Structural diversity, tree species richness and biomass production in new forests on agricultural land

Changes in forest structure (FS) significantly influence forest conditions and dynamics, ecosystem services, including biomass accumulation and biodiversity. Several LiDAR metrics have been described to analyse FS at a high spatial resolution, including vertical and horizontal attributes. This study aimed to investigate FS development in spruce (Picea abies), oak (Quercus robur) and beech (Fagus sylvatica) through the first 50 years after planting and their relationship to biomass stocks and richness of arrival tree species. LiDAR and ground truth data were extracted from 60 plots. To characterize the FS, LiDAR metrics were calculated using a point cloud of 5.4 pulses/m2 under leaf-off conditions from 2019. Moreover, trees were measured in 2022 following the Danish National Forest Inventory protocols to calculate biomass and estimate tree species richness. FS changes and their relationship were analyzed by employing linear mixed models and principal component analysis (PCA). Results show that spruce stands exhibited rapid growth in height and the highest canopy cover values over time. Contrarily, oak stands developed a multi-layer canopy, increasing light availability and tree size variability with age. At later stages, beech stands reached similar height as spruce and showed the highest standard deviation of height (StDH) with age. The PCA revealed that tree maximum height and StDH were primarily age-dependent, while the internal vertical and horizontal variability tree species dependent. Spruce showed a constant homogeneous FS, oak was characterized by a high internal and external FS heterogeneity, while beech gradually decreased the FS variability over time. Old, tall and structurally homogeneous beech and spruce stands supported the highest biomass accumulation. Conversely, more structurally heterogeneous oak stands tended to harbor greater tree species richness. The study underscores the importance of incorporating both vertical and horizontal dimensions to understand FS changes and the trade-offs between biomass production and biodiversity in monoculture afforestation.