Tree height and stem growth dynamics in a Scots pine dominated boreal forest

Tree growth is a key forest ecosystem service and essential for carbon sequestration and biomass production. However, the intra-seasonal dynamics of tree height and stem diameter growth have been difficult to measure hampering the understanding of the interplay between these processes. Here, we investigated the feasibility of a laser scanning system in monitoring tree height development, aiming to study how tree height and stem diameter growth dynamics vary and interact, and how environmental variables explain the tree growth dynamics within a growing season. The experimental design consisted of 40 boreal trees equipped with dendrometers measuring changes in the stem diameter at 15-minutes intervals while a laser scanner fixed to a 35-meter tower was used to measure tree height near-daily during the monitoring period from May to mid-August 2021. We found that vertical changes in the tree-segmented point clouds enabled monitoring of tree height increment and investigation of the temporal dynamics of changes in tree height and stem diameter, when coupled with dendrometer measurements. The experiments revealed that, on average, height increment occurred ahead of diameter increment and deviated more towards the late season. Norway spruce showed more delayed diameter increment than Scots pine during the late season. Silver birch experienced diameter increment ahead of height increment. Based on the dendrometer measurements, we computed a radial change response function that aimed at characterizing the current state of stem diameter development, whether it was increasing or decreasing from its past state. When these radial change responses were compared against environmental variables, we found that the radial change response was mostly controlled by balance between precipitation and evapotranspiration, soil water content, minimum daily temperature, and vapor pressure deficit. Our findings support the utilization of laser scanning time series for measuring intra-seasonal changes in tree height and increase our understanding of the interactions between tree height and stem diameter growth processes.