Fjölrit RALA - 05.12.1999, Side 93

JlA JlONG
91
absence of stress and to examine the effect of disturbance by human impact and graz-
ing by domesticated livestock. The data upon which this paper is based include a se-
ries of permanent plots of desert steppes along the gradient of intensities of grazing
and human impact.
Originally we designed
a series of experiments that
included different intensi-
ties of grazing, human ac-
tivities, and soil erosion.
We had various numbers of
plots, lines, soil cores or
other sampling units within
the series. Changes can be
seen in Figure 2. Plot num-
ber represents progressive
stages of stress. For exam-
ple, DS-1 is the plot with
no disturbance while DS-5
is the one with heavy dis-
turbance.
Changing of bjotic
AND ABIOTIC FACTORS ALONG THE DEGRADATION SERIES
The study begins with the analysis of the changing of structure and function of range-
land ecosystem along a series of desert steppe plots (Figure 2). Based on the analysis
of change of rangeland features on different scales, three groups of variations are se-
lected: rangeland status, natural impact, and human impact. Results of multi-factor
impact analysis suggested that there are obvious changes in plant biomass, floristic
composition, species diversity, soil moisture and other factors along the degradation
gradients.
Plant biomass
Generally, total above-ground biomass is gradually reduced along the disturbance se-
ries, i.e., the heavier the disturbance, the less above-ground biomass. In the gradients,
estimates of above-ground biomass in the absence of grazing ranged ffom 42.3 to
134.1 g/m2 from sites to sites, but the peaks of biomass occur at a certain stage where
vegetation is under light grazing. It is suggested that light disturbance can help to re-
duce the cover of dead shoot and help the regrowth of plants, mainly forbs, in the
spring in desert steppe condition. This could perhaps also be related to the release of
nutrients that would otherwise accumulate in litter on topsoil (Coupland 1979). In the
middle and heavy grazed condition, above-ground biomass was reduced in almost all
sites. The data also suggested that the proportion of perennial biomass reaches peak
values at ungrazed plots, and then declines along the degradation series.
It is argued that grazing time and intensity strongly influence animal performance,
vegetation regeneration, protection, and future production from these grassland. The
later tillering stages of grasses and branching in early flowering stages of legumes are
140
GC DH GB Biodiv(-10) Sn
Figure 2. Change of features along degradation gradient.
* Dom.Sp - Dominant species; Biomass - Above ground
biomass (g/m2); SDI - Species Ðiversity Index;
Sn - Species number.