Expanding Blue Intensity applications: Exploring compression wood proportions in cross-sections of treeline Picea abies seedlings

Abstract

Blue Intensity (BI) has been widely used as a proxy for wood density in dendrochronology, yet its applications remain underexplored in treeline ecology. Moving beyond traditional growth ring analyses, we expanded the Blue Intensity scope by quantifying the high wood density proportions (HighWD), including two HighWD subcategories: latewood and compression wood, in the stems of Picea abies seedlings from Central European treelines. We used BIcounter, a novel tool based on the CooRecorder BI algorithm, to quantify pixel distributions across blue light intensities and estimate HighWD areas as proportions of the total cross-section. Our approach combined BIcounter with densitometry analysis in CooRecorder (both based on BI measurements), allowing us to study compression wood occurrence as a continuous variable, which enhances statistical models' robustness. To estimate and compare means and variance of HighWD, latewood and compression wood proportions, and to quantify those estimates' uncertainty, we constructed Bayesian generalized linear models. HighWD occupied nearly half of the cross-section of treeline seedlings (Mean = 0.39, S.D. = 0.04) and did not differ between treeline sites. Stem eccentricity was generally high and did not correlate with compression wood proportions. As compression wood accounted for up to 43 % of the treeline seedlings' stem cross-section, its occurrence could affect growth and survival, highlighting the importance of considering compression wood proportions in future ecological research. With this study, we outline the potential of BI for applications in and dendroecology, and suggest that future research could explore its use in other dendrochronological sub-disciplines, such as dendrogeomorphology.