Keywords: Rotational grazing, full protection, continuous grazing species richness,
species diversity, soil seed bank, Bayesian methods, Salsola vermiculata, seed
longevity, rangeland management, Syria.
Rangelands represent 70% of the semi-arid and arid Mediterranean land mass. It is a
natural habitat for millions of people whose livelihood depends on animal husbandry.
The revolutionary developments in the animal husbandry and veterinary medicines
resulted in exponential increases in human and livestock population living on and from
dry lands. To respond to population growth, urbanization and transportation means
and expanded road networks, land reform and rural development policies forced
nomads to adopt semi to fully sedentary lifestyles with disintegrated traditional
community structures and organizational frameworks. Under these demographic
changes coupled with national and international border crossing restrictions, there was
an escalation in opportunistic cultivation, and excessive exploitation of the scarce and
slowly renewable vegetation cover resulting in a steep decline in the primary
production components of the rangeland ecosystems. In an attempt to stop and reverse
the degradation process, large-scale re-vegetation programs based on transplanting and
reseeding with perennial shrubs, resting and grazing management systems were
devised and implemented. This study aimed to evaluate the impacts of the
rehabilitation programs on the above-ground vegetation cover and soil seed bank
replenishment in the Syrian rangelands. The underlying assumption of the
rehabilitation program is that with a well-established perennial plant cover and proper
grazing management, top soil is stabilized, soil moisture, nutrients and seed bank are
replenished, organic matter is accumulated and microorganisms’ activity is promoted
resulting in greater abundance, species richness and diversity of annuals. To test the
above hypothesis, field and controlled environment based studies were carried out with
quantitative data collection and processing on plant species abundance, richness and
diversity of above-ground vegetation and soil seed bank for fully protected rotationally
and continuously grazed areas of 10 rangeland sites in northern Syria for three
In addition to the use of frequentist statistical approaches for species diversity
estimation in the studies, the Bayesian method was explored. Moreover, the crucial
issues of seed quality in re-vegetation were tackled through a study on seed viability
and longevity in Salsola vermiculata L., which is a highly palatable, well adapted and
widely used species in the arid Mediterranean rangeland rehabilitation programs.
The major findings are indicated below.
Above ground vegetation cover
The vegetation cover data analysis using ANOVA showed that overall plant density
consistently declined from full protection to rotational and then continuous grazing in
9 out of the 10 sites studied, whereas the trends of change in species richness and
diversity were not consistent.
Pair-wise comparison showed that plant density, species richness and diversity
were lowest for the overall plant population under rotational grazing in which plant
density of perennial grasses was highest. This suggests that rotational grazing has
reshaped the composition of the plant communities under the study areas by promoting
the perennial grasses that are more arid rangeland adapted and ecosystem significant.
Higher plant density under rotational grazing and similarity in species diversity under
the three grazing treatments for perennial grasses imply that a longer period of
rehabilitation and/or probably incorporating inter-seasonal rotational grazing and
variation in herbivore types into the current intra-seasonal rotational grazing system
will be required to cross the line of no return in plant community composition
optimization through the prevalent arid Mediterranean rangelands rehabilitation
Soil seed bank assessment
The soil seed bank data analysis using ANOVA showed no significant differences in
the overall physical and germinable soil seed bank size and diversity along the grazing
gradient. However, there was a significant grazing-by-site interaction for both and a
significant grazing-by-year interaction for germinable seed bank size showing that the
change in seed bank size is moderated by physical and environmental characteristics
and human-induced disturbances. Continuous grazing treatments for some sites were
located near agglomerations of people and animals, main roads and water points.
Under such conditions the more disturbance-adapted ephemerons and non-palatable
plants with limited constraints for seed setting dominated and this resulted in a larger
soil seed bank under continuous compared to rotational and full protection grazing
treatments. For the more human-induced disturbance distanced sites, the soil seed bank
was at larger or similar under full and/or rotational compared to continuous grazing.
Results from pair-wise comparisons showed a simultaneous decline and surge in
physical and germinable soil seed bank size of annuals and those of perennials under
the grazing treatments over sites. This suggests relative differences in root competition
and gap exploitation characteristics among plant functional groups; these differences
could be considered indicative to rangeland status and a guide to vary herbivores in
order to maintain optimum plant species diversity in the target rangelands.
