Abstract
Since the 1980s, vegetated lands have experienced widespread greening at the global scale. Numerous studies have focused on spatial patterns and mechanisms of this phenomenon, especially in the Arctic and sub-Arctic regions. Greening trends in the European Alps have received less attention, although this region has experienced strong climate and land-use changes during recent decades. We studied the rates and spatial patterns of greening in an inner-alpine region of the Western Alps. We used MODIS-derived normalized difference vegetation index (NDVI) at 8-day temporal and 250 m spatial resolution, for the period 2000–2018, and removed areas with disturbances in order to consider the trends of undisturbed vegetation. The objectives of this study were to (i) quantify trends of greening in a representative area of the Western Alps; and (ii) examine mechanisms and causes of spatial patterns of greening across different plant types. We show that 63% of vegetated areas experienced significant trends during the 2000–2018 period, of which only 8% were negative. We identify (i) a climatic control on spring and autumn phenology with contrasting effects depending on plant type and elevation, and (ii) land-use change dynamics, such as shrub encroachment on abandoned pastures and colonization of new surfaces at high elevation. Below 1500 m, warming temperatures promote incremental greening in the transition from spring to summer, but not in fall, suggesting either photoperiod or water limitation. In the alpine and sub-alpine belts (>1800 m asl), snow prevents vegetation development until late spring, despite favorable temperatures. Instead, at high elevation greening acts both in summer and autumn. However, photoperiod limitation likely prevents forested ecosystems from fully exploiting warmer autumn conditions. We furthermore illustrate two emblematic cases of prominent greening: recent colonization of previously glaciated/non vegetated areas, as well as shrub/tree encroachment due to the abandonment of agricultural practices. Our results demonstrate the interplay of climate and land-use change in controlling greening dynamics in the Western Alps.
Highlights
Since the 1980s, vegetated lands have experienced widespread greening at a global level [1]
The Breaks For Additive Seasonal and Trend (BFAST) method applied to the 2000–2018 normalized difference vegetation index (NDVI) imagery on Aosta Valley revealed that 31% of pixels showed a disturbed time series characterized by one or more break points (Table 2)
Different plant functional types were characterized by distinct absolute trend values; deciduous broadleaf forests (DBF) and meadows had the highest rates of greening, followed by evergreen and larch forests (DNF)
Summary
Since the 1980s, vegetated lands have experienced widespread greening at a global level [1]. The Alps represent a key area where climate and land-use changes are already strongly influencing ecosystem processes, and shifts in vegetation structure have immediate consequences for services provided to densely populated alpine valleys. Temperature increase is about double than the global average [9,10], and snow cover duration has decreased above 2000 m in the last 50 years [11], climatic trends since the 1990s are weaker [12]. A recent study [18] documented widespread greening in grassland, low-shrub, and scree slopes of a protected area in the French Alps and proposed rising temperatures and decreased snow-cover duration as the main drivers. 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]
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