Seasonal patterns between wood formation and non-structural carbohydrate in two conifers with distinct life-history traits

Understanding the dynamics of wood formation and non-structural carbohydrates (NSCs) in trees is essential for unraveling mechanisms underlying carbon allocation and storage. While temperature is widely acknowledged as a key driver of growth initiation in temperate trees, the relative contributions of air and soil temperatures remain poorly quantified. In addition, the role of temperature in regulating the cessation of wood formation has received limited attention. In this study, we monitored intra-annual wood formation of Picea koraiensis and Larix olgensis across an altitudinal gradient and assessed seasonal variation in NSC concentrations in leaves, xylem, and phloem in Changbai Mountain. Growth onset was delayed by 3.3 per 100 m increase in altitude for Picea and 2.5 days for Larix. Picea initiated growth earlier and ceased later than Larix, reflecting species-specific phenological strategies. Growth onset was primarily driven by cumulative thermal conditions, particularly growing degree days, whereas cold degree days emerged as the sole temperature-related factor significantly associated with growth cessation. Despite contrasting seasonal patterns of NSC dynamics between deciduous Larix and evergreen Picea, the mobilization of NSCs was closely synchronized with cambial activity in both species. Starch concentrations reached their annual minima, and sugar concentrations peaked during periods of maximum width of cell enlargement and wall thickening, suggesting a conserved physiological mechanism for carbon utilization during xylem development. These findings reveal the interplay between temperature, phenology, and NSC dynamics in regulating xylem growth and carbon allocation. By elucidating these processes, this study advances our understanding of how temperate forests may respond to shifting climatic conditions.