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
Why savannas?
• Relevant ecosystem: 20%of Earth's cover• Tree-grass coexistence model,
non-equilibrium vs equilibrium systems• Bush encroachment threatens livelihoods
Why savannas?
4
• Relevant ecosystem: 20%of Earth's cover• Tree-grass coexistence model,
non-equilibrium vs equilibrium systems• Bush encroachment threatens livelihoods
Why savannas?
8
• Relevant ecosystem: 20%of Earth's cover• Tree-grass coexistence model,
non-equilibrium vs equilibrium systems• Bush encroachment threatens livelihoods
and nature conservation
Lower Omo Valley: the place and the Mursi people
12
• 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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Palaeoecology Spatial ecology Anthropology
Pollen, charcoal analyses
Vegetation surveys
Interviews to locals
Is the Mursi landscape management leading to the observed bush encroachment?
Research question
16
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)
Assessing current Mursi management: methods
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Mapping every woody element (xy) and biometric variables (dbh, canopy, height, age status)
Assessing current Mursi management: methods
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Mapping every woody element (xy) and biometric variables (dbh, canopy, height, age status)
Assessing current Mursi management: methods
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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)
Assessing current Mursi management: results
27
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
Assessing current Mursi management: results
28
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
Assessing current Mursi management: results
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
0.04
0.06
0.08
0.1
0.12
0.14
0.16
Random pattern
Pair
corr
elat
ion
fun c
ti on
-Ri p
l ey's
K
Assessing current Mursi management: results
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
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
Aggregated pattern over the 3-4 m around each stem
Pair
corr
elat
ion
fun c
ti on
-Ri p
l ey's
K
Assessing current Mursi management: results
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
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
Aggregated pattern on the first 2 meter
Pair
corr
elat
ion
fun c
ti on
-Ri p
l ey's
K
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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