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.