Fjölrit RALA - 05.12.1999, Page 95

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more humid condition, but in drier condition forb species are usually reduced with
perennial species (Hustor 1979).
Stipa breviflora
Carex enervis
Oxytropis aciphylla
Eragrostis poaeoides
Pappophorum boreale
Peganum harmala
Corispermum sp.
Cleistoaenes songori
Lespedeza potaninii
Solsola collina
Heteropappus altaicu
Astragalus scaberrim
Artemisia scoparia
Allium tenuissimum
Artemisia friaida
Astragalus melilotoi
Oxytropis psammochar
Tribulus terrestris
Stipa gobica
Allium polyrrhizum
Aneurole.chinensis
Agropyron cristatum
Iris tenuifolia
Gueldenstaedtia vern
Asparagus gobicus
Ajania achilleoides
Lagochilus il:cifoIi
Astragalus efoliolat
Haplophyllúm davuric
Cymbaria dahurica
Artemisia blepharole
Tragus mongolorum
Setaria viridis
Serratula centauroid
Aristida adscensioni
Convolvulus ammannii
Astragalus galactite
Cleistogenes squarro
Figure 4. Distribution spectrum of species along the gradient.
Soil moisture
There were no significant differences in soil moisture along the degradation gradients.
Seasonal change in soil moisture was the most pronounced feature of the soil moisture
regime in the Ordos rangeland. Mean soil moisture ranged from 3.8-7.5% in the des-
ert steppe site to 15.2-32.9% in the salt meadow site during the rainy season (June-
October). Although grazing has no significant impact on top soil moisture regime,
there was evidence that this affected the bottom soil moisture regime. In the sites, the
bottom soil moisture was lower at the heavy grazed plots than at the lisht grazed plots
during the rainy season. This suggests that perennial grasses such as ipa breviflora
and Cleistogenes songorica use more water in the upper bottom soil nile the invad-
ers such as Artemisia songorica use more water in the lower bottom soil. The pattem
of moisture recharge reflected this differential utilization of soil moisture between
grazed and ungrazed plots. Generally, the soils at the opened plots had lower moisture
and higher bulk density than those at other plots.
CONCLUSIONS AND DISCUSSION
Results of multi-factors impact analysis suggest that heavy grazing can alter the spe-
cies composition of rangeland sites to annuals and sometimes small shrubs. Most of
the damage to range resources occurs because of combined effects of overgrazing and
drought. Heavy grazing alters community stmcture by eliminating species that are less
resistant to grazing and promoting resistant species, thereby reducing diversity.
Drought never removes a dominant species entirely. Recovery following drought is