Data and code from: Pre-industrial land-use limits contemporary shrub encroachment in the French Alps

Shrub encroachment has become a global phenomenon in recent decades. While
global warming in the Arctic is often cited as the primary cause,
human-managed mountain regions have experienced intense historical
land-use that may also play a considerable role. Shrub encroachment has
significant implications for biodiversity, carbon storage, and
transformation of mountain landscapes. However, disentangling between the
effects of present-day environmental conditions and historical land-use
trajectories in explaining present-day shrubland distribution remains
challenging. We use early 19th-century land registry records — rare
historical sources documenting past land-use and vegetation cover — to
assess whether these legacy conditions influence present-day shrub
distribution. By combining these archive data with dendrochronological
sampling and a diachronic analysis of shrub expansion based on historical
aerial photographs, we provide a comprehensive analysis of shrub
colonisation. Our results reveal an increase in shrub cover over the past
two centuries, primarily driven by the expansion of common juniper
(Juniperus communis). Dendrochronological analyses from never mown or
ploughed plots indicate that shrub recruitment began around 1870,
intensified until the 1970s, and then slowed slightly thereafter. While
shrubs preferentially establish on steep, south-facing slopes and
topographically rough terrain, their distribution remains highly
heterogeneous and is strongly shaped by pre-industrial land-use patterns.
Parcels that were ploughed or mown 200 years ago remain largely devoid of
shrubs, even decades after mowing ceased. In contrast, surrounding areas —
whether grazed or unused — have experienced shrub encroachment. We
hypothesize that the low shrub density on former ploughed lands and hay
meadows may reflect persistent legacy effects of dense, competitive
herbaceous communities, which hinder the establishment of woody species.
This study demonstrates that pre-industrial land-use continues to
constrain both the distribution and recent dynamics of shrub expansion,
highlighting the importance of accounting for land-use legacies when
interpreting contemporary shrub encroachment.