Comparison of Carbon Fluxes Over Three Boreal Black Spruce Forests in Canada O. Bergeron §, H.A....
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Comparison of Carbon Fluxes Over Three Boreal Black Spruce Forests in Canada O. Bergeron §, H.A. Margolis §, T.A. Black †, C. Coursolle §, A.L. Dunn ф,
Comparison of Carbon Fluxes Over Three Boreal Black Spruce
Forests in Canada O. Bergeron , H.A. Margolis , T.A. Black , C.
Coursolle , A.L. Dunn , A.G. Barr , S.C. Wofsy Facult de Foresterie
et de Gomatique, Universit Laval, Qubec, Qubec, G1K 7P4 Canada;
Faculty of Agricultural Sciences, 135-2357 Main Mall, University of
British Columbia, Vancouver, B.C., V6T 1Z4 Canada Department of
Earth and Planetary Science, Harvard University, 29 Oxford St.,
Cambridge, Massachusetts, 02138 USA ; Climate Research Branch,
Meteorological Service of Canada, 11 Innovation Blvd., Saskatoon,
Saskatchewan, S7N 3H5 Canada Black spruce forests are the most
dominant cover type in the boreal forest of North America. Mature
black spruce ecosystems store large amounts of carbon that can be
released back to the atmosphere depending on intra- and
inter-annual variability of environmental conditions and the
occurrence of fire or forest harvest. These forests are impacted by
a broad range of environmental and site conditions in Canada.
Previous efforts to study C exchanges over mature boreal forests in
North America have been concentrated in central Canada as part of
BOREAS and BERMS programs. However, the wetter climate in eastern
Canada exposes black spruce forests to different light,
temperature, water, and nutrient conditions. It is not clear to
what extent the response of C exchange to climate variability may
vary across the Canadian boreal forest. In 2004, year-round eddy
covariance flux measurements were made for the first time over an
Old Black Spruce (OBS) forest in eastern Canada (EOBS, Quebec)
concurrently with two other pre-existing OBS sites in central
Canada (SOBS, Saskatchewan and NOBS, Manitoba). The current study
took advantage of this initial opportunity to compare and contrast
three OBS sites located in different climatic regions of Canada.
1.Quantify the annual NEP, R, and GEP of three mature black spruce
stands located in three different climatic regions of Canada ;
2.Differentiate the contribution of R and GEP to NEP that occurred
in spring, summer, autumn, and winter between sites ; 3.Isolate
environmental factors at the three flux sites that can explain the
variability of R and GEP occurring at different time scales.
Introduction Conclusions This study was supported by CFCAS, NSERC
and BIOCAP Canada. Poster presented at the FCRN 4 th Annual
Meeting, Victoria, British Columbia, 24-26 Feb 2006.
SiteNOBSSOBSEOBS LocationManitobaSaskatchewanQuebec Stand Age
(years)~130 ~100 Mean Tree Height (m)9.17.213.8 Tree Density
(trees/ha)545059004770 LAI4.84.24.0 Mean Annual Temperature (C)
-3.20.40.0 Mean Annual Precipitation (mm) 517.4424.3961.3
Objectives Table 1. Annual totals of NEP, R, and GEP in g C m -2 y
-1. Uncertainties correspond to gap filling error estimates.
Environmental Conditions EOBS received lower amounts of incident
light from May to July (Fig. 1a). Air temperatures were higher at
SOBS in April and May (Fig. 1b). The snowpack was thicker and
persisted longer at EOBS (Fig. 1c). The soil (5 cm depth) was
generally cooler at NOBS and did not freeze in winter at EOBS (Fig.
