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Fire effects on regeneration cycles in northern boreal forests

Fire effects on regeneration cycles in northern boreal forests. Dr. Jill Johnstone Biology, University of Saskatchewan. Northern boreal forest. Conifer dominated Cool soils, slow growth & decomposition Widespread fire disturbance. Stocks et al. 1998. Fire and Global Change.

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Fire effects on regeneration cycles in northern boreal forests

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  1. Fire effects on regeneration cycles in northern boreal forests Dr. Jill Johnstone Biology, University of Saskatchewan

  2. Northern boreal forest • Conifer dominated • Cool soils, slow growth & decomposition • Widespread fire disturbance

  3. Stocks et al. 1998 Fire and Global Change Chapin et al. 2005

  4. How are changing fire regimes likely to alter forest dynamics?

  5. Local seed rain Cool soils, low nutrients Conservative growth Regeneration Feedbacks Dominant conifers Interactions Recruitment

  6. overlap zones: rapid disturbance return historic fire recent fire Studies of fire frequency using overlapping fires image courtesy of David Milne, Yukon Gov.

  7. Repeat fires alter tree regeneration *** *** *** ns Johnstone & Chapin (2006) Ecosystems 9: 268-277.

  8. Effects of fire frequency • Short fire intervals can short-circuit conifer regeneration pathways • Plant types that regenerate by resprouting are strongly favored • Net effect is to shift trajectories to increased deciduous dominance

  9. Fire severity and forest regeneration 9

  10. Fire severity affects seedbed quality Burning of organic soils influences patterns of post-fire recruitment 10

  11. Seeds & Seedlings of: • Pinus contorta (lodgepole pine) • Picea glauca (white spruce) • Picea mariana (black spruce) • Populus tremuloides (aspen) Patch-scale Experiments Burn severity treatment singed (organic) burned (severe) Johnstone & Chapin (2006) Ecosystems 9: 14-31.

  12. Recruitment declines with increased organic depth Standardized germination rate Residual organic layer depth (cm)

  13. Meta-analysis shows species differ in sensitivity to severity Johnstone & Chapin (2006) Ecosystems 9: 14-31.

  14. Patch effects of fire severity Low severity (organic) • Poor seedbeds • Recruitment requires high seed inputs • Strongly favors conifers High severity (mineral) • Higher quality seedbeds • Creates opportunity of deciduous establishment

  15. Scaling up: From small plots to a single wildfire 15

  16. August June 14 Natural variations in fire severity:10,000 ha burn in interior Alaska Johnstone & Kasischke (2005) Can. J. For. Res. 35: 2151-2163.

  17. Spruce • Weak negative response • Contrary to experiments • Aspen • Strong positive response • Consistent with experiments Tree seedling responses 17

  18. Community response to burn severity 4 Woody shrubs & graminoids 3 2 1 Aspen & mosses Vegetation community index (PCA1) 0 -1 -2 -3 -2 -1 0 1 2 3 Burn severity (PCA1) Community responses 18

  19. Scaling fire severity effects to large landscapes 19

  20. Interior Alaska • Widespread 2004 fires • 3 fire complexes • 90 black spruce sites

  21. Field measurements • Environmental conditions • Site moisture • Elevation • Potential insolation • Pre-fire stand structure • Stem density • Stem basal area • % Standing after fire • Fire severity • Canopy consumption • Composite Burn Index (CBI) • Post-fire organic layer depth • Post-fire recruitment • Tree seedling density • 4 years post-fire 21

  22. Spruce dominance of seedling regeneration Boosted regression tree model (n=90, prediction error=0.42) Johnstone et al., in prep.

  23. Spruce resilience reduced when: • Fires consume the organic layer • Reduces BS reproductive advantage • Fires burn after a short interval • Insufficient seed availability • Sites are well drained • Severe fires more likely • Warm, dry soils favor aspen growth • Permafrost thaw -> drainage changes • Loss of resilience can tip regeneration balance away from black spruce

  24. High local seed rain; Tolerance of organic soils Species dominant Cool soil, Low nutrients Interactions Recruitment Conservative growth Regeneration Feedbacks & Changing Fire Regime X X

  25. Area = ~ 2500 Area = ~1000 KEY: Green & Yellow = Low Sev. Red = High Sev. in HSS Black = High Sev. in Mix + HSS Potential feedbacks to fire behavior: ALFRESCO simulation model • Succession modeled as a function of fire severity • Fire severity increases with fire size • 3 Severity Scenarios: • Low (LSS): All fires burn with low severity (spruce replacement) • High (HSS): Maximum plausible extent of high severity (mixedwoods) • Mix: Intermediate proportion of high and low severity

  26. Cumulative area burned under different climate and severity scenarios High warming Low warming

  27. directional change tundra black spruce deciduous Disturbance & climate interact to alter black spruce resilience dynamic equilibrium

  28. Conclusions • Fire is both catalyst and driver of change • Critical & sensitive post-fire window • Both frequency and severity have critical effects • Landscape context => vulnerability to change • Understanding the drivers of resilience is key to predicting future change 28

  29. Thank You! Collaborators: Terry Chapin Teresa Hollingsworth Scott Rupp Eric Kasischke Michelle Mack Bonanza Creek 29

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