An elevation gradient affects temperature, precipitation, soil properties, and other environmental factors important for plant growth, resulting in differential responses of plant functional traits within and between species. Here, three transect lines were established along an elevation gradient of 1500 to 2900 m that spanned four vegetation types: desert grassland, low mountain shrub zone, subalpine coniferous forest, and subalpine shrub zone in a semiarid mountain ecosystem (western part of the Helan Mountains, Northwest China). Nine functional plant traits — leaf nitrogen (LN), leaf carbon (LC), leaf thickness (LT), specific leaf area (SLA), leaf phosphorus, leaf dry matter content, and the leaf carbon/leaf nitrogen (C/N), leaf carbon/leaf phosphorus (C/P), leaf nitrogen/leaf phosphorus (N/P) — were quantified to investigate (1) how the community-weighted means (CWM) and unweighted means (CM) vary; (2) how inter- and intraspecific functional traits vary; and (3) how the climate, topography and soil properties affect plant functional traits at the community level. We found that with rising elevation, SLA and LT increased and then decreased, while C/P and N/P showed opposite trend for both CWM and CM. Additionally, the higher LN and lower C/N were found in subalpine shrub zone. The leaf functional traits between CWM and CM showed no significant difference but a close relationship between them. Generally, all the leaf traits were explained better by interspecific variation than by intraspecific variation, except for N/P. The covariance analysis indicated that LP and LC showed negative covariance effects, whereas all other leaf traits showed positive covariance effects. The contribution to plant leaf traits from altitude was slightly higher than vapor pressure and soil organic carbon. Our major finding emphasize that plant communities in the western Helan Mountains were assembled along elevation primarily via interspecific variation.