Seasonal Changes of Soil and Ecosystem Respiration in a Young Forest J Hunt, T McSeveny and F Kelliher 4-6 th Feb 2008, Weintal Resort, Tanunda, Australia.

Seasonal Changes of Soil and Ecosystem

Respiration in a Young Forest J Hunt, T McSeveny and F Kelliher

4-6th Feb 2008, Weintal Resort, Tanunda, Australia.

Carbon storage

• New Zealand is a signatory to the Kyoto Protocol– Required to create inventory of its

GHG sources and sinks– Reduce GHG emissions to a fixed

amount

• NZ GHG emissions are increasing– Increased fuel use– Increased reliance on coal power

stations– Increase in methane emitters– Reduction in pine plantations

• Effort going into carbon storage in marginal lands, esp. re-afforestation with kanuka)

The forest

• Kanuka (Kunzia ericoides, Myrtaceae)

• Short lived (60-100 yrs)• Fast growth habit, light

demanding• Secondary invader, reclaims

old pasture• Exotic grass/herb understory• 7m tall trees, 3 500 ha-1

• First forest on site, 35 yrs old • 49 tC ha-1,equivalent to a net

gain of 1.4 tC ha-1 y-1

Site Location

• Island in the middle of the braided Rakaia River • Forest occupies 800 m x 2000 m• 50 km SW of Christchurch (43 ºS)• 60 masl• Soils 100 yrs old• NW foehn + katabatic winds

X

Instrumentation• Solar powered, closed-path, eddy

covariance • Profiling system• Background met.• Soil moisture and temperature• Monthly soil surface respiration and

biomass measurements

Seasonal understory biomass

Month

Aug Oct Dec Feb Apr Jun Aug

Und

erst

ory

Bio

mas

s (g

m-2

)

0

100

200

300

400

500

Live Dead Total

• Tree leaf biomass was ~ 200 g m-2

• Unimodal change in total understory biomass• Change in live:dead ratio between spring and autumn

Seasonal ecosystem respiration

Soil temperature (C)

4 6 8 10 12 14 16 18 20

Spring

< 0.15Autumn

Oct Dec Feb Apr Jun Aug

Month

Eco

syst

em r

esp

irat

ion

( m

ol C

O2

m-2

s-1

)

0

1

2

3

4

5

6

7(a) (b)

• Summer drought reduced ecosystem respiration by up to 7-fold• Spring and autumn respiration have similar response to temperature

Ecosystem respiration under well watered conditions

Spring

Soil temperature (0C)

6 8 10 12 14 16

Eco

syst

em R

esp

iratio

n (

mol

C

O2

m -2

s-1)

0

1

2

3

4

5

6

7

R10 = 3.12

r2 = 0.64***

R10 = 2.76

r2 = 0.38Autumn

Soil temperature (0C)

6 8 10 12 14 16 18

R10 = 2.75

r2 = 0.38***

• At same temperature, autumn respiration 13% lower then in spring• Suggests change in carbon substrate supply or quality

Response of ecosystem respiration to changes in soil VWC

Root-zone water content (m3

m-3

)

0.05 0.10 0.15 0.20 0.25 0.30

Ec

osy

ste

m R

10

( m

ol C

O2

m-2

s-1)

0

1

2

3

4

5

• Fit R10 to all data to remove temperature effect• If VWC < 12% then linear response of R10 to increasing soil moisture

Comparison between soil surface and ecosystem respiration

Re

sp

ira

tio

n r

ate

( m

ol C

O2

m-2

s-1

)

0

1

2

3

4

5

6

Rs SoilRe Ecosystem Col 1 vs Soil Flux Col 1 vs Soil Flux

Month

Rs

/ Re

(%)

40

50

60

70

80

90

Sep Nov Jan Mar May Jul

• Soil and ecosystem respiration bimodal • Ratio of Rs/Re remained stable until rewetting in April• After rewetting 3-fold increase in Rs

• Sustained increase in Rs for 2 months

Conclusions

• Response of ecosystem respiration to temperature greater in spring than autumn

• Due to increased substrate availability with active growth and greater biomass

• When VWC > 12%, temperature and substrate availability were main drivers of ecosystem respiration

• Rs/Re varied from 50-90%, not caused by differences in soil-canopy temperatures

• Dynamic forest understorey can influence magnitude and seasonal maxima of ecosystem respiration