savanna encroachment in sw ethiopia: spatial analyses at decadal time scales

Bush encroachment and long-term savanna dynamics: the case of the Mago National Park and the Mursi community

Graciela Gil-Romera, Yolanda Pueyo, Henry F. Lamb, Miguel Sevilla-Callejo, David Turton@gilromera | [email protected] | tiny.cc/gilromera

mailto:[email protected]

Why savannas?

• Relevant ecosystem: 20%of Earth's cover• Tree-grass coexistence model,

non-equilibrium vs equilibrium systems• Bush encroachment threatens livelihoods

Why savannas?

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Why savannas?

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Why savannas?

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Why savannas?

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Why savannas?

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Why savannas?

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and nature conservation

Why savannas?

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Lower Omo Valley: the place and the Mursi people

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• Conservation narrative: grazing animals remove the grass layer facilitating tree expansion

• African Parks uses this argument to suggest restrictions on the use pastoralist can make of the savanna

Research question

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Is the Mursi landscape management leading to the

observed bush encroachment?

Research question

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Palaeoecology Spatial ecology Anthropology

Pollen, charcoal analyses

Vegetation surveys

Interviews to locals

Is the Mursi landscape management leading to the observed bush encroachment?

Long-term perspective

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Hysteresis loop,Self-thinning process?

Encroached savanna

Open savanna

Research question

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Is the Mursi landscape management leading to the observed bush encroachment?

1. Does woody cover change with use?

2. Is there a tree/bush spatial pattern change based on management?

→ Aggregation=Closed landscape → Repulsion/competition=Open landscape

Research question

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Open ended interviews to knowledgeable people on the use they make of their land, frequency of burning, purpose and grazing practices

Assessing current Mursi management: methods

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• Used/Burnt every year (T1)• Managed every 5-7 years (T2)• Not burnt/used over the last 20 years (T3)


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50x50 plots (17) and 3x50 m transects (5)


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Mapping every woody element (xy) and biometric variables (dbh, canopy, height, age status)


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Mapping every woody element (xy) and biometric variables (dbh, canopy, height, age status)


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Q1.: Does woody cover change with use?Estimating cover differences by measuring total basal areaQ2.:Is there a tree/bush spatial pattern change based on management? Univariate point-pattern analyses (Ripley's K function | Heterogeneus poisson process as null model)


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Programita by T. Wiegand

Assessing current Mursi management: results

T1: used every year


T2: used every 5-7 years


T3: not used over the last 20 years


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Cover estimation for each plot within each management treatment

Management Plot

III P11 33 0.943III P12 29 0.346III P15 25 0.200III P16 38 0.173III P17 47 0.028III P18 45 0.178III P22 61 0.162Mean 39.71 4.850SD 12.338 0.860

No. woody elements

Cover (m2/ha)

Management Plot

I P01 32 3.181I P04 14 0.090I P05 24 0.134I P06 22 0.246I P08 254 0.238I P20 201 0.016I P21 33 0.006Mean 82.86 0.558SD 100.191 1.160

No. woody elements

Cover (m2/ha)

Management Plot

II P02 196 0.943II P03 134 0.346II P07 78 0.200II P09 65 0.173II P10 49 0.028II P13 286 0.178II P14 124 0.162II P19 169.00 4.850Mean 137.625 0.860SD 78.640 1.636

No. woody elements

Cover (m2/ha)

Woody cover is actually larger in areas that haven't been used in the last 20 years


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Cover estimation for each plot within each management treatment

Management Plot

III P11 33 0.943III P12 29 0.346III P15 25 0.200III P16 38 0.173III P17 47 0.028III P18 45 0.178III P22 61 0.162Mean 39.71 4.850SD 12.338 0.860

No. woody elements

Cover (m2/ha)

Management Plot

I P01 32 3.181I P04 14 0.090I P05 24 0.134I P06 22 0.246I P08 254 0.238I P20 201 0.016I P21 33 0.006Mean 82.86 0.558SD 100.191 1.160

No. woody elements

Cover (m2/ha)

Management Plot

II P02 196 0.943II P03 134 0.346II P07 78 0.200II P09 65 0.173II P10 49 0.028II P13 286 0.178II P14 124 0.162II P19 169.00 4.850Mean 137.625 0.860SD 78.640 1.636

No. woody elements

Cover (m2/ha)

Number of stems is larger in T2 intermediate state on →the savanna equilibrium


T1: used every year

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 260

0.02

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Random pattern

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T2: used every 5-7 years

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 260

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Aggregated pattern over the 3-4 m around each stem

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T3: not used in the last 20 years

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 260

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Aggregated pattern on the first 2 meter

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Assessing current Mursi management: discussionObserved pattern: Trees show an aggregation pattern some years after use- Encroachment l inked to a temporal scale?

Assessing current Mursi management: discussionObserved pattern: Trees show an aggregation pattern some years after use- Encroachment l inked to a temporal scale?

Potential process 1- Abandonment leads to tree spreading as frequent fires are absent (high number of trees,~cover) 2- Reached a certain level of encroachment, self-thinning process or large natural fires open the landscape as reported by locals. (~ number of trees, high cover only the big ones survive) →

Assessing current Mursi management: discussionHysteresis might be a process occurring at multidecadal, centennial and millenial time-scales. Encroached savanna

Open savanna

Concluding remarks

Q1: Does woody cover change with use?

- Yes, it does, but not linearly with time after use as there seems to be a threshold response

Q2: Is there a tree/bush spatial pattern change based on management?- Yes and the change seems to be driven by initial aggregation after abandonment followed by inter/intra-specific interactions or natural landscape opening

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Concluding remarks

Still to do...- Bivariate statistics: inter/intra-specific interactions- Analyses of finite size elements

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Concluding remarks

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Acknowledging

Mursi community

Melaku Wondafrash (Botanical Garden Addis Abbeba)

African Studies Centre at Uox and IGES at Aberystywyth University

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Thanks for listening!

savanna encroachment in sw ethiopia: spatial analyses at decadal time scales

Environment