Repeated Airborne Laser Scanning for Analyzing Drought-Related Crown Dynamics in Mature Norway spruce

Repeated airborne laser scanning (ALS) offers new opportunities to quantify individual-tree structural dynamics over time. In this study, we analysed annual to multi-year changes in height growth and crown structure of mature Norway spruce in southern Sweden using repeated ALS acquired in 2016, 2017, 2019, and 2022. Individual trees were delineated from normalized point clouds, and tree height increment, maximum crown radial extension, crown projection area, and crown-boundary metrics were derived to evaluate temporal structural change. To interpret the results in relation to recent climate extremes, the observation period was divided into pre-drought (2016–2017), during-drought (2017–2019), and post-drought (2019–2022) phases, and structural changes were expressed on a yearly basis. Tree height increased significantly in all periods. Annualized median height growth was 24.9 cm yr^-1 before drought, 26.5 cm yr^-1 during drought, and 16.8 cm yr^-1 after drought. In contrast, annualized maximum crown radial expansion was limited before drought (2.4 cm yr^-1), peaked during drought (19.0 cm yr^-1), and was nearly absent after drought (0.17 cm yr^-1). Crown-boundary metrics suggested an upward shift of the upper crown and a reorganization of the middle crown over time, lower-crown estimates were more uncertain. Driver analyses showed that height growth was mainly related to tree size before and during drought, whereas post-drought growth became more influenced by stand competition. Overall, this study shows that repeated ALS is a useful tool for analysing crown structural dynamics during and after drought, providing a promising basis for monitoring how tree architecture responds to stress.