Effects of Forest Thinning on CO2 Efflux

Peter Erb, Trisha Thoms, Jamie Shinn

Biogeochemistry 2003: Block 1

Our project…

•Measured soil CO2 efflux at the Catamount Institute in both control and thinned plots, using the Li-Cor 6200.

Aim of Study

•To understand the effects of forest thinning on CO2 efflux at the Catamount Institute.

What is CO2 efflux?

• Rate of CO2 released from soil. (gC/m2s)

• Caused by microbial, soil animal and root respiration.

• Important part of global C cycle.

Why study CO2 efflux?

• Soil CO2 efflux produces 10 times more atmospheric CO2 than fossil fuels.

• Concern that respiration and decomposition rates will increase exponentially, accompanied by a linear growth rate of plant biomass.

Soil Respiration and NPP v. Temperature

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0 2 4 6 8 10 12

Temperature

NPP

soil respiration

Positive Feedback Cycle

Temperature

DecompositionCO2 efflux

Global Warming

Hypothesis

•Control plots will have a higher CO2

efflux due to more litterfall and greater root density.

Methods Study site: Catamount upper montane mixed conifer. Plots 1-3

divided into thinned & control.

Technique: Li-Cor 6200 (infrared absorption)

Field process

Statistical analysis: T-test

Graphical analysis

T-test: Total plots showed significance, within 90% confidence.

P = .08

Hotspots: Plot one is driving total.

Carbon Efflux: Thinned vs. Control

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Plot 1 Plot 2 Plot 3 Total Plots

Carbon Efflux (µgC/m^2s)

Control

Thinned

Results

T-test: Total plots showed significance, within 95% confidence. P=.02

Soil Temperature: Thinned vs. Control

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5

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15

Plot 1 Plot 2 Plot 3 Total PlotsSoil Temperature (C)

Control

Thinned

Results

Low correlation: .0348

Log scale: .027

CO2 Efflux vs. Soil Temperature

R2 = 0.0348

0204060

80100120140

0 5 10 15

Soil Temperature (C)

CO2 Efflux (µgC/m^2s)

Carbon Efflux(µgC/m^2s)

Expon. (Carbon Efflux(µgC/m^2s))

Results

Importance of slope

Correlation significant: .2486

CO2 Efflux vs. Slope

R2 = 0.2486

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30

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15 20 25 30 35 40

Slope (degrees)

Average CO2 Efflux

(µgC/m^2s)

CO2 Efflux

Linear (CO2 Efflux)

Discussion

Results differ from hypothesis: Thinned CO2 efflux is greater than control

•Investigate major components of CO2 efflux:

-Litterfall

-Root respiration

-Temperature

Litterfall• Soil respiration is directly related to

aboveground litterfall• Thinned sites: fewer trees, less litterfall

However…

•Thinning occurred ~20 years ago•Time for re-growth•Species composition: higher quality litter in thinned?

Root Respiration• Contributes to efflux:

1. Site of plant respiration

2. Highly decomposable fine roots

• Thinned sites: fewer trees, less root respiration

Considerations…•Re-growth period•Root expansion due to low nutrient availability•Root density may not differ much

Temperature• Warm temperatures stimulate microbial

activity and root respiration• Thinned: open canopy, sunlight warms soils

Carbon Efflux: Thinned vs. Control

0

20

4060

80

100

Plot 1 Plot 2 Plot 3 Total Plots

Carbon Efflux (µgC/m^2s)

Control

Thinned

Soil Temperature: Thinned vs. Control

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5

10

15

Plot 1 Plot 2 Plot 3 Total PlotsSoil Temperature (C)

Control

Thinned

•Temperature seems to have greatest impact in this ecosystem on CO2 efflux

Soil Moisture• Thinned sites are drier

• Reduces decomposition, but slows NPP more

• Speed decomposition with temperature, while slowing NPP with reduced water…

Positive Feedback Loop!-Carbon sinks cannot keep up with C flux to the atmosphere

Spatial and Temporal Heterogeneity

Things to Consider

• Recently thinned forests v. our study site• Diurnal fluctuations• Slope• Hotspots: - location relative to vegetation

- exposure to sunlight - ground cover

All these factors contribute to great uncertainty

Conclusion

• CO2 Efflux higher in thinned: due to

high soil temperature

• Litter quality, quantity, and root

biomass may not differ

• Loss of CO2 sink

• Future Research

• Management implications for upper

montane mixed conifer