Monitoring complex vegetation trajectories in open and semi-open habitats of the Mediterranean Pyrenees

Mountainous regions are characterised by unique biodiversity and provide essential ecosystem services. However, they are also particularly vulnerable to global change. In the Mediterranean mountains, climate change has a significant impact on plant productivity and phenology, through an increase in extreme drought events and changes in snowfall and temperature patterns. Additionally, the progressive abandonment of pastoralism is leading to profound changes in open and semi-open habitats (subalpine grasslands, heathlands and forest ecotones) through the densification and recolonisation of woody species (ericaceous shrubs and conifers in particular). There is an urgent need to develop a more profound understanding of these complex vegetation dynamics in order to inform the development of coherent guidelines for stakeholders in mountainous regions, including livestock farmers and nature reserve managers, to help them adapt their practices to preserve both foraging resources and rich biodiversity. In order to monitor past and current vegetation dynamics in open and semi-open habitats, we propose applying a method of classifying ecological trajectories into nine highly interpretable categories. This method involves adjusting a second-degree polynomial function and provides insight into the direction and acceleration of the studied trajectories (Rigal et al., 2020). To describe changes in vegetation functioning and composition, we calculated the Dynamic Habitat Index (DHI) using Landsat and Sentinel-2 time series data. Finally, we aimed to identify the impact of potential climate-related drivers in different contexts of agro-pastoral management. Our approach was applied to the alpine and subalpine landscapes of the Mediterranean Pyrenees in southern France, with the results highlighting the existence of differential trajectories depending on the habitat considered, and potentially the pastoral management practices in Catalan nature reserves with high biodiversity values. Our results encourage us to move beyond traditional remote sensing approaches based on calculating greening/browning trends to monitor vegetation dynamics patterns, providing a more detailed framework for analysing the complex changes affecting mountain ecosystems.