Influences of forest canopy on snowpack accumulation and isotope ratios

Abstract

The stable water isotopes, 2 H and 18 O, can be useful environmental tracers for quantifying snow contributions to streams and aquifers, but characterizing the isotopic signatures of bulk snowpacks is challenging because they can be highly variable across the catchment landscape. In this study, we investigate one major source of isotopic heterogeneity in snowpacks: the influence of canopy cover. We measured amounts and isotopic compositions of bulk snowpack, throughfall, and open precipitation during seven campaigns in mid-winter 2018 along forest-grassland transects at three different elevations (1196, 1297, and 1434 m above sea level) in a pre-Alpine catchment in Switzerland. Snowpack storages under forest canopies were 67 to 93% less than in adjacent open grasslands. On average, the water isotope ratios were higher in the snowpacks under forest canopy than in open grasslands (by 13.4 ‰ in δ 2 H and 2.3 ‰ in δ 18 O). This isotopic difference mirrored the higher isotope values in throughfall compared with open snowfall (by 13.5 ‰ in δ 2 H and 2.2 ‰ in δ 18 O).Although this may suggest that most of the isotopic differences in snowpacks underforests versus in open grasslands were attributable to canopy interception effects, the temporal evolution of snowpack isotope ratios indicated preferential effluxes of lighter isotopes as energy inputs increased and the snowpack ripened and melted.Understanding these effects of forest canopy on bulk snowpack snow water equivalent and isotopic composition are useful when using isotopes to infer snowmelt processes in landscapes with varying forest cover.

Vegetation water sources in California's Sierra Nevada (USA) are young and change over time, a multi‐isotope (δ18O, δ2H, 3H) tracer approach.

“…Samplers were distributed within 70 m of each other and the tower (Figure 1 and Figure S1). P‐caps were distributed beneath forest canopy and in open areas because snow stable isotope δ 18 O and δ 2 H values can be influenced by canopy cover (Freyberg et al, 2020; Gustafson et al, 2010). In 2016 six P‐Caps were located beneath tree canopy, in three clusters, and three P‐Caps were in open areas, in two clusters ( n = 9).…”

“…Distinct snowmelt isotopic values can be compared to vegetation water isotopic values to observe where vegetation is accessing water when snowmelt input and transpiration are in phase. Throughfall δ 18 O and δ 2 H values can be affected by canopy interception (Freyberg et al, 2020;Gustafson et al, 2010;Saxena, 1986); similarly, canopy cover can affect snowmelt 18 O and δ 2 H isotopic values. The distinct δ 18 O and δ 2 H values of snowmelt can be compared with δ 18 O and δ 2 H values of vegetation to observe vegetation water sources when transpiration and snowmelt occur in phase.…”

Sierra Nevada isotope hydrology, tracking links between forest water sources, subsurface storage and major river runoff

“…On a finer scale, snow melt stable isotope signatures were measured every ten days over two snowmelt seasons. Snowmelt 2 signatures beneath canopy interaction and in forest gaps were significantly different, providing evidence that canopy interception can affect the input of snow isotopic signatures similarly to other areas in the world von Freyberg et al, 2019).…”

“…In addition to LMWLs, observed seasonality in precipitation stable isotope signatures is useful in determining seasonal contributions to vegetation, streams and groundwater and can be captured using sine waves Jasechko et al, 2014;. On a finer scale, sub-canopy forest heterogeneities have been found to affect snow and the stable isotope signatures of snow (von Freyberg et al, 2019).…”

Arctic Snow Isotope Hydrology: A Comparative Snow-Water Vapor Study

“…However, while many studies have investigated site-specific controls on local precipitation isotopes [7,35,36,43], few consider if/how the snowpack distorts the original vapor and snowfall isotope signal [49,50]. Postdepositional processes, such as snow sublimation [51][52][53][54][55], isotope exchange in vapor flow [56][57][58] and canopy interception [59,60], can each modify the snowpack isotope composition from individual precipitation events. However, the cumulative influence of these isotope fractionation processes on snowpack isotope stratigraphy over winter is poorly known [61][62][63][64], with studies focusing on meltwater percolation in the snowpack [65][66][67] or isotope diffusion in firn profiles [47,68].…”