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.