Accatino european conferenceoftropicalecology2

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

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Accatino european conferenceoftropicalecology2

  1. 1. Spatial ecological processes allow tree-grass coexistence in savanna despite repeated fires Francesco Accatino, Kerstin Wiegand, Carlo De Michele, David Ward
  2. 2. 0 20001000 1500500 20 40 60 80 100 A global view:Treecover[%] Mean annual rainfall [mm/yr] Data from Hirota et al. (2011) Science
  3. 3. 0 20001000 1500500 20 40 60 80 100 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. 4. 0 20001000 1500500 20 40 60 80 100 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. 5. A look in detail: Let’s now look at moist savannas more in detail considering the dynamical interactions between their components.
  6. 6. Moist savannas: seasonality Wet season In the wet season abundant rainfall allows the accumulation of grass
  7. 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. 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. 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. 10. GRASSFIRE TREES Shade effect of groups of trees Concepts
  11. 11. GRASSFIRE TREES Fire spread follows the distribution of fuel (grass) Concepts
  12. 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. 13. INITIALIZATIO N TREE GROWTH GRASS GROWTH FIRE SPREAD TREE MORTALITY GROWTHseasonDRYseasonModel description
  14. 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. 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. 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. 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. 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. 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. 20. Resprouters If burned they can go into the class gulliver and can resprout Gullivers
  21. 21. Resprouters Year 3 Burnt Unburnt Gullivers Trees
  22. 22. Resprouters Year 3 Year 22 Burnt Unburnt Gullivers Trees
  23. 23. Resprouters Year 3 Year 22 Burnt Unburnt Gullivers Trees Some clusters are forming
  24. 24. Resprouters Year 3 Year 22 Year 29 Burnt Unburnt Gullivers Trees
  25. 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. 26. Resprouters Year 3 Year 22 Year 29 Year 31 Burnt Unburnt Gullivers Trees
  27. 27. Avoiders They are very vulnerable to fire but they grow very fast.
  28. 28. Results: Netlogo screenshots avoiders The situation remains like this until there is a year without fire Year with no fire
  29. 29. Results: Netlogo screenshots avoiders Year 1 + 2 Years Year with no fire
  30. 30. Results: Netlogo screenshots avoiders Year 1 + 2 Years + 5 Years Year with no fire
  31. 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. 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. 33. Resisters They grow very slowly but they are very resistant to fire even in the young ages Strong spatial competition
  34. 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. 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. 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. 37. Take-home messages Trees can invade the grass stratum despite repeated fires by: • Collective strategy: clustering • Individual strategy: resisting
  38. 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. 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 0 20001000 1500500 20 40 60 80 100 Mean annual rainfall [mm/yr] Woodycover[%]
  40. 40. Thank you!

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