fire, nitrogen loss, and nitrogen availability in black spruce forests of interior alaska

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