Introduction
Subalpine wetlands provide a large range of ecosystem services within mountains and nearby lowlands, making water and carbon cycling within these systems important to understand. Although mountain terrain covers only 20% of the Earth’s land mass, they contribute 40 to 60% of annual surface flow, making them an important regulator of the global water cycle (Ives & Messerli, 1999; Grusson et al. 2015). As a result, many of the world’s major rivers begin in alpine headwaters, where runoff from snowmelt may entirely comprise downstream flow (Viviroli et al. 2011). In areas that receive low summer precipitation, such as the semi-arid Western United States and Canada, alpine headwaters provide a natural and continuous water source for irrigation and municipal water supplies to over 60 million people (Barnett, Adam, & Lettenmaier, 2005; Bales et al. 2006). Because of their large hydrological contributions, literature often refers to mountains as the “Water Towers of the World” (European Environment Agency, 2009; Immerzeel, 2008). The Canadian Rocky Mountains represent Western Canada’s Water Tower, since they store and distribute large quantities of water resources to 13 million people across the western Canadian Prairie Provinces and American north-central states (Fang et al. 2013).
Alpine wetlands, typically found within intermountain basins and upper mountain valleys (Windell et al. 1986), provide many important hydrological and ecological functions, such as flood mitigation, water for consumption and irrigation, support for important ecological habitats and carbon storage (Aber, Pavri, & Aber, 2012). An important component of the alpine hydrological cycle is evapotranspiration (ET). Although in general, alpine ET has been extensively researched, ET and other physical processes within alpine wetlands remain poorly characterized (Souch, Wolfe, & Grimmond, 1996). Wetlands are also an important contributor to carbon (C) storage, storing 12 to 15% of the global C pool (Cao et al. 2017), of which alpine wetlands contribute 2.5% (Zhao et al. 2010). Regionally the contribution of alpine ecosystems to local C-storage can be even greater. For example, in the Western United States alone, 70% of the carbon sink is located above 750 m.a.s.l, in landscapes that are 85% hills and/or mountains (Schimel et al. 2002; Desai et al. 2011). Distribution of carbon within alpine regions is extremely diverse. Hotspots of high soil organic carbon (SOG) have been found in moist to wet meadows, moderate SOG in dry meadows, and low SOG in fellfield (i.e. alpine tundra) (Knowles, Burns, Blanken, & Monson, 2015). Yet, relatively few studies have observed carbon cycling in alpine wetlands, making them some of the least understood wetlands in the world (Wickland, Striegl, Mast, & Clow, 2001; Cao et al. 2017). Although models indicate that globally wetlands may shift from sink to source of C under future climates, minimal research has been done on carbon source/sink dynamics in alpine wetlands and in understanding their controlling variables (Cao et al. 2017).
Ecosystem water use efficiency (WUE) is a useful metric to analyse the interaction of water and C fluxes of an ecosystem, as it is a proxy that quantifies photosynthetic carbon-uptake (GPP) per gram of water lost either through transpiration (T) or evapotranspiration (ET) (Rosenberg, Blad, & Verma, 1983), depending on the scale and WUE-definition used. WUE has been measured on numerous scales, including: ecosystem, plant, and leaf level and is often used in agricultural and crop sciences (Medrano et al. 2015). WUE has been used in studies on mountain wetlands to help evaluate seasonal water resources (Hu et al. 2008; Han, Luo, Li, Ye, Chen, 2013; Strobl et al. 2017; Quan et al. 2018). Research has shown that WUE decreases with increased water availability, often resulting in lower WUE at high elevations in the subalpine zone (Han et al. 2013).
Strobl et al. (2017) also showed that ecosystem WUE adapts to environmental conditions, like microclimate and available energy, over the course of a day, making it a useful metric to help evaluate the influence of microclimates on C and water fluxes in mountain environments. Shade is a common feature in alpine basins, where complex terrain causes parts of the landscape to pass in and out of shadows over daily and seasonal time scales ( Marsh, Pomeroy, & Spiteri, 2012) . Shade can significantly reduce the available energy in mountain ecosystems during the snow-free period and create distinct microclimates compared to non-shaded areas (Hrach, Van Huizen, Khomik, & Petrone, 2021). However, little research has been conducted on the impact of shade on water and carbon cycling within mountain landscapes, especially wetlands.
The main objectives of this research were to: 1) characterize growing season carbon and water dynamics of an alpine wetland located in complex terrain; 2) explore if and how horizon shade (Essery & Marks, 2007; Marsh et al. 2012), caused by the local complex terrain, influences water use efficiency of the wetland; and 3) investigate which key environmental factors, including shade, could explain the temporal variability in observed carbon and water fluxes. Note, in this study we define WUE as the ratio between GPP and ET. Given that water fluxes are related to available energy through latent heat of vaporization, we hypothesized that ET will be negatively affected by horizon shade and cause an increase in or unchanged WUE, if C-fluxes are unaffected or if C-fluxes also decrease with shade, respectively.