Fjölrit RALA - 05.12.1999, Blaðsíða 94

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Rangeland degradation in Northern China
the best time for grazing to begin. After a period of grazing, the grasses should be al-
lowed to regrow at least 15 cm before further grazing. Liu (1990) recommends that
grazing be forbidden during the first 12-18 days after the grass tums green in spring
and for 30 days after the cessation of growth at the end of the season.
Plant species composition
The botanical composi-
tion of rangeland
changed greatly along
the spatial gradient; in
the ungrazed condition
the steppes were domi-
nated by palatable per-
ennial grasses, such as
Stipa breviflora, S. bun-
geana and Cleistogenes
squarrosa (Figure 3).
But in the middle-heavy
disturbance condition,
invading species were
common, especially an-
nual plants, such as Setaria viridis, Peganum harmala and Artemisia scoparia. These
species are probably successful invaders under grazing because of abundance of their
seed in the soil, and favourable fertility conditions.
Grazing and other disturbances greatly affected the floristic composition, especially
the dominant species of the sites. In the desert steppe sites, under light-middle distur-
bance the dominant species of Stipa breviflora was replaced by Artemisia songorica,
which was later replaced by Peganum harmala and Eragrostis poaeoides under heavy
disturbance. The effect of grazing on floristic composition appears to be greater in de-
sert steppe and sandy vegetation than in salt meadows and true steppe. In true steppe
sites, the obvious change of dominant species occurred only under heavier distur-
bance. This differential effect of grazing may be associated with the degree to which
the composition of plant water tolerance type is modified by grazing. We have already
noted a marked trend towards meso-species in grazed areas in drier sites that is not
present to the same extent eastward (where xero-species are less important constitu-
ents of the vegetation). It is probable that there is an interaction between water supply
and grazing (Parsons et al. 1983).
Species diversity
Species diversity (Shannon-Wiener Index) was also impacted by grazing and other
disturbance. Species diversity index was reduced by 30% as a result of heavy grazing.
At some steppe sites, the species diversity index increased slightly under light-middle
grazing pressure and reached its peaks at stage II or even III (Figure 4). It is suggested
that certain disturbance can reduce the dominance of current dominant species and
create more opportunities for the individuals of different species to live together in a