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SER Summer School Restoration Ecology 2009 -
Münster, Germany
Soil seed banks in relation to degradation of
aridgrazing lands in southern Namibia
Niels DreberBiocentre Klein Flottbek Botanical
Garden, University of Hamburg
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INTRODUCTION
the study area
NAMA-KAROO _670 240 km2 (23 of the southern
African subregion)
x
Source Cowling, R.M., Richardson, D.M., Pierce,
S.M., 1997. Vegetation of Southern Africa,
Cambridge University Press.
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INTRODUCTION
the study area
NAMA-KAROO _670 240 km2 (23 of the southern
African subregion) _open grassy dwarf
shrubland
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INTRODUCTION
the study area
NAMA-KAROO _670 240 km2 (23 of the southern
African subregion) _open grassy dwarf
shrubland _rainfall 100-200 mm coefficient
of variation 80
x
study sites
Source MAWRD, mean annual rainfall
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INTRODUCTION
the study area
NAMA-KAROO _670 240 km2 (23 of the southern
African subregion) _open grassy dwarf
shrubland _rainfall 100-200 mm coefficient
of variation 80 _carrying capacity 1
SSU / 5 ha small stock farming
study sites
x
Source MAWRD, mean above ground biomass estimate
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INTRODUCTION
the study area
Land degradationis a widespread problem in the
region,especially in communal areas!
The main driving factors(i) a lack in
regimentations(ii) high population stock
densities(iii) the highly variable climate
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INTRODUCTION
the study area
fenceline contrast a natural experiment
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INTRODUCTION
degradation facts
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INTRODUCTION
degradation facts
Such profound regime shifts in savannah
rangelands might be attributable to to a loss of
ecosystem resilience due to sustained grazing
pressure
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INTRODUCTION
degradation facts
Is the degraded state reversible?
40 years x
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INTRODUCTION
study questions
Assessment of seed banks is a constructive tool
to assess rangeland condition and restoration
potential,especially in combination with habitat
appraisal. Jones Esler 2004,
Biodiversity and Conservation 13
Central study questions 1 To what extent
has the structure of the viable soil seed bank
changed after long- term
over-utilization of the communal rangeland? 2
What is the impact of different microhabitats on
the seed bank patterning? 3 Are there
microhabitats functioning as potential seed
sources for the recovery of target
plant species which are absent from the standing
vegetation?
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SOIL SEED BANKS DEGRADATION
methods
THE SAMPLING DESIGN
Stratification of the area selection of
comparabel habitats regarding bedrock soil
type geomorphology woody character
species homogeneous vegetation regarding species
composition structure
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SOIL SEED BANKS DEGRADATION
methods
THE SAMPLING DESIGN
after seed shet, before any germination event
Under shrub canopy seed bank (macro safe sites)
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SOIL SEED BANKS DEGRADATION
methods
THE SAMPLING DESIGN
after seed shet, before any germination event
Seed bank of the inter-shrub matrix
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SOIL SEED BANKS DEGRADATION
methods
THE SAMPLING DESIGN
Inter-shrub matrix vegetated patches (micro
safe sites) bare patches
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SOIL SEED BANKS DEGRADATION
methods
DIRECT SEEDLING EMERGENCE METHOD
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SOIL SEED BANKS DEGRADATION
first results
2007 W1-62, W3-852008 W1-62, W3-81
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SOIL SEED BANKS DEGRADATION
first results
Total species richness of the soil seed bank
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SOIL SEED BANKS DEGRADATION
first results
Total seed density of the soil seed bank
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SOIL SEED BANKS DEGRADATION
first results
p lt 0.05
Mean seed densities / m2 of the germinable soil
seed bank
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SOIL SEED BANKS DEGRADATION
first results
Plant functional groups in the soil seed bank
_species on REF and DEG are similar distributed
within the plant functional groups _proportions
of germinants within the single groups are highly
different among the sites
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SOIL SEED BANKS DEGRADATION
first results
Species Abundance DistributionsSAD-Curves for
the soil seed bank of the inter-shrub matrix
(MISs BPs)
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SOIL SEED BANKS DEGRADATION
first results
Species Abundance DistributionsSAD-Curves for
the soil seed bank of the inter-shrub matrix
(MISs BPs)
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SOIL SEED BANKS DEGRADATION
first results
Species Abundance DistributionsSAD-Curves for
the soil seed bank of the inter-shrub matrix
(MISs BPs)
_dominance of small-seeded, annual plant
species _antipodal species abundance pattern
comparing REF and DEG species common under
low grazing pressure are seldom on the heavily
grazed site vice versa
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SOIL SEED BANKS DEGRADATION
first results
Species Abundance DistributionsSAD-Curves for
the soil seed bank vegetation of the
inter-shrub matrix
_species composition of the soil seed bank
standing herbage is similar _proportional
composition of the vegetation mirrors the
condition of the soil seed bank
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SUMMARY
Structural heterogeneity of the
environment causes a horizontal patterning of
the seed bank provides effective microsites for
seed retention and accumulation contributes to
the presence of species diverse seed pools of
high seed numbers
Long-term over-utilization of the communal
rangeland has altered the size, species
composition and plant functional
composition of the soil seed bank has reduced
(and partially depleted) desirable seed banks
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CONCLUSIONS in a restoration context
Evaluation of the restoration potential the
seed bank is dominated by single species of low
grazing value which recruit in high
numbers the seed bank contain a few residual
species of the previous vegetation, but in
very low proportions
safe sites (shrubs) contain high seed numbers
(30.000/m2) comprise species rich seed
banks, but provide no material for restoration
as target plant species are lacking the lack
in valuable species such as perennial grasses
is attributable to a failure in seed
production a reduction of adequate safe sites
for establishment
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CONCLUSIONS in a restoration context
the natural regeneration capacity of the
rangeland towards an improved postdisturbance
vegetation with species common under
sustainable management is limited an
improvement of site coditions is unlikely to
occur in appropriate time as the seed bank is
degraded
passive restoration practices (e.g. resting) are
not an option active accelerating interventions
such as seeding brush packing
ploughing soil transfer creation of
refuges implemented in a long-term strategical
restoration framework have to be applied
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Thank you