1d). Soil temperatures were highest at EOBS from July through
December. Soil water content (5 cm depth) decreased only at EOBS
during the growing season (Fig. 1e). Water tables were near the
surface at SOBS throughout the growing season. At NOBS and EOBS,
water table depth was similar from May to July, but sharply
declined in August at EOBS (Fig. 1f). Fig. 1. Selected
environmental variables. In 2004, NOBS and SOBS were weak C sinks
of similar strength, while EOBS was C neutral. The annual sums of
GEP and R were highest at SOBS, intermediate at EOBS, and lowest at
NOBS, which was consistent with growing season length. However, GEP
totals at NOBS and EOBS were similar. Fig 2. Time series of monthly
totals of (a) NEP, (b) R, and (c) GEP. Annual C Budgets Site
Characteristics Seasonal Patterns NEP seasonal pattern was similar
among sites : Sites were C sinks from May through September ;
Maximum NEP occurred in June at all three sites ; NEP depression in
July yielded similar NEP totals among sites. Sites specificities: C
losses were higher at EOBS from January to March. C gains began
first at SOBS in April and last at NOBS in May. GEP and R totals
were lower in June and July at EOBS. GEP and R totals peaked in
July at SOBS and NOBS and in August at EOBS. GEP and R decline in
autumn was fastest at NOBS and slowest at EOBS. Sites GEPR GEP GEP
max Q 10 R 10 NOBS0.057 b 12.4 a 3.80 c 6.21 c SOBS0.036 a 17.3 c
3.00 b 4.97 b EOBS0.034 a 14.5 b 2.78 a 3.71 a Physiological
Parameters Table 2. Light response curve parameters and
coefficients of respiration response to soil temperature.
Relationships developed with half-hour non-gap-filled measurements.
Superscripts indicate significant differences. SitesNEP RGEP NOBS27
11538 10565 12 SOBS29 5661 5690 6 EOBS 4 8580 9584 7 GEP response
to light was characterised with a rectangular hyperbolic function (
GEP = ( GEP GEP max PPFD) / ( GEP PPFD + GEP max ) ) using daytime
NEP R measurements. Only measurements made under optimal
environmental conditions ( 15 5 C, VPD 0.15 m 3 m -3 ) were used to
factor out intra-annual climatic differences between sites. NOBS
showed higher light use efficiency ( GEP ) and lower maximum
photosynthetic capacity (GEP max ). Highest maximum photosynthetic
capacity was observed at SOBS. These differences could reflect site
specific climate adaptation. R response to near surface soil
temperature was characterised with a Q 10 function ( R = R 10 Q 10
[(Ts-10) / 10] ) using night-time and cold season NEP measurements.
Respiration temperature sensitivity (Q 10 ) and basal respiration
(R 10 ) were higher at NOBS and lower at EOBS. Inversely, mean
annual soil temperatures where lower at NOBS and higher at EOBS.
Daily Response to Temperature Monthly Response to Temperature Fig
4. The response of total daily (a) R and (b) GEP to mean daily
temperature. (2 C bin averages SE) The response of daily R to soil
temperature was significantly different among sites (Fig 4a). These
daily differences were also reflected at the half hour scale. Fig
3. Response of total daily GEP to light. (monthly averages SE)
Daily GEP was independent of light during the cold season (Dec Apr)
but also during the first part of the growing season (Apr Jul)
(Fig. 3). Air temperature explained most of the variability in GEP
at the daily time scale. The relationships were not significantly
different among sites using a linear function on log-transformed
GEP data. The temperature response of GEP was best described using
a logistic function (Fig 4b). Relative GEP (ratio of daily GEP :
maximum value of daily GEP) was used to take into account the
pattern towards the maximum photosynthetic capacity. According to a
residual analysis, incident light affected the response of daily
GEP to air temperature The responses of monthly R and GEP to soil
and air temperatures, respectively, were not different among sites.
The response of R to temperature was exponential, while GEP
increased linearly with temperature (Fig. 5). From residual
analysis, soil water content was the only variable significantly
affecting the response of R to temperature (Fig. 6). 1.In 2004,
SOBS and NOBS were weak C sinks and EOBS was C neutral. Total
annual GEP and R were highest at SOBS and lowest at NOBS.
2.Overall, all three sites showed very similar seasonal patterns of
C fluxes. However, EOBS showed higher winter C losses associated
with warmer soil under thicker snowpack and SOBS began fixing C
almost one month before NOBS. 3.From half-hour measurements, all
three sites showed different light response curve parameters and a
specific R response to soil temperature. 4.Temperature drove both
GEP and R at the daily and monthly scales. 5.At the daily scale, R
response to soil temperature was site specific. GEP response to air
temperature was not different among sites and was affected by the
quantity of incident light. 6.At the monthly time scale, all three
sites showed a similar response to temperature. Soil water content
affected the response of R to soil temperature. Fig 5. Response of
total monthly (a) R and (b) GEP to temperature. In (b), months with
mean air temperature below -5 C were discarded to attain
homoscedasticity. Fig 6. Residuals of total monthly R as a function
of soil water content. Dec Apr Jul