3. RESULTS
3.1. Seasonal and inter-annual variations of meteorological
Environmental factors varied greatly in different seasons from January
2007 to December 2016 (Fig. 1). The peaks of monthly environmental
factors (Ta, Ts, GDD, PPFD, PPT, RH and VPD) were not consistent (Fig.
1). The maximum value of monthly GDD occurs in July (Fig. 1a). The
average annual air temperature (Ta) and soil temperature (Ts) were -1.0
± 0.5 and 3.0 ± 0.8°C, respectively. Only in April, May and June, the
average monthly Ta is higher than monthly Ts (Fig. 1a). This may be
caused by melting of snow and ice, which absorbed heat from the soil.
Based on a threshold deviation from average annual Ta of 20%, the years
2014 and 2015 were classified as warmer years (>0.8 Ta),
the years 2007, 2010, 2013, 2016 were considered normal years, and the
years 2008, 2009, 2011, 2012 were defined as cooler years
(<1.2 Ta) (Fig. 1). The average annual precipitation (PPT) was
456.5 ± 86.2 mm. The minimum annual rainfall was 323.1 mm occurred in
2012 year, and the maximum annual rainfall was 578.1 mm occurred in
2013. The years 2007, 2013, 2014 were classified as wetter years
(>1.1 PPT) and 2012, 2015, 2016 were classified as drier
years (<0.9 PPT) (Fig. 1).
3.2. Seasonal and inter-annual variations of GPP, RES and NEE
Both monthly GPP (162.7 ± 8.4 g C
m−2month−1, mean ± S.E., with the
same notation hereon in) and monthly NEE (−61.1 ± 6.3 g C
m−2month−1) peaked in July, while
the maximum monthly RES (109.4 ± 8.8 g C
m−2month−1) appeared in August (Fig.
2a). The wetland ecosystem is a carbon sink from June to August with a
negative NEE (Fig. 2a). During the growing season, linear regression
analysis indicated that compared to monthly RES (r2=
0.44, p < 0.001), monthly GPP (r2= 0.88, p
< 0.001) played a more important role in controlling the
change of monthly NEE.
Annual GPP was 500.3 ± 59.4g C
m−2year−1 and annual RES was 620.7 ±
74.2g C m−2year−1, respectively
(Fig. 2b). Both monthly GPP (r2= 0.11, p =0.33) and
monthly RES (r2= 0.28, p = 0.12) in the peak growing
season (July and August) were not predominantly determined for
variations of annual GPP and annual RES. The10-year mean NEE was
120.4±34.8 g C m−2year−1 and a range
of 76.3 g C m−2year−1 in 2010 to
184.8 g C m−2year−1 in 2014 (Fig.
2b). The change of annual NEE was neither directly controlled by annual
RES (p = 0.06) nor directly controlled by annual GPP (p = 0.61), which
was controlled more strong by annual RES to a certain extent compared to
annual GPP. The sum of monthly NEE of July and August was also not
predominantly determined for variations of annual NEE
(r2= 0.12, p =0.32). The non-growing season RES
accounts for a small proportion of annual RES, but it is significantly
positively correlated with annual NEE (r2= 0.50, p
< 0.05) and annual RES (r2= 0.45, p
< 0.05). This finding suggested that CO2fluxes in alpine wetland during the non-growing season were crucial for
annual carbon balance of alpine wetland.
3.3. Influence of environmental factors on seasonal variations of GPP,
RES and NEE
The value of proportional reduction in error is more than 0.92,
indicating that the analysis results of CART on monthly
CO2 fluxes (GPP, RES and NEE) are acceptable. The
results of CART indicated that GDD was the main control factor that
affecting the change of monthly GPP and NEE. GDD accounted for 78% and
79% change of monthly GPP (Fig. 3a) and NEE (Fig. 3c), respectively.
Furthermore, the splitting values of GDD were all around 230℃d, and
appeared at the beginning of July or the end of August, indicating that
thermal condition was the main control factor that determined the change
of monthly GPP and NEE during the peak of plant growth stage. The factor
of the second right root node of GPP is Ta, suggesting that Ta was
crucial to the photosynthetic and carbon fixation capacity of plants in
wetland ecosystem during the peak growth period. Variability in monthly
RES was predominantly determined by Ts in the whole year (Fig. 3b). The
factor of the second right root node of NEE was Ts, suggesting that the
carbon dynamics of wetland ecosystem to some extend was controlled by
RES more than GPP in non-peak growth period. In addition, during the
growing season, the influence of GDD on variations of monthly GPP was
stronger than the change of monthly RES (p < 0.001) (Fig. 4).
Furthermore, the growing season NEE had no significant correlation with
growing season Ta (r2=0.06, p=0.48) and PPT
(r2= 0.15, p=0.27). However, the non-growing season
NEE significantly correlated with non-growing season Ta (Fig. 5a) and
PPT (Fig. 5b).
3.4. Influence of environmental factors on annual GPP, RES and NEE
Annual GPP and RES had no significant correlation with annual
environmental factors (p > 0.05, Table.1). The correlation
analysis showed annual GPP had significant positive and negative
correlation with the growing season Ta (p<0.05) and PPT
(p<0.05), respectively, suggesting that the increase of
temperature and decrease of precipitation in growing season was
beneficial to the improvement of GPP. The result was noteworthy that the
Ts during the non-growing season had significant impact on annual GPP of
the following year (p<0.05), indicating that the warmer Ts
during the non-growing season promoted the activities of microorganisms
and accelerates the decomposition of litters and soil organic matter.
Therefore, the soil of wetland contained a lot of nutrients and promoted
the photosynthesis and carbon sequestration of vegetation in the
following year. The further partial correlation analysis showed that
annual RES had significant positive correlation with the Ta and Ts
(p<0.05) of the non-growing season. Annual PPT exhibited
significant impact on variation of annual NEE (Table 1), and linear
regression analysis indicated that the non-growing season Ta (Fig. 5c)
and PPT (Fig. 5d) had a major impact on the change of annual NEE.
However, the further partial correlation analysis showed that
non-growing season air temperature (Ta, p = 0.05), rather than
precipitation (PPT, p = 0.25) was a predominant determinant on the
change of annual NEE. Overall, hydrothermal condition especially in
non-growing season was the most important variable for describing
variation on annual carbon fluxes.