1. Fire, nitrogen loss, and nitrogen availability in black spruce forests of Interior Alaska Michelle C. Mack 1, Leslie A. Boby 1, Edward A.G. Schuur 1, Jill F. Johnstone 2, Teresa N. Hollingsworth 3 and F.S. Chapin, III 3 1 University of Florida 2 University of Saskatchewan 3 University of Alaska Fairbanks

2. - Energy balance ↑[ CO 2 ] ↑ T ↑ Fire NECB ? ↓ N pools ↓ productivity ↑ fire severity and area burned

3. ↑[ CO 2 ] ↑ T ↑ Fire NECB + ↓ N pools ↓ productivity ↑ fire severity and area burned

4. Question: How does fire severity affect N loss and how does this interact with ecosystem structure? 70% of forest area Natural fire regime Severe, stand replacing Return interval: 70-150 yrs Majority of ecosystem N stored in forest floor Plant productivity is N-limited throughout succession 2004: 6.4 million acres burned Study system: black spruce (Picea mariana) forests of Interior Alaska

5. Study design: 82 extensive sites in 3 large fire complexes that burned in 2004 32 intensive sites chosen to max. variation in drainage and burn severity Measurements in 2005: Burn severity C and N pool loss and structure Vegetation recovery and tree recruitment N cycling and plant productivity

6. Severity: Moisture: Dry Wet Low High

7. How does increasing burn severity affect soil N loss ? Observation: Severity and N loss will be positively related. H 1 : Wet sites will have similar absolute N loss to dry sites BUT lower relative N loss than dry sites. H 2 : Absolute amount of N loss will be positively related to pre-fire N pool size. H 3 : Relative N loss will be negatively related to pre-fire N pool.

8. 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] Calculating soil N loss

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

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

11. DryWet Moisture class 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.11 0.12 N loss (kg N m -2 ) No effect of moisture on absolute N loss supports H 1. Soil organic layer N loss across sites High Low Burn severity Moisture:F=0.69, P=0.41 Severity:F=4.62. P=0.04 M x S:F=0.19, P=0.67 *

12. DryWet Moisture class 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.11 0.12 N loss (kg N m -2 ) Mean =80 ± 4 g ·N m -2 15-100 % of organic layer N pool Soil organic layer N loss across sites High Low Burn severity Moisture:F=0.69, P=0.41 Severity:F=4.62. P=0.04 M x S:F=0.19, P=0.67 N inputs are low Alder fixation max. Lichen/moss norm. (~0.1 g·N m -2 yr -1 ) Mean stand age: 94 ± 5.4 yrs Mining N from past fire cycles *

13. Pre-fire N pool (kg N m -2 ) N loss (kg N m -2 ) N loss versus pre-fire N pool What does predict N loss across sites? Linear comb. of tree density and drainage class predicted 20% of variation. Dry, high severity sites: R 2 =0.61, P<0.02 0.00.10.20.30.4 0.5 0.00 0.05 0.10 0.15 0.20 Dry, high Dry, low Wet, high Wet, low No support for H 2 in wet sites, or low severity dry sites

14. Relative N loss (% of pre-fire N pool) High Low Burn severity DryWet Moisture Class 10 20 30 40 50 60 70 80 90 100 110 Rel.N loss (%) Moisture n.s. Severity *** M x S n.s. No support for H 1. ***

15. 0.00.10.20.30.40.5 10 20 30 40 50 60 70 80 90 100 110 Rel.N loss (%) Pre-fire N pool (kg N m -2 ) R 2 =0.64, P<0.001 Relative N loss versus pre-fire N pool Reinforces patterns of heterogeneity in N accumulation across the landscape

16. Summary Mean N loss from forest floor was 0.08 kg N m -2. High severity sites lost more N than low severity sites. Absolute N loss was positively related to pre-fire N pools, but only in dry, high severity sites. Relative N loss was negatively related to pre-fire N pool size across all sites.

17. Conclusions Substantial N loss mines N accumulated over centuries, even in low severity burns. Severity reinforces landscape patterns of organic soil layer N accumulation.

18. Thanks to: Field crew: Adrienne Frisbee, Laura Gutiarrez, Emily Tissier Lab crew: Grace Crummer, Jennie DeMaro, Ashley Gordon Funding: Interagency Joint Fire Sciences Program, Mellon Foundation