Responses of plant community coverage to simulated warming and nitrogen addition in a desert steppe in Northern China

Grassland communities occupy about 25 % of global land area and global warming could alter grassland plant community coverage. A field study was conducted to investigate the impact of soil warming (surface soil temperature increased by 1.3 A degrees C) and nitrogen addition (100 kg N ha(-1) year(-1)) on a desert steppe community in Inner Mongolia, China, from 2006 to 2011. Although there were strong intra- and inter-annual variations, in general, warming had little effect on coverage of the four major species (two cool season C-3 species: Convolvulus ammannii and Stipa breviflora, and two warm season C-4 species: Cleistogenes songorica and Kochia prostrata) over six growing seasons. The C-3/C-4 coverage ratio consistently decreased with warming, with the decrease being statistically significant for one-third of sampling dates. Warming caused a reduction (from 27.6 to 25.1 %) in peak season total plant community coverage. N addition did not affect the four main species, four functional groups (perennial grass, perennial forb, shrub and annual-biennial) or the total plant community over the 6 years, except when precipitation was high (231 mm in 2008). The small reduction in plant community coverage demonstrated the resilience of desert steppe to warming. There could be a shift of C-3 to C-4 species because of warming, as indicated by the decrease in C-3/C-4 ratio, but confirmation of this trend requires further study.