From Roots to Leaves: Tree Growth Phenology in Forest Ecosystems

Purpose of Review: This review synthesizes recent advancements and identifies knowledge gaps in the tree growth phenology of both belowground and aboveground organs in extra-tropical forest ecosystems. Phenology, the study of periodic plant life cycle events, is crucial for understanding tree fitness, competition for resources, and the impacts of climate change on ecosystems. By examining the phenological processes of various tree organs, the review aims to provide a comprehensive understanding of how these processes are interconnected and how they influence overall tree growth and ecosystem dynamics. The review aims to provide a comprehensive overview of current knowledge, highlight recent technological advancements, and identify critical areas where further research is needed. Recent Findings: The review highlights significant progress in monitoring leaf and canopy phenology, thanks to advancements in remote sensing and automated observation systems. These technologies have enhanced our ability to track seasonal changes in leaf development and canopy dynamics more accurately and over larger areas. There has also been a substantial increase in research on wood formation in stems, expanding beyond northern hemisphere conifers to include a broader range of functional groups. However, despite these efforts, identifying the precise drivers of wood formation remains challenging, necessitating further integration of molecular and eco-physiological insights. A critical area of focus is root phenology, encompassing both primary and secondary growth. Despite the fundamental role of roots in tree physiology and ecosystem dynamics, our understanding of root phenology remains limited, primarily due to the inherent difficulties in monitoring root growth. The review emphasizes the need for more detailed studies on root growth processes and the development of new methodologies and technologies to improve root phenology assessments. Summary: The review highlights the importance of incorporating eco-physiological insights into phenological assessments. Leaf and canopy phenology would benefit from more studies focusing on autumnal events. Indeed, compared to the onset of the growing season, much less is known about its end, despite its critical importance for understanding processes such as carbon uptake and nutrient cycle. Advancing knowledge of wood growth phenology will require greater focus on angiosperms, as research on xylogenesis has historically been centered on gymnosperms. This will likely necessitate the development of new, tailored methodologies to address the characteristics of angiosperm wood formation. Similarly, further exploration of phloem phenology is essential to better understand the links between phenological processes across different organs. Finally, compared to other organs, root growth remains less well understood, underscoring the need for deepening the investigation on root phenology in the coming years.