Transcript Folie 1 - uni

SER Summer School Restoration Ecology 2009 - Münster, Germany
Soil seed banks
in relation to degradation of arid
grazing lands in southern
Namibia
Niels Dreber
Biocentre Klein Flottbek & Botanical Garden, University of Hamburg
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.
INTRODUCTION the study area
NAMA-KAROO
_670 240 km2
(23% of the southern African
subregion)
_open grassy dwarf shrubland
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
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%
study sites
x
Source: MAWRD, mean above ground biomass estimate
_carrying capacity:
~ 1 SSU / 5 ha
small stock farming
INTRODUCTION the study area
Land degradation
is 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
INTRODUCTION the study area
fenceline contrast – ‘a natural experiment‘
Gellap-Ost:
Nabaos:
governmental
communal
rotational grazing
open access pasture
karakul sheep
goats
low grazing pressure
high grazing pressure
30 years +
stable land tenure
70 years +
no seasonal restrictions or stocking
limitations
INTRODUCTION degradation facts
Above ground degradation on location
increase in bare soil
reduction in plant total cover
loss of valuable plant species
increased topsoil crusting & erosion
changes in diversity & abundance of rodents & ground beetles
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
INTRODUCTION degradation facts
Is the degraded state reversible?
40 years x
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 longterm 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?
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
SOIL SEED BANKS & DEGRADATION
methods
THE SAMPLING DESIGN
after seed shet, before any germination event
Under shrub canopy seed bank (macro safe sites)
SOIL SEED BANKS & DEGRADATION
methods
THE SAMPLING DESIGN
after seed shet, before any germination event
Seed bank of the inter-shrub matrix
SOIL SEED BANKS & DEGRADATION
methods
THE SAMPLING DESIGN
x
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x
x
x
x
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x xx
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x
vegetated patch
(n=45)
bare patch (n=45)
x
sampling point
Inter-shrub matrix:
vegetated patches (micro safe sites) & bare patches
SOIL SEED BANKS & DEGRADATION
methods
DIRECT SEEDLING EMERGENCE METHOD
SOIL SEED BANKS & DEGRADATION
first results
Total number of
Year
germinants
species
2007
5207
36
2008
6417
45
Overall
11880
49
2007: W1-62%, W3-85%
2008: W1-62%, W3-81%
SOIL SEED BANKS & DEGRADATION
first results
Total species richness of the soil seed bank
SOIL SEED BANKS & DEGRADATION
first results
Total seed density of the soil seed bank
SOIL SEED BANKS & DEGRADATION
first results
p < 0.05
REF: 626.60 ±717.25
DEG: 1519.76 ±1332.12
REF: 1206.00 ±1050.13
DEG: 3383.83 ±3059.15
REF: 2083.89 ±1708.28
DEG: 4305.22 ±2199.34
Mean seed densities / m2 of the germinable soil seed bank
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
SOIL SEED BANKS & DEGRADATION
first results
Species Abundance Distributions
SAD-Curves for the soil seed bank of the inter-shrub matrix (MISs & BPs)
Helichrysum candolleanum
Microcharis disjuncta
Stipagrostis uniplumis
Dicoma capensis
SOIL SEED BANKS & DEGRADATION
first results
Species Abundance Distributions
SAD-Curves for the soil seed bank of the inter-shrub matrix (MISs & BPs)
Indigastrum argyroides
Trianthema parvifolia
>80%
Aristida adscensionis
Kohautia caespitosa
Schmidtia kalahariensis
SOIL SEED BANKS & DEGRADATION
first results
Species Abundance Distributions
SAD-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
SOIL SEED BANKS & DEGRADATION
first results
Species Abundance Distributions
SAD-Curves for the soil seed bank & vegetation of the inter-shrub matrix
Helcan, Micdis, Lotpla
Indarg, Tripar
Stipagrostis uniplumis
Limarg, Tepdre
Aristida adscensionis
Kohautia caespitosa
Schmidtia kalahariensis
_species composition of the soil seed bank & standing herbage is similar
_proportional composition of the vegetation mirrors the condition of the soil seed bank
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
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
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
Thank you