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The patterns and consequences of post-fire successional trajectories in Alaska’s boreal forest

The patterns and consequences of post-fire successional trajectories in Alaska’s boreal forest. The “Generators” - Fire severity - Abiotic and biotic site characteristics. The consequences for -Nitrogen cycling, carbon storage and other key ecosystem processes

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The patterns and consequences of post-fire successional trajectories in Alaska’s boreal forest

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  1. The patterns and consequences of post-fire successional trajectories in Alaska’s boreal forest The “Generators” - Fire severity - Abiotic and biotic site characteristics The consequences for -Nitrogen cycling, carbon storage and other key ecosystem processes -Future landscape and vegetation patterns (including biome shifts)

  2. JFSP Study design • 90 sites established in 2004 burns along Dalton, Taylor, and Steese Highways • 32 intensive study sites • arranged across combinations of high-low site moisture & high-low burn severity • 7 treeline sites • Detailed pre-fire stand data available for 14 sites • Reconstruction of pre-fire conditions at remaining sites

  3. JFSP Study design High Low Dry Wet

  4. What drives natural post-fire seedling recruiment? Differential sensitivity of functional groups to site characteristics and fire regime -Spruce recruitment (as measured by post-fire seedling density) is most influenced by elevation, pre-fire spruce density, and site moisture.Fire severity (CBI) and stand age had weaker effects -50 % of Deciduous recruitment can be explained by fire severity; also important were elevation, latitude, moisture, and distance to nearest unburned deciduous stand. - The ratio of spruce/deciduous recruitment is driven by the relationship between deciduous and fire severity. Important role of fire severity in “tipping” the balance between coniferous and deciduous dominance Johnstone, Hollingsworth, Chapin, and Mack (in prep) Global Change Biology

  5. What drives post-fire vegetation composition? Bernhardt, Hollingsworth, Chapin, and Viereck (in prep) Journal of Vegetation Science

  6. ↑[CO2] ↑ T ? NECB - Energy balance ↓ productivity ↑ Fire ↓ N avail. Frequency, intensity, and area

  7. ↓ productivity ↓ N avail. ↑[CO2] ↑ T + NECB ↑ Fire Frequency, intensity, and area

  8. FireN availability • N pool size • N turnover time • Environment for decomposition • Plant species composition • Pool size • TT • Environment • Uptake and use

  9. Calculating soil N loss Net loss of N from forest floor/organic soil = Pre-fire forest floor N pool - Remaining N pool [+ Ash from plants and upper layers] [- Leaching, erosion, gaseous loss]

  10. Adventitious root Mean offset between adv. roots and moss across 30 unburned sites: 3.2 ± 0.3 cm

  11. N pool in missing layers = root collar depth x empirical relationships derived from unburned stands Residual organic soil depth, bulk density and [N] Adventitious root collar to burned soil = depth of organic matter combusted

  12. Soil organic layer N loss across sites 0.12 • Mean =80 ± 4 g·N m-2 Burn severity 0.11 Low * High • 1-94 % of pre-fire organic layer N pool 0.10 0.09 0.08 • N inputs are low • Alder fixation max. • Lichen/moss norm. (<0.1g·N m-2 yr-1) N loss (kg N m-2) 0.07 0.06 0.05 0.04 • Mean stand age: 94 ± 5.4yrs 0.03 Dry Wet Moisture class • Mining N? Alder inputs? Occult N? Moisture: F=0.69, P=0.41 Severity: F=4.62. P=0.04 M x S: F=0.19, P=0.67

  13. Comparing the boreal forest to the Arctic www.gina.alaska.edu

  14. Anaktuvuk River Fire 2007 An opportunity for synthesis between a fire-naïve versus a fire-experienced biome

  15. Research Questions • How much carbon and nitrogen was lost during the Anaktuvuk River Fire? • Do our known relationships between seedling regeneration, vegetation composition, and site characteristics hold true for tundra fires?

  16. Estimating pre-fire soil organic matter pools R2=0.94, P=<0.01 Organic layer depth (cm) Meristem to mineral soil depth (cm)

  17. Pre-fire pools and fire-driven losses of nitrogen from tundra and taiga Pools MAT Black spruce Pre-fire O layer Mean: 280 200 (g N m-2) Range: 206 to 578 50 to 429 N: 20 90 O layer N loss 33 103 (g N m-2) 7to 209 30 to 180 % O layer N loss 10.4 50 2 to 36 1 to 94

  18. Wet, low Wet, high Dry, low Dry, high 0.0 100 200 300 400 500 N loss versus pre-fire N pool Black spruce taiga Moist Acidic Tundra N loss (kg N m-2) Pre-fire N pool (g N m-2) All sites R2=0.86, P<0.001 High-Dry R2=0.61, P<0.02 No relationship for other categories

  19. Plant methods • 9 transects across the Anaktuvuk River fire sampled • 10 points along each transect sampled and combined for a) mineral soil and b) organic soils • Germinated alder and spruce seedlings • Autoclaved Tanana River silt as growth substrate • Innoculated with organic or mineral soil from 9 sites (18 treatments) • Minimal fertilizer (want them to survive but not thrive)

  20. Preliminary results Alder Spruce a a b Spruce Alder

  21. Conclusions • In taiga, sites with more accumulated N lost a smaller % of total N, while sites with more N lost a larger % • Fire driven N loss reinforces landscape patterns of N accumulation • In arctic tundra, sites with more accumulated N lost more N • Fire smoothes landscape heterogeneity in N accumulation • In arctic tundra, effects of mycorrhizal innoculum on plant height growth is related to soil horizon, not fire severity

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