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Soil seed banks in relation to degradation of arid grazing lands in southern Namibia Niels Dreber Biocentre Klein Flottbek & Botanical Garden, University of Hamburg SER Summer School Restoration Ecology 2009 - Münster, Germany

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Southern Africa. SER Summer School Restoration Ecology 2009 - Münster, Germany. Soil seed banks in relation to degradation of arid grazing l ands in southern Namibia. Niels Dreber Biocentre Klein Flottbek & Botanical Garden, University of Hamburg. Southern Africa. INTRODUCTION. - PowerPoint PPT Presentation

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Page 1: Southern Africa

Soil seed banksin relation to degradation of aridgrazing lands in southern Namibia

Niels DreberBiocentre Klein Flottbek & Botanical Garden, University of Hamburg

SER Summer School Restoration Ecology 2009 - Münster, Germany

Page 2: Southern Africa

INTRODUCTION the study area

NAMA-KAROO

_670 240 km2

(23% of the southern African subregion)

Source: Cowling, R.M., Richardson, D.M., Pierce, S.M., 1997. Vegetation of Southern Africa, Cambridge University Press.

x

Page 3: Southern Africa

INTRODUCTION

NAMA-KAROO

_670 240 km2

(23% of the southern African subregion)

_open grassy dwarf shrubland

the study area

Page 4: Southern Africa

INTRODUCTION

x

Source: MAWRD, mean annual rainfall

study sites

NAMA-KAROO

_670 240 km2

(23% of the southern African subregion)

_open grassy dwarf shrubland

_rainfall 100-200 mm coefficient of variation: 80%

the study area

Page 5: Southern Africa

INTRODUCTION

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

x

Source: MAWRD, mean above ground biomass estimate

the study area

study sites

Page 6: Southern Africa

INTRODUCTION the study area

The main driving factors:(i) a lack in regimentations(ii) high population & stock densities(iii) the highly variable climate

Land degradationis a widespread problem in the region,especially in communal areas!

Page 7: Southern Africa

INTRODUCTION

Gellap-Ost:

governmental

rotational grazing

karakul sheep

low grazing pressure

stable land tenure 70 years +

Nabaos:

communal

open access pasture

goats

high grazing pressure 30 years +

no seasonal restric- tions or stocking limitations

the study area

fenceline contrast – ‘a natural experiment‘

Page 8: Southern Africa

INTRODUCTION

Above ground degradation on location

increase in bare soilreduction in plant total coverloss of valuable plant speciesincreased topsoil crusting & erosionchanges in diversity & abundance of rodents & ground beetles

degradation facts

Page 9: Southern Africa

INTRODUCTION

Such profound regime shifts in savannah rangelands might be attributable to

to a loss of ecosystem resilience due to sustained grazing pressure

degradation facts

Page 10: Southern Africa

INTRODUCTION

Is the degraded state reversible?

40 years x

degradation facts

Page 11: Southern Africa

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?

Page 12: Southern Africa

SOIL SEED BANKS & DEGRADATION methods

THE SAMPLING DESIGN

Stratification of the area:

selection of comparabel habitats regarding

bedrocksoil typegeomorphologywoody character species

homogeneous vegetation regarding species composition & structure

Page 13: Southern Africa

SOIL SEED BANKS & DEGRADATION methods

THE SAMPLING DESIGN

Under shrub canopy seed bank (macro safe sites)

after seed shet, before any germination event

Page 14: Southern Africa

SOIL SEED BANKS & DEGRADATION methods

THE SAMPLING DESIGN

Seed bank of the inter-shrub matrix

after seed shet, before any germination event

Page 15: Southern Africa

SOIL SEED BANKS & DEGRADATION methods

THE SAMPLING DESIGN

Inter-shrub matrix: vegetated patches (micro safe sites) & bare patches

xx

x

xx

xxx

x x

xx

x

x xvegetated patch (n=45)

bare patch (n=45)

xsampling point

Page 16: Southern Africa

SOIL SEED BANKS & DEGRADATION methods

DIRECT SEEDLING EMERGENCE METHOD

Page 17: Southern Africa

SOIL SEED BANKS & DEGRADATION first results

4911880Overall

4564172008

3652072007

speciesgerminantsYear

Total number of

2007: W1-62%, W3-85%2008: W1-62%, W3-81%

Page 18: Southern Africa

SOIL SEED BANKS & DEGRADATION first results

Total species richness of the soil seed bank

Page 19: Southern Africa

SOIL SEED BANKS & DEGRADATION first results

Total seed density of the soil seed bank

Page 20: Southern Africa

SOIL SEED BANKS & DEGRADATION first results

Mean seed densities / m2 of the germinable soil seed bank

REF: 1206.00 ±1050.13

DEG: 3383.83 ±3059.15

REF: 626.60 ±717.25

DEG: 1519.76 ±1332.12

REF: 2083.89 ±1708.28

DEG: 4305.22 ±2199.34

p < 0.05

Page 21: Southern Africa

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

Page 22: Southern Africa

SOIL SEED BANKS & DEGRADATION first results

Species Abundance DistributionsSAD-Curves for the soil seed bank of the inter-shrub matrix (MISs & BPs)

Stipagrostis uniplumis

Helichrysum candolleanum

Microcharis disjuncta

Dicoma capensis

Page 23: Southern Africa

SOIL SEED BANKS & DEGRADATION first results

Species Abundance DistributionsSAD-Curves for the soil seed bank of the inter-shrub matrix (MISs & BPs)

Indigastrum argyroidesTrianthema parvifolia>80%

Aristida adscensionis

Schmidtia kalahariensis

Kohautia caespitosa

Page 24: Southern Africa

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

Page 25: Southern Africa

SOIL SEED BANKS & DEGRADATION first results

Species Abundance DistributionsSAD-Curves for the soil seed bank & vegetation of the inter-shrub matrix

Helcan, Micdis, Lotpla

Stipagrostis uniplumis

Limarg, Tepdre

Indarg, Tripar

Aristida adscensionisKohautia 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

Page 26: Southern Africa

SUMMARY

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

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

Page 27: Southern Africa

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

Page 28: Southern Africa

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

Page 29: Southern Africa

Thank you