Using leaf mass fractions and height/mass ratios to explain the positive response of Gramineae to nitrogen addition on a sub-alpine meadow

Nitrogen addition tends to increase the dominance of Gramineae in grassland ecosystems. However, the underlying mechanism is rarely reported. We analyzed the morphological characteristics and responses of functional groups to nitrogen addition to explore the adaptation mechanism of Gramineae to nitrogen addition on a sub-alpine meadow on the Qinghai-Tibet plateau. Results showed that the leaf mass fraction (LMF) and height/mass ratio (HMR) were significantly correlated with the plant height for a representative species (P < 0.05). The LMF and HMR of Gramineae were 77.86% and 34.32 cm·g^–1, respectively, which is significantly higher (P < 0.05) than those of forbs (66.12% and 7.20 cm·g^–1, respectively). Nitrogen addition (15 g·m^–2) significantly increased the aboveground biomass (P < 0.001), plant height (P < 0.01), coverage (P < 0.01) and, chlorophyll content (P < 0.05) of the community, for which only the biomass proportions of only Gramineae and sedge increased significantly (both P < 0.001), and coverage of only Gramineae increased significantly (P < 0.01). Plant height of Gramineae was always higher than that of the other functional groups whether nitrogen was added or not (P < 0.001, respectively). Therefore, a high LMF and HMR of Gramineae enhances their competition potential for aboveground resources, and enables them to respond quickly and effectively to nitrogen addition.