Variability in Subalpine Wetland Carbon Flux
The C flux varied greatly over the study period as the system quickly
shifted between a C source and sink in response to changing
environmental conditions following Snow Melt . Considering all
seasonal phases, the study period was a cumulative net sink of 63
gCm-2 and was comparable to snow-free period values
reported in other mountain ecosystems, such as sub-alpine meadows (Kato
et al. 2003), and wetlands (Zhao et al. 2010; Millar, Cooper, Dwire,
Hubbard, and von Fischer, 2017) studies. Average growing season NEE in
subalpine alpine wetlands varies from strong sinks at the Qinghai Tibet
Plateau (46 to 212 gCm-2) (Kato et al. 2003; Kato et
al. 2006; Zhao et al. 2010), to strong sources and sinks (-342 to 256
gCm-2) in the Rocky Mountains of Wyoming and Colorado
(Knowles et al. 2015; Millar et al. 2017). Although alpine systems show
varying results in ecosystem source/sink strength, winter C flux can
cause alpine meadow and wetland ecosystems to be a strong C sources, due
to the insulating properties of snow cover that keeps the ground surface
unfrozen and soil respiration (Rs) active (Zhao et al.
2010; Knowles et al. 2015; Lange, Allaire, Castillo, and Dutille, 2016).
In our study, snowpack kept soil temperature (2, 5, and 10 cm) above -5
°C during Snow Melt, meeting the threshold required for
Rs to occur (Brooks, Schmidt, and Williams, 1997). This
indicated that Reco was representative of
Rs during the Snowmelt period, when vegetation
was not productive. However, it was most likely that the warming period
in Snowmelt promoted degassing from melted ice lenses in the
snowpack (Lange et al. 2016), causing “burps” in Recoover time. Therefore, Rs likely continued over the
winter months at Bonsai, possibly making it a small annual net source of
CO2; however, an annual multi-year investigation will be
required to quantify its average yearly net C contributions and confirm
or refute this hypothesis.
The measurement period began with “pulses” of carbon emission duringSnowmelt because of degassing air pores in surface ice lenses
that melted during rainfall and warm temperatures (Lange et al. 2016).
Because of a lengthy melt process from shading (Hrach et al. 2021), theGreen Up period lasted longer at our study site compared to those
in other alpine ecosystems. Green Up represented the increase in
net seasonal CO2 flux, which led to peak production
(maximum GPP and Reco) during the Peak Growing
Season . The 2018 Green Up at Bonsai lasted from 24 June to 20
July, ending several weeks later than Green Up at similar
ecosystems reported in literature (Kato et al. 2003; Zhao et al. 2010;
Knowles et al. 2015), which began Green Up in June and ended by
early July. Despite these subalpine wetlands being located at lower
latitude (35-40N) compared to Bonsai (~50N) and
experiencing fewer daylight hours due to less solar input in June and
July, they greened up faster in comparison. Our results suggest that the
microclimate resulting from horizon shade, extended the Green Upand shortened the Peak Growing Season periods at our site by
approximately two weeks.
We found that in general, during the period of increasing shade, the C
flux had a statistically significant relationship with horizon shade.
During the period of Dynamic Shade , GPP and Recohad a statistically significant negative relationship with shade, while
NEE had a statistically significant positive relationship with shade.
Each hourly increase in shade per day during Dynamic Shade ,
decreased GPP by 15% and increased NEE by 18% (p<0.001,
R2=0.65), indicating that shade negatively impacted C
uptake at our subalpine wetland. Our finding found agreement with Cao et
al. (2017), which concluded that NEE has a statistically significant
negative linear relationship with Rg (p<0.05,
R2=0.27). However, what we also found is that shade
potentially increases or mitigates the overall reduction in available Rg
at our site, by allowing plants to utilize more diffuse radiation during
shaded periods (similar to cloudy conditions), thus increasing the
overall photosynthetic rate of the ecosystem which would not be observed
in direct light. However, further research on this topic is required at
our study site.
Beyond shade and Rg, the literature has identified that other
environmental variables like VMC (Kato et al. 2003), Ts(Kato et al. 2003; Zhao et al. 2010; Cao et al. 2017) , and
Ta (Zhao et al. 2010; Cao et al. 2017) play a strong
role in regulating the C flux. Past studies in mountain meadow (Knowles
et al. 2015), wetland (Millar et al. 2017), and forest ecosystems
(Monson et al. 2002) identified temperature and moisture as key controls
of seasonal patterns in NEE. Our wetland was a net source in Snow
Melt and early Green Up, but a sink at the end of Green
Up and throughout Peak Growing Season. NEE increased alongside
Ts through Green Up and Peak Growing
Season , and quickly responded to an abnormal spike in
Ta and Ts on September
7th in Late Growing Season. However, GPP at our
site during Peak Growing Season was not related to any
environmental variable, other than Rg.