Abstract:

Aims Our objective was to determine ecological mechanisms of plant community productivity performance based on study of effects of species diversity, species identity and fertility level on productivity.
Methods We carried out an experiment with combinations of three plant species (Elymus nutans, Roegneria nutans and Festuca sinensis) under three different fertility levels in an area of subalpine meadow in Hezuo County, Gansu Province, China. Using seeds collected in the area, we sowed seeds in monoculture with sowing densities of 20, 40 and 60 seeds·pot^-1, in mixtures with sowing densities of each pair of species of 20/20, 20/40 and 40/20 seeds·pot^-1 and in mixtures of all three species at a sowing density of 20/20/20 seeds·pot^-1. We fixed seedling density at half of the sowed density. The pots were fertilized with 0, 2.5 and 5.0 g of phosphorate ammonium at tillering and jointing stages. There were five replicates of each treatment. We regularly removed weeds by hand. Aboveground biomass in each pot was harvested, dried and measured by species by early October.
Important findings Under conditions of no-fertility and fertilizing 5.0 g diammonium phosphate per pot, aboveground biomass did not increase significantly with increasing species diversity; however, under condition of fertilizing 10.0 g diammonium phosphate per pot, aboveground biomass increased significantly with increasing species diversity. Compared with Festuca sinensis, both Elymus nutans and Roegneria nutans made larger contributions to community productivity. But the effects of different fertility levels and sowing density were not the same. Results suggest that effects of species diversity on productivity varied because of variation of soil fertility. Productivity of plant communities may be mainly influenced by species identity, in correspondence with resource use under the conditions of the habitat. The high productivity in the high fertility level was caused by species or species assembly, which can better adapt to the high soil fertilization, rather than by species diversity. Therefore, plant community productivity does not necessarily relate to plant species diversity, but relates to soil fertility level and species identity determined by the soil fertility.

Key words: species identity, species diversity, soil fertility, community productivity