Relationship between Spatiotemporal Variations of Climate, Snow Cover and Plant Phenology over the Alps—An Earth Observation-Based Analysis

Abstract

Alpine ecosystems are particularly sensitive to climate change, and therefore it is of significant interest to understand the relationships between phenology and its seasonal drivers in mountain areas. However, no alpine-wide assessment on the relationship between land surface phenology (LSP) patterns and its climatic drivers including snow exists. Here, an assessment of the influence of snow cover variations on vegetation phenology is presented, which is based on a 17-year time-series of MODIS data. From this data snow cover duration (SCD) and phenology metrics based on the Normalized Difference Vegetation Index (NDVI) have been extracted at 250 m resolution for the entire European Alps. The combined influence of additional climate drivers on phenology are shown on a regional scale for the Italian province of South Tyrol using reanalyzed climate data. The relationship between vegetation and snow metrics strongly depended on altitude. Temporal trends towards an earlier onset of vegetation growth, increasing monthly mean NDVI in spring and late summer, as well as shorter SCD were observed, but they were mostly non-significant and the magnitude of these tendencies differed by altitude. Significant negative correlations between monthly mean NDVI and SCD were observed for 15–55% of all vegetated pixels, especially from December to April and in altitudes from 1000–2000 m. On the regional scale of South Tyrol, the seasonality of NDVI and SCD achieved the highest share of correlating pixels above 1500 m, while at lower elevations mean temperature correlated best. Examining the combined effect of climate variables, for average altitude and exposition, SCD had the highest effect on NDVI, followed by mean temperature and radiation. The presented analysis allows to assess the spatiotemporal patterns of earth-observation based snow and vegetation metrics over the Alps, as well as to understand the relative importance of snow as phenological driver with respect to other climate variables.

Climatic Drivers of Greening Trends in the Alps

“…Snow and temperature regimes control the shape of greening in this region ( Figure 5). The recent increase in air temperature recorded all over the Alps results generally in an earlier start of the growing season [20]. In our study, we confirmed this pattern below 1500 m (Figure 5b), where snow melts before March 1st and plant development can take advantage of warmer spring temperatures.…”

“…Above 1800 m the snow cover prevents the vegetation development until late spring. While a trend toward an earlier snow melt was reported at an alpine-wide scale by [20] for the 2000-2018 period, longer records suggest little evidence of earlier snow melt in the Alps after the 1990s [46]. For this reason, we hypothesize plants above 1800 m cannot take advantage of warmer springs due to the presence of snow cover [34].…”

“…Forested ecosystems could also be subjected to greening trends via biomass increase, forest densification, and shifts in species composition. An alpine-scale study on land surface phenology showed extensive lengthening of the growing season and demonstrated the role of snow duration on phenology trends in the last 20 years [20].…”

Deriving Regional Snow Line Dynamics during the Ablation Seasons 1984–2018 in European Mountains

“…Flora and fauna in high mid-latitude mountain regions are influenced by the snowline dynamics. The spatiotemporal snow dynamics affect the plant phenology [64][65][66], and animal activity [67][68][69].…”

Greening Implication Inferred from Vegetation Dynamics Interacted with Climate Change and Human Activities over the Southeast Qinghai–Tibet Plateau

“…Soil moisture is also affected by climate change and simultaneously involved in a number of feedback loops at the local, regional, and global scales [32,33], and the effects of soil moisture on vegetation growth have also been analyzed. Unequivocally, great spatial heterogeneity exists in vegetation responses to precipitation and surface air temperature, which is especially apparent in the high-altitude regions with significant sensitivity and vulnerability to environmental changes [34][35][36].…”