Monitoring the Seasonal Hydrology of Alpine Wetlands in Response to Snow Cover Dynamics and Summer Climate: A Novel Approach with Sentinel-2

Abstract

Climate change in the European Alps during recent years has led to decreased snow cover duration as well as increases in the frequency and intensity of summer heat waves. The risk of drought for alpine wetlands and temporary pools, which rely on water from snowmelt and provide habitat for specialist plant and amphibian biodiversity, is largely unknown and understudied in this context. Here, we test and validate a novel application of Sentinel-2 imagery aimed at quantifying seasonal variation in water surface area in the context of 95 small (median surface area <100 m2) and shallow (median depth of 20 cm) alpine wetlands in the French Alps, using a linear spectral unmixing approach. For three study years (2016–2018), we used path-analysis to correlate mid-summer water surface area to annual metrics of snowpack (depth and duration) and spring and summer climate (temperature and precipitation). We further sought to evaluate potential biotic responses to drought for study years by monitoring the survival of common frog (Rana temporaria) tadpoles and wetland plant biomass production quantified using peak Normalized Difference Vegetation Index (NDVI). We found strong agreement between citizen science-based observations of water surface area and Sentinel-2 based estimates (R2 = 0.8–0.9). Mid-summer watershed snow cover duration and summer temperatures emerged as the most important factors regulating alpine wetland hydrology, while the effects of summer precipitation, and local and watershed snow melt-out timing were not significant. We found that a lack of summer snowfields in 2017 combined with a summer heat wave resulted in a significant decrease in mid-summer water surface area, and led to the drying up of certain wetlands as well as the observed mortality of tadpoles. We did not observe a negative effect of the 2017 summer on the biomass production of wetland vegetation, suggesting that wetlands that maintain soil moisture may act as favorable microhabitats for above treeline vegetation during dry years. Our work introduces a remote sensing-based protocol for monitoring the surface hydrology of alpine wetland habitats at the regional scale. Given that climate models predict continued reduction of snow cover in the Alps during the coming years, as well as particularly intense warming during the summer months, our conclusions underscore the vulnerability of alpine wetlands in the face of ongoing climate change.

Earlier Snowmelt Advances Breeding Phenology of the Common Frog (Rana temporaria) but Increases the Risk of Frost Exposure and Wetland Drying

“…On the contrary, ponds at the low elevation site remained wet during all summers, presumably because they are deeper and benefit from a larger upstream watershed and a more diverse water supply. In the future, we expect that the increase of summer heatwave frequency and intensity (Corona-Lozada et al, 2019) combined with a decrease in snow cover depth and duration (Klein et al, 2016) may lead to a higher probability of wetland pools drying out, especially in high elevation sites that are highly dependent on snowpack (Carlson et al, 2020). Analysis of the effects of snow depth and duration, precipitation, and temperature on the water regime of wetlands in mountain watersheds are consequently highly valuable to better understand the impact of climate change on these ecosystems and on frog populations (Carlson et al, 2020).…”

“…In the future, we expect that the increase of summer heatwave frequency and intensity (Corona-Lozada et al, 2019) combined with a decrease in snow cover depth and duration (Klein et al, 2016) may lead to a higher probability of wetland pools drying out, especially in high elevation sites that are highly dependent on snowpack (Carlson et al, 2020). Analysis of the effects of snow depth and duration, precipitation, and temperature on the water regime of wetlands in mountain watersheds are consequently highly valuable to better understand the impact of climate change on these ecosystems and on frog populations (Carlson et al, 2020). It is known that amphibian populations can develop under wetland drying conditions (Laurila and Kujasalo, 1999) and that the upper thermal tolerance of juvenile frogs slightly increases with elevation (Enriquez-Urzelai et al, 2020).…”

“…Species must grow, reproduce and acquire resources during a shorter and colder growing season than at low elevations (Ryser, 1996;McCaffery and Maxell, 2010;Muir et al, 2014). In addition, for amphibian species, their full development is highly dependent on water availability in wetland pools, which itself depends on climate parameters as well as potential water storage in snowpack and glaciers (Carlson et al, 2020).…”

“…Alongside increases in mean air temperature, the frequency, intensity and length of summer heatwaves have increased in the French Alps, especially since 2000 (Corona-Lozada et al, 2019). Heatwaves have been shown to promote evapotranspiration and contribute to drier wetlands (Carlson et al, 2020). In parallel, the decrease of snow cover duration (Klein et al, 2016) and snowpack in mountain environments leads to a decrease in water supply (Hanzer et al, 2018) especially during the summer period.…”

Monitoring Dynamic Braided River Habitats: Applicability and Efficacy of Aerial Photogrammetry from Manned Aircraft versus Unmanned Aerial Systems

“…Since these technologies are relatively new, there is only a handful of scientific papers discussing their operational complexities [3]. It will be useful to have a comparative summary of available technology with information about their applicability and efficiency for a given purpose such as habitat monitoring [4][5][6], wildlife monitoring [7][8][9], vegetation change analysis [10,11], forest inventory [12], monitoring agricultural productivity [13,14] etc.…”