Figure 1 . Differences between the MANIPULATION runs (S1-S4) and
the HISTORICAL runs (S0) for winter GT (°C; left column), non-winter GT
(°C; middle column), and winter snow depth (%; right column), at each
site.
3.3.2 Impacts on biogeochemical variables
Our results further showed large WWE-induced impacts on all
biogeochemical variables, with magnitudes often comparable to impacts
deriving from altered winter climatologies (Figures 2 and G3). GT and
water availability are key drivers of the ecosystem C cycle, as they
influence the start of the growing season, nutrient availability,
vegetation dynamics, and soil Rh.
The modelled impacts of the manipulation experiments on biogeochemical
variables generally followed the same direction as those observed for GT
and GWC. We noted substantial reductions in GPP (Figure 2i-l) under the
WWE experiments S1 and S3 of up to 25% at the fen, 20% at the peat
plateau, and 10% at the birch site, and smaller reductions under ROS
events alone (S2). The tundra site showed weaker reductions in GPP than
the other sites, ranging from a few percent to up to 5% in the milder
WWE experiments (driven by SSP119), and increases of up to 10% under
CanESM5 SSP585. In contrast, for S4, the model simulated weak changes in
GPP in the mildest CMIP6 scenarios, but sizable increases in GPP in the
CanESM5 SSP585: up to 75% in the peat plateau, c. 50% in the birch
forest, c. 25% in the tundra, and c. 10% in the fen site. The impacts
on Ra mirrored those described for GPP (Figure 2e-h).
Noticeably, the relationship between GT, and both GPP and
Ra, is not linear because the processes involved in
plant dynamics are multiple and complex. For example, observational and
manipulation studies have reported considerable vegetation damage,
delays in bud phenology, and reductions in vegetation greenness due to
phenological and frost stress following WWEs (Bokhorst et al., 2009,
2010). These impacts on vegetation are not explicitly represented in the
model, as it does not yet take into account these winter/spring
stress-related processes.
Increases in GT can stimulate microbial activity and accelerate litter
and soil decomposition rates, resulting in increased Rh(Natali et al., 2019). Accordingly, winter Rh increased
or decreased by <5% at the fen, and up to 25% at the tundra
and peat plateau, following the GT responses (Figure G1). Contrastingly,
in response to the CanESM5 SSP585 scenario at the birch forest site,
despite the overall lower winter GT, winter Rh increased by up to
>200%. This may be explained because the modeled winter
Rh in the birch forest is very low (6 g C
m-2, compared to >30 g C
m-2 at the other sites), and even small increases in
Rh during WWEs can result in substantial relative
increases in winter emissions. A recent synthesis of in situ
observations across the Arctic indicates that winter Rhaccounts for a substantial portion of the arctic’s annual C budget
(Natali et al., 2019). Winter Rh alone currently offsets
~40% of the measured annual vegetation C uptake at our
sites. Applying the winter Rh response curve to GT by
Natali et al., (2019) (Q10 = 2.9), WWE-induced impacts on winter GT of
the magnitudes reported here could change total winter
Rh by up to 25%. Hence, realistically simulating the
effects of WWEs on GT is important for improving estimates of GHG
emissions in high latitudes.
The modelled CH4 emissions at the peat plateau and fen
sites decreased under all WWE experiments due to WWE-induced decreases
in GT and GWC (Figure G2). At the fen site, emissions halved mostly due
to lower water tables in the non-winter season.
Overall, the year-round ecosystem C exchange (i.e. NEE) of the birch
forest, tundra, and peat plateau sites decreased (i.e., became a smaller
C sink) considerably under most WWE experiments (Figure G3), generally
between 20% and 50%, and up to 90% occasionally in the tundra site,
due to larger reductions in vegetation C assimilation compared to the C
losses from Rh. Contrastingly, NEE at the fen site
increased weakly (i.e., became a stronger C sink), mostly due to large
reductions in Rh. Noticeably, the changes in NEE caused
by WWEs (S1-S3) are substantial and of similar magnitude to those caused
by shifts in future winter climatologies (S4). This further indicates
that WWEs, despite their short duration, may have the potential to
induce changes in high-latitude ecosystem C cycling of magnitudes
comparable to those induced by long-term climatic trends.
Given the observed discrepancies in modeled vs measured GT responses to
WWEs (section 3.2.1), the modeled impacts on ecosystems C fluxes
reported here should not be interpreted as a direction prediction of
future impacts of WWEs, but rather as a sensitivity test of the current
model’s responses to altered levels of WWEs.