Combined effects of topography, soil moisture, and snow cover regimes on growth responses of grasslands in a low- mountain range (Vosges, France)

Abstract. Growth responses of low-mountain grasslands to climate warming are poorly understood despite very large surfaces being covered in central Europe. These grasslands are characterized by still-present agricultural exploitation and complex topographical features that limit species migration and increase differences in snow regimes. This study examined MODIS surface reflectances between 2000 and 2020 across the Vosges mountain grasslands to investigate trends and drivers of spatial patterns in the annual maximum NDVI (normalized difference vegetation index). We found that a majority of grasslands indicate no significant trends, which possibly reveals several environmental and ecological compensatory effects in response to warming in the Vosges Mountains. We also noted hotspots of browning grasslands (a decrease in annual maximum NDVI), largely overrepresented compared to the greening ones (an increase in annual maximum of NDVI), a pattern which contradicts most productivity signals highlighted in European high-mountain grasslands. Spatial patterns of browning are enhanced on north-facing slopes and at low elevations (<1100 m), where high-producing grasslands with dominant herbaceous communities prevail. A low soil water recharge also appears to be pivotal in explaining the probability of browning at the study site. Through the use of winter habitat indices, we noted high responsiveness of low-mountain grasslands to differences in intra-seasonal snow regimes, partly modulated by topographic features. Prolonged and time-continuous snow cover promotes higher peak productivity and shortened green-up periods. A high number of frost events results in lower peak productivity and prolonged green-up periods. We hypothesize that observed growth responses in the Vosges Mountains are indicative of long-term future responses to climate warming in high-mountain ranges. With shorter and more discontinuous snow cover, we expect higher diversity of growth responses in European low-mountain grasslands due to strong contextual effects and high terrain complexity.