Abstract
Mountain regions are an important regulator in the global water cycle
through their disproportionate water contribution. Often referred to as
the “Water Towers of the World”, mountains contribute 40 to 60% of
the world’s annual surface flow. Shade is a common feature in mountains,
where complex terrain cycles land surfaces in and out of shadows over
daily and seasonal scales. This study investigated turbulent water and
carbon dioxide fluxes over the snow-free period in a subalpine wetland
in the Canadian Rocky Mountains, from June 7th to
September 10th, 2018. Shading had a significant and
substantial effect on water and carbon fluxes at our site. Each hourly
increase of shade per day reduced evapotranspiration (ET) and gross
primary production (GPP) by 0.42 mm and 0.77 gCm-2,
equivalent to 17% and 15% per day, respectively, over the entire study
period. However, during only peak growing season, when leaves were fully
out and mature, shade caused by the local complex terrain, reduced ET
and increased GPP, likely due to enhanced diffuse radiation. The overall
result was increased water use efficiency at the site during periods of
increased shading during the peak growing season. In addition to
incoming solar radiation (Rg), temporal variability in ET was found to
relate to temporal variability in soil temperature, moisture and vapour
pressure deficit. Shade impacted the curvature and intercept of the
nonlinear ET-Rg relationship at our site. In contrast, temporal
variability in GPP at our site was dependent largely on Rg only. Our
findings suggest that shaded subalpine wetlands can store large volumes
of water for late season runoff and are productive through short growing
seasons.
Keywords: shading, complex terrain, wetlands,
evapotranspiration, carbon dioxide fluxes, gross primary production,
water use efficiency, Canadian Rocky Mountains