2.2 Sample site design
The study site consists of a relatively flat and even area of 600 m by
300 m, fenced since 1979 to prevent grazing by large animals (Li et al.,
2015). In 1981, the area was separated
into ten replicate blocks (60 m × 300
m each). Community aboveground biomass was surveyed in the middle of
every month throughout the growing season (from May to September) each
year by clipping all plants within a 1 m × 1 m quadrat that was randomly
located within each block, over 1981–2011. After harvesting, all living
vascular plants were sorted into species, and oven-dried at 65 ℃ to a
constant weight. Species richness was calculated as the total number of
species present in ten blocks in a year. All species were classified
into five plant functional groups primarily on the basis of life forms
(Bai et al., 2004): perennial rhizome grass (PR), perennial bunchgrasses
(PB), perennial forbs (PF), shrubs and semi-shrubs (SS), and annuals and
biennials (AB). Functional group biomass was determined as the biomass
sum of all the species in each functional group. Then, the proportion of
aboveground biomass of each functional group to the community total
aboveground biomass was calculated and used for the analysis of biomass
complementarity among different functional groups (Bai et al., 2004).
Data on productivity and diversity were collected monthly between May
and September over a period of 29 years from 1981 to 2011, excluding the
missing data from the years 1995–1996 (Ma et al., 2010).