belonging to 106 families and 339 genera were recorded from 96 sampling plots along the elevational gradient in the Gyirong Valley. Out of all the vascular plants, 158 were woody (28.99%) and 387 were herbaceous (71.01%), 337 were temperate (61.83%) and 112 were tropical (20.55%) species. For each of 300-m elevational band, the species accumulation curves reached plateau (Fig. 2), and the non-parametric estimated species richness were highly correlated with the observed species richness (Choa2, R2=0.974,p <0.01; Jackknife2, R2=0.988,p <0.01; Table S2), suggesting that the sampling effort was adequate.

3.2 Elevational trends of species range size and environmental variables

For all groups of vascular plants, species range size correlated positively with elevation (β>0.7, p <0.01), and shown uniform increasing trends along the elevational gradient as predicted by Rapoport’s rule (Fig. 3). Woody species and tropical species were found having relatively stronger range size-elevation relationship with higher regression coefficient.
With increasing elevation, MAT and MAP decreased steeply whereas MATR and TS increased monotonically, TC and PC showed a similar pattern which generally decreased with an intermediate trough at 2400 m a.s.l., POP and AALU showed a similar bimodal pattern with peaks at 2700 and 4200 m a.s.l. corresponding to the Gyirong town and Zongga town, SR showed a left-skewed hump-shaped pattern with peak at 3000 m a.s.l. (Fig. 4). MDE of all groups of vascular plants showed a mid-peak pattern (Fig. 4, Fig. S1).

3.3 Relationship between species range size and environmental variables

OLS models and SAR models showed similar results about the relationship between species range size and environmental variables, though the correlation would decrease when the spatial autocorrelation was taken into account (Table 1). For all groups of vascular plants, almost all environmental variables were significantly correlated with species range size along the elevational gradient, except for MDE, POP, and AALU. Among them, MAT, MAP, TC, PC, and SR showed negative relationship with species range size, whereas MATR and TS showed positive relationship with species range size.

3.4 Relative importance of each environmental variables

The Random Forest models explained 58.09%, 60.24%, 51.86%, 52.05%, and 41.93% of the variation of range size in overall species, woody species, herbaceous species, temperate species, and tropical species, respectively. Generally, MATR, TS, MAT and MAP are the most important variables in explaining the elevational variation of range size across all groups of vascular plants (Fig. 5). SR, TC, and PC also play supplementary roles in determining the range size of vascular plants, whereas MDE, POP, and AALU appear to be weak explanations of range size.