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