Automated analysis of intra-annual density fluctuations reveals climate-sensitive and genetically variable wood traits

Understanding how trees adjust their wood structure to increasing climate variability is critical for predicting forest resilience. In this study, we examined intra-annual density fluctuations (IADFs) in the boreal black spruce as indicators of plastic and genetic responses to water stress. IADFs are abrupt deviations in the density of wood formed within a growth ring and reflect temporary disruptions in cambial activity. We assessed their occurrence in 24-year-old trees from controlled-cross families planted in two climatically distinct common gardens in eastern Québec. Using an automated method, we quantified both the frequency and the structural characteristics of IADFs, including their width, height, and area. Our results show that earlywood IADF frequency was strongly correlated with local climate indices related to water deficit, indicating that IADFs are sensitive biomarkers of both current-year and lagged drought conditions. IADF frequency was positively associated with radial growth but negatively with earlywood and total wood density, suggesting a physiological trade-off between maintaining growth under water stress and investing in wood structural properties. Moreover, moderate levels of genetic control indicated that these traits are partially heritable. By integrating IADFs, climatic, and genetic data, our approach provides new insights into how a major boreal tree species modulate wood structure in response to environmental stress. It thus represents a promising framework for studying drought-response and support the selection of more climate-resilient trees.