Abandonment of less productive grasslands in European mountains leads to colonization by shrubs, which climate change is expected to further favour. These changes in plant biodiversity and associated biota profoundly alter grassland biogeochemical functioning and ecosystem services (ES). LUCSES aims to develop understanding of nitrogen (N) and water-cycling processes in shrub-encroached mountain grasslands to support novel trait-based models for predicting global change effects on mountain grasslands. The core hypothesis of LUCSES is that shrub colonization induces a tipping point in relationships between plant economics traits and processes of N- and water-cycling, which can be related to specific shrub traits and to mycorrhizal functions. LUCSES will take significant steps towards reducing limited trait and functional knowledge for mountain shrubs. While a first step has been taken across the cold biome based largely on ‘soft’ traits, LUCSES aims to address more directly functional processes affecting soil N and water dynamics using ‘harder’ traits and considering mycorrhizal associations. In particular, the biochemical nature and functions of alpine shrub secondary compounds are poorly elucidated, due to the technical barriers of their analyses in the field. LUCSES will improve methods for screening large numbers of plant samples and for detecting intraspecific variation in experimental conditions by combining biochemical analysis and IR spectrometry. Functions of mountain shrub mycorrhizae and their responses to critical climate modifications such as early snowmelt and drought have never been described. Seasonal dynamics of N and mycorrhizae remain both a scientific and a technical challenge due to their lability, and functional effects of changes in snow cover are a research frontier. By analyzing at our French and Austrian long-term research sites variation along encroachment gradients of plant traits specific to deciduous vs. evergreen shrubs and mechanisms associated with their mycorrhizae, we will elucidate pathways of effects of combined land use and climate change. We will use lysimeters to analyze full water balance, overcoming the scientific barrier of linking changes of functional traits to water-related ecosystem functions. Resulting structural equation models will be used to develop trait-based ES models, parameterised from field surveys and climate manipulations. ES models application to land use change trajectories from past to present, and to future scenarios will highlight trade-offs and adaptation options from shrub encroachment at our two sites and for the Ecrins National Park (France). The LECA (lead: B. Mouhamadou) and University of Innsbruck (lead: G. Leitinger) teams comprise extensive and complementary scientific and technical expertise, and very experienced staff in all relevant aspects for the project of plant functional ecology, microbial ecology, climate change experiments and ecosystem service modelling. LUCSES results will help building capacity on poorly known, yet inevitable ecosystem transformations and support management and decisions by our socio-economic partners. We thus intend to support medium- and long-term climate change adaptation in mountain regions by providing decision makers and the public with evidence on options from managing ecosystems and landscapes.