Distribution, species diversity and life-form spectra of plant communities along an altitudinal gradient in the northern slopes of Qilianshan Mountains, Gansu, China

We studied the distribution pattern, species diversity and life-form spectra of plant communities along an altitudinal gradient in the mid-section of the northern slopes of Qilianshan Mountains by means of multivariate analyses. Two data sets (167 species x 75 plots, 10 environmental variables x 75 plots), originated from the fieldworks in 1998-1999, were subjected to TWINSPAN and DCCA, resulting in 8 major plant communities: 1) Asterothamnus centraliasiaticus-Halogeton arachnoideus desert grassland on azonal substrates from 1450 to 1600 m and 2) zonal Reaumuria soogorica desert grassland on gravels from 1470 to 1900 m; 3) Stipa przewalskii-Stipa purpurea montane grassland from 2200 to 2900 m; 4) Polygonum viviparum alpine grassland from 2900 to 3700 m; 5) Caragana stenophylla-Ajania fruticulosa dry-warm shrubland from 2350 to 2800 m; 6) Sabina przewalskii mid-wet warm forest from 2700 to 3300 m; 7) Picea crassifolia cold coniferous forest from 2450 to 3200 m; 8) Caragana jubata wet-cold alpine shrubland from 3100 to 3700 m. Species diversity and species richness of both grasslands and forests peaked at the intermediate portion of the elevational gradient. Evenness might be strongly influenced by either biotic or abiotic factors at a local scale, while seems quite independent of an elevational gradient at landscape scales. Beta-diversity decreased from 1500 to 3700 m, indicating that species turnover declined with increased elevation. Both richness of life-form and total species richness in a given altitudinal belt (gamma-diversity) peaked at intermediate elevations, while relative species richness of different life-form varied differently along the altitudinal gradient.