Soil seed banks with no seeds of perennial grasses generated 208 seedlings m-2 of
germinable soil seed bank under continuous grazing. This is probably due to seed
setting failure resulting from overgrazing compensated by vegetative reproduction.
The widely used phanerophytes in the rangeland rehabilitation program had a physical
soil seed bank of 59.7 to 119 seed m-2 and a zero germinable one. This shows high
complementarity between physical and germinable seed testing methods for rangeland
High Morisita-Horn and Sørensen similarities were recorded between the quadrat and
point intercept measurements of the above ground vegetation and with each of
physical and germinable soil seed banks. However, the similarity indices of the above
ground vegetation measurements were higher with the germinable soil seed bank than
with the physical soil seed bank. This suggests that the germinable soil seed bank is
more suitable for monitoring arid Mediterranean rangeland than the physical soil seed
Based on the coefficients of determination (CDs), the variation in plant density and
seed bank size accounted for a significant portion of the variations in spcies richness
but not of the diversity indices. However, plant density and species diversity
consistently and significantly declined during the season with the lowest mean annual
precipitation showing the crucial role of precipitation in the dynamics of the yet active
soil seed banks of the study areas. The CDs for the germinable soil seed bank size
tested under optimum soil moisture with species richness were also significant
throughout the grazing treatments reflecting the dependence of seed bank dynamics on
For the physical soil seed bank (PSSB), CDs of its size with species richness
were only significant under rotational grazing implying positive impacts of grazing
management on soil seed bank replenishment. Morover, the CDs of PSSB richness
with the diversity indices were only significant for Singletons but not for Shannon and
Simpson. This is attributable to the fact that the Singleton index is more sensitive to
rare species than Shannon and Simpson. The non-significant correlations between
plant density and species diversity reflect a need to incorporate inter-seasonal
rotational grazing and herbivore variation to the current intra-seasonal rotational
grazing for greater plant community integrity.
Using two above and two underground vegetation data collection methods, a total of
137 species, including 102 annuals and 35 perennials from 36 families of 11
chorotypes, were recorded. The number of species recorded were 56, 66, 68 and 98
from physical seed extraction, point intercept, quadrat and grow out test, respectively.
These results showed the superiority of the growing out test over the three other
methods. With its easiness and relative flexibility of application in terms of time and
space, the grow out test seems to be the best method for arid Mediterranean rangelands
monitoring and assessment of rehabilitation impacts in which the frequency of good
rainy season is one out of four years.
The rotational grazing component of the rangeland rehabilitation program resulted
in a change in plant community composition shown by an increase in low
proportional abundance perennial grasses with greater arid Mediterranean
Continuous grazing reduced plant density but not richness and diversity, indicating
that maximum diversity is not a sign of rangeland health and integrity. This also
suggests that inter-seasonal rotational grazing and herbivore variation could
probably improve the shaping effects of grazing on the arid rangeland
Capturing more species and higher similarity indices with the above ground
measurements, the simple and flexible, germinable soil seed bank test seems to be
a good monitoring and evaluation method for arid Mediterranean rangelands.
However, results from the tedious and less accurate physical seed extraction
method could be crucial to capture the species with long seed dormancy.
Larger Bayesian estimates of diversity, smaller standard errors, lower p-values and
more significance of differences in diversity between grazing treatments compared
to the frequentist approaches were observed, thus indicating clear merits for the
approach in estimating diversity.
The seed longevity study showed that under relatively higher seed moisture
content, longevity increased suggesting that desiccation susceptibility is probably
the cause of short seed longevity of Salsola vermiculata L. Moreover, drying and
packaging alone increased longevity, thus providing a simple, cost-effective and
environmentally friendly method for rangeland rehabilitation programs.
|Qualification||Doctor of Philosophy|
|Award date||3 Dec 2013|
|Place of Publication||Wageningen|
|Publication status||Published - 2013|
- arid grasslands
- rotational grazing
- botanical composition
- biodiversity assessment
- seed longevity