Spatial ecological processes allow tree-grass coexistence in savanna despite repeated fires.

1. Spatial ecological processes allow
tree-grass coexistence in savanna
despite repeated fires
Francesco Accatino, Kerstin Wiegand,
Carlo De Michele, David Ward

2. 0 20001000 1500500
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40
60
80
100
A global view:Treecover[%]
Mean annual rainfall [mm/yr]
Data from Hirota et al. (2011) Science

3. 0 20001000 1500500
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40
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Above ~800 mm/yr tree cover is
bimodally distributed
Relative frequency
Mean annual rainfall [mm/yr]
A global view:
Data from Hirota et al. (2011) Science
Treecover[%]

4. 0 20001000 1500500
20
40
60
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Mean annual rainfall [mm/yr]
Many of these points can be classified as moist
savannas or grasslands: potentially forests but tree
cover is kept low by recurrent fires.
Relative frequency
A global view:
Data from Hirota et al. (2011) Science
Treecover[%]

5. A look in detail:
Let’s now look at moist savannas more in detail
considering the dynamical interactions between
their components.

6. Moist savannas: seasonality
Wet season
In the wet season abundant rainfall allows the
accumulation of grass

7. Moist savannas
Wet season Dry season
In the dry season grass dries out and becomes fuel for
fires. Fires usually kill juvenile trees.

8. Research question
Is fire alone able to stop the closure of the canopy
in moist savannas?
We explore with a simulation model some
ecological mechanisms related to:
• Space
• Tree life history traits

9. GRASSFIRE
TREES
Concepts
We model the dynamic interactions between these
elements:
In particular we include the role of space and tree life
history traits

10. GRASSFIRE
TREES
Shade effect of
groups of trees
Concepts

11. GRASSFIRE
TREES
Fire spread
follows the
distribution of
fuel (grass)
Concepts

12. GRASSFIRE
TREES
Trees grow or die differently according to:
- Whether they are reached or not by fire
- Their age and life history traits
Seedlings Juveniles Adults
Concepts

13. INITIALIZATIO
N
TREE
GROWTH
GRASS
GROWTH
FIRE SPREAD
TREE
MORTALITY
GROWTHseasonDRYseasonModel description

14. INITIALIZATIO
N
TREE
GROWTH
GRASS
GROWTH
FIRE SPREAD
TREE
MORTALITY
GROWTHseasonDRYseason
A spatial domain (discretized in
cells) is initialized with a few tree
seedlings
Model description
200 m
200m

15. INITIALIZATIO
N
TREE
GROWTH
GRASS
GROWTH
FIRE SPREAD
TREE
MORTALITY
GROWTHseasonDRYseason
Trees have a
probability to go to
an upper height
class each growth
season
Spatial competition
between tree and tree
is accounted for
Model description
New tree seedlings
are introduced in the
spatial domain

16. INITIALIZATIO
N
TREE
GROWTH
GRASS
GROWTH
FIRE SPREAD
TREE
MORTALITY
GROWTHseasonDRYseason
Grass is assigned to the cells
according to the proximity to
trees
Model description

17. INITIALIZATIO
N
TREE
GROWTH
GRASS
GROWTH
FIRE SPREAD
TREE
MORTALITY
GROWTHseasonDRYseason
Fire occurs almost every year
An algorithm simulates fire spread
following grass distribution
Model description

18. INITIALIZATIO
N
TREE
GROWTH
GRASS
GROWTH
FIRE SPREAD
TREE
MORTALITY
GROWTHseasonDRYseason
Trees have a
probability to go
to lower height
class or die,
depending if
they are burnt
or not
Model description

19. Scenarios & Results
Three scenarios with three types of trees,
distinguished according to:
• Speed of growth
• Vulnerability to fire
• Spatial competition
These types of trees are called:
• Resprouters
• Avoiders
• Resisters

20. Resprouters
If burned they
can go into the
class gulliver
and can
resprout
Gullivers

21. Resprouters
Year 3
Burnt
Unburnt
Gullivers
Trees

22. Resprouters
Year 3 Year 22
Burnt
Unburnt
Gullivers
Trees

23. Resprouters
Year 3 Year 22
Burnt
Unburnt
Gullivers
Trees
Some clusters are forming

24. Resprouters
Year 3 Year 22 Year 29
Burnt
Unburnt
Gullivers
Trees

25. Resprouters
Year 3 Year 22 Year 29
Burnt
Unburnt
Gullivers
Trees
Clusters enlarge and create areas
where fire cannot arrive
Gullivers are distributed in the parts of
the domain burnt by fire

26. Resprouters
Year 3 Year 22 Year 29 Year 31
Burnt
Unburnt
Gullivers
Trees

27. Avoiders
They are very
vulnerable to fire but
they grow very fast.

28. Results: Netlogo screenshots avoiders
The situation remains like this until
there is a year without fire
Year with
no fire

29. Results: Netlogo screenshots avoiders
Year 1
+ 2 Years
Year with
no fire

30. Results: Netlogo screenshots avoiders
Year 1
+ 2 Years + 5 Years
Year with
no fire

31. Results: Netlogo screenshots avoiders
Year 1
+ 2 Years + 5 Years
Year with
no fire
Tree clusters
Some areas not occupied by trees
are protected from fires

32. Results: Netlogo screenshots avoiders
Year 1
+ 2 Years + 5 Years
Year with
no fire + 10 Years
At the end all the spatial domain is
occupied

33. Resisters
They grow very
slowly but they are
very resistant to fire
even in the young
ages
Strong spatial
competition

34. Results: Netlogo screenshots avoiders
Year 3 Year 7 Year 11 Year 25
Population growth is very slow, but no
clustering patterns are observed

35. 250
500
750
Trees
perhectare
Results summary
20 40 60 20 40 6020 40 60
Resprouters Avoiders Resisters
Time [yr] Time [yr]Time [yr]

36. 250
500
750
Clustered
Regular
Random
Trees
perhectare
20 40 60
Results summary
20 40 6020 40 60
Time [yr] Time [yr]Time [yr]
Resprouters Avoiders Resisters

37. Take-home messages
Trees can invade the grass stratum despite
repeated fires by:
• Collective strategy: clustering
• Individual strategy: resisting

38. Take-home messages
Once trees invade, fires disappear from the
ecosystem and the process is not reversible (with
the elements considered in the model)

39. Take-home messages
Tree-grass co-existence can be:
A transient state of the ecosystem
Due to strong spatial competition between trees
Due to other mechanisms not included in the model
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Mean annual rainfall [mm/yr]
Woodycover[%]

40. Thank you!