Fire, nitrogen loss, and nitrogen availability in black spruce forests of
Interior Alaska
Michelle C. Mack1, Leslie A. Boby1, Edward A.G. Schuur1, Jill F. Johnstone2, Teresa N. Hollingsworth3 and F.S. Chapin, III3
1 University of Florida2 University of Saskatchewan
3 University of Alaska Fairbanks
- Energy balance
↑[CO2]
↑ T
↑ Fire
NECB?
↓ N pools
↓ productivity
↑ fire severity and area burned
↑[CO2]
↑ T
↑ Fire
NECB+
↓ N pools
↓ productivity
↑ fire severity and area burned
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
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
Severity:M
ois
ture
:
Dry
Wet
Low High
How does increasing burn severity affect soil N loss ?
Observation: Severity and N loss will be positively related.
H1: Wet sites will have similar absolute N loss to dry sites BUT lower relative N loss than dry sites.
H2: Absolute amount of N loss will be positively related to pre-fire N pool size.
H3: Relative N loss will be negatively related to pre-fire N pool.
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
Mean offset between adv.
roots and moss across 30 unburned sites: 3.2 ±
0.3 cm
Adventitious root
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
Dry WetMoisture 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 H1.
Soil organic layer N loss across sites
HighLow
Burn severity
Moisture: F=0.69, P=0.41Severity:F=4.62. P=0.04M x S: F=0.19, P=0.67
*
Dry WetMoisture 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
HighLow
Burn severity
Moisture: F=0.69, P=0.41Severity:F=4.62. P=0.04M 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
*
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 predicted20% of variation.
• Dry, high severity sites: R2=0.61, P<0.02
0.0 0.1 0.2 0.3 0.4 0.50.00
0.05
0.10
0.15
0.20
Dry, highDry, lowWet, highWet, low
• No support for H2 in wet sites, or low severity dry sites
Relative N loss (% of pre-fire N pool)
HighLow
Burn severity
Dry WetMoisture 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 H1.
***
***
0.0 0.1 0.2 0.3 0.4 0.510
20
30
40
50
60
70
80
90
100
110
Rel.N
loss
(%
)
Pre-fire N pool (kg N m-2)
R2=0.64, P<0.001
Relative N loss versus pre-fire N pool
• Reinforces patterns of heterogeneity in N accumulation across the landscape
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.
Conclusions
• Substantial N loss mines N accumulated over centuries, even in low severity burns.
• Severity reinforces landscape patterns of organic soil layer N accumulation.
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
