Seven-year UAV LiDAR time series reveals shrub dynamics and herbivory effects in a rewilded grassland

Rewilding initiatives are currently being intensely debated, as a measure to restore natural processes and enhance biodiversity. However, it is yet to be clarified which are the best conditions to achieve the goals of a dynamic and self-sustaining ecosystem as a result of removing anthropogenic pressures. A prominent case of this debate is shrub encroachment of grasslands which, in temperate mesic North European grasslands, is widely seen as a threat to plant biodiversity and a balanced ecosystem nutrient cycling. In this study, we use an unstaffed aerial vehicle-based light detection and ranging (UAV LiDAR) system to examine: (i) the year-to-year horizontal spread of shrubs in a rewilded grassland, influenced by processes such as resprouting, colonization, and recruitment; (ii) changes in the vertical biomass distribution of individual plants due to ontogenetic shifts in biomass allocation, herbivory, and growth; (iii) allometric changes of individual plants caused by phenology, by analyzing in detail biomass shifts within 1-year cycle; and (iv) the shifts in vegetation biomass allocation under varying grazing regimes, by regulating large herbivore access through the use of exclosures. Specifically, by means of UAV LiDAR surveying and herbivory exclosure treatments, we monitored the 3-dimensional dynamics of woody shrubs in a grassland rewilding project (Mols Bjerge, Denmark), with a focus on the N-fixing leguminous species Cytisus scoparius. We monitored vegetation dynamics over a 7-year period (2018–2024) with annual time resolution, including a 1-year interval (2020–2021) during which surveys were conducted every two months. This enabled us to analyze interannual shrub encroachment as well as seasonal biomass dynamics within a single year. The results revealed that: at the single shrub level, the C. scoparius community stabilized its canopy height after the 4th year surveyed (2021), coinciding with an increased thicket canopy closure. At the group level, shrub thickets enhanced overall coverage after thicket canopy closure, and vertical growth shows spatial clustering. Finally, at the plot level, the analysis of experimental plots shows that (i) herbivory has a stabilizing role both in interannual and within-year vegetation dynamics, but most prominent in the latter; (ii) different temporal herbivory exclosure treatments result in statistically different vertical distributions of overall vegetation; and (iii) the herbivory effect on vegetation diminishes following particularly environmentally harsh years (i.e. drought). In sum, this study presents a data-driven approach to understanding the role of herbivory in shaping naturally occurring vegetation dynamics in grasslands, providing a foundation for (i) ecosystem modeling for forecasting rewilded scenarios, (ii) vegetation modeling by tracking shrub encroachment, and (iii) developing best practices for adaptive sustainable ecosystem restoration.