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Fire and Forest Dynamics in Northern Boreal Forests

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This webinar was presented by Jill Johnston on Oct 28, 2010. For more information about this webinar, visit the Alaska Fire Science Consortium website at http://akfireconsortium.uaf.edu

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Fire and Forest Dynamics in Northern Boreal Forests

  1. 1. Fire and Forest Dynamics in Northern Boreal Forests Jill Johnstone Biology, University of Saskatchewan
  2. 2. Northern boreal forest • Conifer dominated • Cool soils, slow growth & decomposition • Resistant to change?
  3. 3. Fire and Global Change Chapin et al. 2005 Stocks et al. 1998
  4. 4. Can we expect changes in forest composition? What are those likely to be?
  5. 5. Resilience and Response Dynamics dynamic equilibrium directional change
  6. 6. Resilience & Ecosystem Feedbacks Dominant species Disturbance Functional traits Interactions Competition, herbivory Recruitment
  7. 7. Alternate successional cycles A. Black spruce domain B. Broadleaf forest domain Broadleaf dominant Black spruce dominant High moisture High moss Cool soils FIRE Local seed & Resprouting Slow growth Poor quality seedbeds (organic soil) Slow nutrient turnover Low competition Low moisture Low moss Warm soils Rapid growth Rapid nutrient turnover High competition Johnstone et al. 2010, Can. J. Forest Research FIRE Resprouting & Seed dispersal High quality seedbeds (mineral soil)
  8. 8. Alternate successional cycles A. Black spruce domain B. Broadleaf forest domain Broadleaf dominant Black spruce dominant High moisture High moss Cool soils FIRE Local seed & Resprouting Slow growth Poor quality seedbeds (organic soil) Slow nutrient turnover Low competition Low moisture Low moss Warm soils Rapid growth Rapid nutrient turnover High competition Johnstone et al. 2010, Can. J. Forest Research FIRE Resprouting & Seed dispersal High quality seedbeds (mineral soil)
  9. 9. How do fire characteristics shape patterns of forest resilience? • Why study fire? – Ubiquitous in western boreal region – Sensitive to climate – Post-fire recovery determines future forest composition
  10. 10. Fire and successional trajectories in black spruce forests
  11. 11. Fire severity affects seedbed quality Burning of organic soils influences patterns of post-fire recruitment
  12. 12. Patch effects of fire severity Low severity (organic) – Poor seedbeds – Recruitment requires high seed inputs – Favors serotinous conifers High severity (mineral) – Higher quality seedbeds – Creates opportunities for deciduous establishment
  13. 13. How does this influence forest dynamics across heterogeneous landscapes?
  14. 14. Fire severity and post-fire recovery • Alaska 2004 fires • 90 black spruce sites • Initial stand recovery
  15. 15. • Environmental conditions Field Data – Potential site moisture – Elevation – Potential insolation • Pre-fire stand structure – Stem density – Stem basal area • Fire severity – Composite Burn Index (CBI) – Residual organic layer depth • Post-fire recruitment – Tree seedling density – 4 years post-fire
  16. 16. Spruce seedling density Boosted regression tree, prediction error=0.54 Johnstone et al. 2010, Global Change Biology
  17. 17. Deciduous seedling density Boosted regression tree, prediction error=0.44 Johnstone et al. 2010, Global Change Biology
  18. 18. Relative spruce dominance: Recovery of spruce trajectory Boosted regression tree, prediction error=0.42 Johnstone et al. 2010, Global Change Biology
  19. 19. Controls on spruce forest resilience • Severe fires reduce the competitive advantage of spruce and favor deciduous species • Severe fires alter soil microclimate • Site moisture – Warm, dry soils favor aspen – Severe fires are also more likely • Young stands vulnerable to change
  20. 20. Studies of fire frequency using overlapping fires historic fire overlap zones: rapid disturbance return recent fire image courtesy of David Milne, Yukon Gov.
  21. 21. Repeat fires alter tree regeneration 40000 stem density (#/ha) 35000 *** Burned at >80 yr. Burned at <30 yr. 30000 25000 20000 *** 15000 *** 10000 ns 5000 0 total Picea Pinus Populus Johnstone & Chapin 2006, Ecosystems
  22. 22. Seed rain Brown & Johnstone, unpublished
  23. 23. Seedling establishment Brown & Johnstone, unpublished
  24. 24. How old does a stand need to be before there is sufficient cone production to support regeneration?
  25. 25. Cone Production (log scale) Cones/tree (Log10 scale) n=14, p<0.001, r=0.723 Number of Cones Present on Tree n=170, p<0.001, r=0.360 2.5 2 1.5 1 0.5 0 0 20 40 60 80 Tree Age Viglas & Johnstone, unpublished
  26. 26. Fire interval effects • Repeat fires interrupt conifer regeneration cycles – Reduced cone production • Confers a regeneration advantage to winddispersed seeds • Net effect is to shift trajectories to deciduous dominance
  27. 27. Shifts in resilience cycles Black spruce dominant Deciduous dominant Fire High moisture Cool soils Slow turnover Low moisture Warm soils Rapid turnover Organic seedbeds Slow growth Low competition Mineral soil seedbeds Resprouting & seed dispersal Rapid growth High competition
  28. 28. Shifts in resilience cycles Black spruce dominant High moisture Cool soils Slow turnover long fire interval Deciduous dominant Fire severe or short-interval fire Low moisture Warm soils Rapid turnover Organic seedbeds Slow growth Low competition Mineral soil seedbeds Resprouting & seed dispersal Rapid growth High competition
  29. 29. Why is this important? • Changes in forest cover affect: – Carbon storage – Energy and water transfer – Wildlife and subsistence resources – Feedbacks to future fire behavior
  30. 30. Fire severity and succession: Impacts on future fire behavior • High fire severity transforms black spruce to deciduous forest • Deciduous forest has lower flammability Can fire-initiated changes create a negative feedback to climate-driven increases in fire activity?
  31. 31. ALFRESCO simulation experiment • Spatial simulation model for boreal landscapes • Succession influenced by fire severity • 3 Severity Scenarios: – Low (LSS): All fires burn with low severity (spruce trajectory) – High (HSS): Maximum extent of high severity (decid. trajectory) – Mix: Intermediate scenario • High and moderate scenarios of climate warming Area = ~ 2500 KEY: Green & Yellow = Low Sev. Red = High Sev. in HSS Black = High Sev. in Mix + HSS Area = ~1000
  32. 32. Cumulative area burned High warming Low warming Johnstone, Rupp, et al., in review
  33. 33. Disturbance & climate interact to alter black spruce resilience dynamic equilibrium directional change tundra black spruce deciduous
  34. 34. Future Research • Mechanistic understanding of plant-soilmicrobial feedbacks • Quantifying thresholds and tipping points • Landscape prediction of vulnerability to change
  35. 35. Conclusions • Fire is both catalyst and driver of change • Critical post-fire reorganization phase • Both frequency and severity shape future succession • Landscape context => vulnerability to change • Understanding the drivers of resilience is key to predicting future change
  36. 36. Acknowledgements Co-authors: Carissa Brown Terry Chapin Teresa Hollingsworth Michelle Mack Mark Olsen Scott Rupp Ted Schuur David Verbyla Jayme Viglas

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