WUE of a Subalpine Wetland Exposed to Horizon Shade
Water use efficiency at our site varied in response to changes in plant
productivity and water availability throughout the study period. Over
the course of the study, average WUE was comparable to a mountain meadow
at 3,293 m.a.s.l. (Zhu et al. 2014) and wetland at 3,160 m.a.s.l. (Hu et
al. 2008) literature. Maximum WUE at our site (5.3 gC (kg
H2O)-1) occurred on August
20th and was an order of magnitude greater and
occurred several weeks later in the season than the max WUE (2.84 gC (kg
H2O)-1) reported by Hu et al. (2008)
on DoY 200 (July 19th) in their study of an alpine
wetland. Although each site was at a greater elevation, Hu et al. (2008)
had a similar temperature (annual -2 °C, July 10 °C), precipitation
(annual 580 mm), and canopy height (40 cm). Zhu et al temperature
(annual -1.7 °C), precipitation (annual 570 mm), canopy height (60-70
cm), VMC (0.3% in June with steady decline to August). The minimum WUE
at our study site (1.4 gC (kg H2O)-1)
occurred from June 24th to June 30th, a similar
timeframe (June 17th to June 30th)
but different magnitude and value (~1.5 μmol
mol-1 ) as the subalpine forest WUE reported
in Monson et al. (2010). From Green up to Late GrowingSeason Bonsai showed a greater WUE as the season progressed,
despite decreasing soil moisture, suggesting that shaded subalpine
wetlands remain productive through short growing seasons. Recent studies
found that during the growing season, WUE increased linearly with lower
Rg and that fluctuations in light availability increased plant
productivity (Gao et al. 2018; Kromdijk et al. 2016). Our findings
agreed with the literature, that decreased Rg led to a higher WUE (Niu
et al. 2011; Han et al. 2013; Gao et al. 2018), but unlike those studies
we found that ET and Rg have a non-linear relationship and that diffuse
radiation produced by shade can enhance GPP, adding to the reduction in
WUE with decreased Rg. Reflecting on our hypothesis, we conclude
that WUE was affected by shade at
our study site, because shade reduced ET and increased GPP. We
hypothesized that GPP might remain the same or be reduced by shade, but
we found evidence to suggest shade may actually increase GPP. It cannot
be definitively concluded that shade helped increase GPP, since this was
not the focus of this study. However, past studies showed that plants
are more efficient at photosynthesizing in diffuse radiation (such as
during cloudy conditions) as opposed to direct radiation (Gu et al.,
2002). Thus, shade did potentially contribute to increased GPP duringDynamic shade at our site, but more detailed studies should be
conducted to assess the direct contribution of diffuse radiation on
photosynthesis and any leaf-area/greenness changes that occur throughout
the growing season congruently.
Under future climate uncertainties, water storage will become
increasingly important in the subalpine zone, because wetlands supply
forests, which are experiencing increased disturbance (Pomeroy Fang, and
Ellis, 2012) and decreased winter snow accumulation (Harpold et al.
2015; Pomeroy, Fang, and Rasouli, 2015). As a result, shaded wetlands
provide an opportunity to store large volumes of water for late season
runoff, while remaining productive within short growing windows.
Therefore, shade may delay or negate any rapid change(s) to subalpine
wetland water storage and use, which will help balance anticipated water
losses from alpine forests. However, further research is required on
snow water equivalence at our wetland, to help understand how snowmelt
runoff is stored and its contribution to downstream runoff.