Significance: Forest trees can live for hundreds to thousands of years, and they play a critical role in mitigating global warming by fixing approximately 15% of anthropogenic CO ₂ emissions annually by wood formation. However, the environmental factors triggering wood formation onset in springtime and the cellular mechanisms underlying this onset remain poorly understood, since wood forms beneath the bark and is difficult to monitor. We report that the onset of wood formation in Northern Hemisphere conifers is driven primarily by photoperiod and mean annual temperature. Understanding the unique relationships between exogenous factors and wood formation could aid in predicting how forest ecosystems respond and adapt to climate warming, while improving the assessment of long-term and high-resolution observations of global biogeochemical cycles.

Late frost can cause damage to trees, especially to the developing bud of broadleaf species in spring. Through long-term adaptation, plants adjust leaf phenology to achieve an optimal trade-off between growing season length and frost avoidance. In this study, we aim to assess ecotypic differentiation in leaf development of sugar maple populations planted in a common garden. A total of 272 sugar maple seedlings from 29 Canadian provenances were planted at the northern boundary of the natural range, and the phenological phases of bud and leaf development were monitored during spring 2019. The wide geographical area under evaluation showed a complex seasonal pattern of temperature, with spring warming occurring later in the north and close to the sea. Overall, leaf development lasted between 20 and 36 days, from the end of May to end of June. We observed different timings and rates of leaf development among provenances, demonstrating the occurrence of ecotypes in this species. Minimum April temperatures of the original sites were able to explain such differences, while maximum April temperatures were not significant. Seedlings from sites with colder minimum April temperatures completed leaf development earlier and faster. On average, leaf development diverged by up to 6 days among provenances, with minimum April temperatures ranging from −3 to 3 °C. Our results demonstrated that the avoidance of late spring frost is a driving force of leaf development in sugar maple populations. In the colder sites, the growing season is a limiting factor for tree growth. Thus, when thermal conditions become favorable in spring, an earlier growth reactivation and high metabolic activity ensure a fast leaf emission, which maximizes the period available for photosynthesis and growth. These patterns demonstrate the long-term phenological adaptation of sugar maple populations to local climatic conditions and suggest the importance of frost events for leaf development.

The allocation of plant biomass above and below ground reflects their strategic resource utilization, crucial for understanding terrestrial carbon flux dynamics. In our comprehensive study, we analyzed biomass distribution patterns in 580 broadleaved and 345 coniferous forests across China from 2005 to 2020, aiming to discern spatial patterns and key drivers of belowground biomass proportion (BGBP) in these ecosystems. Our research revealed a consistent trend: BGBP decreases from northwest to southeast in both forest types. Importantly, coniferous forests exhibited significantly higher BGBP compared to broadleaved forests (p < 0.001). While precipitation and soil nutrients primarily influenced biomass allocation in broadleaved forests, temperature and soil composition played a pivotal role in coniferous forests. Surprisingly, leaf traits had a negligible impact on BGBP (p > 0.05). Climatic factors, such as temperature and rainfall, influenced biomass partitioning in both strata by altering soil nutrients, particularly soil pH. These findings provide valuable insights into understanding carbon sequestration dynamics in forest ecosystems and improving predictions of the future trajectory of this critical carbon cycle component.

To understand the impact of climate change on the tree radial growth in Southeast China’s subtropical evergreen broadleaved forest, comparative research on the radial growth–climate associations of Schima superba was conducted. This dominant evergreen broadleaved tree species was examined at both its southern and northern distribution margins through dendroclimatology. The results showed that the radial growth of S. superba stands at a high elevation in the southern margin and stands in the northern margin were positively correlated with springtime temperatures, mostly in April (e.g., mean temperature: r = 0.630, p < 0.05) and May (e.g., maximum temperature: r = 0.335, p < 0.05), respectively. Meanwhile, the temperature in the late rainy season had a significant negative effect on the radial growth of S. superba stands in the southern margin, including high-elevation stands (e.g., the mean temperature in previous and current September: r = −0.437 and −0.383, p < 0.05) and low-elevation stands (e.g., the mean temperature in previous August and October: r = −0.577 and −0.348, p < 0.05). It was shown that temperature was the key climatic factor affecting the radial growth of S. superba, and the response of radial growth to temperature had obvious spatial differences. The findings indicate that the radial growth of S. superba stands in warm growth environments will be negatively impacted by future climate warming. On the contrary, the radial growth of S. superba stands growing in relatively cold growth environments may benefit from warmer spring. The results enhance the understanding of tree growth responses to climate change in the subtropical forests of China.

Atmospheric nitrogen (N) deposition and increasing precipitation affect carbon sequestration in terrestrial ecosystems, but how these two concurrent global change variables affect xylem growth in trees (i.e., independently or interactively) remains unclear. We conducted novel experiments in central China to monitor the xylem growth in a dominant species (Quercus acutissima Caruth.) in response to N addition (CN), supplemental precipitation (CW) or both treatments (CNW), compared with untreated controls (C). Measurements were made at weekly intervals during 2014–15. We found that supplemental precipitation significantly enhanced xylem growth in the dry spring of 2015, indicating a time-varying effect of increased precipitation on intra-annual xylem growth. Elevated N had no significant effect on xylem increment, xylem growth rate, and lumen diameters and potential hydraulic conductivity (K_s) of earlywood vessels, but K_s with elevated N was significantly negatively related to xylem increment. The combination of additional N and supplemental precipitation suppressed the positive effect of supplemental precipitation on xylem increment in the dry spring of 2015. These findings indicated that xylem width was more responsive to supplemental precipitation than to increasing N in a dry early growing season; the positive effect of supplemental precipitation on xylem growth could be offset by elevated N resources. The negative interactive effect of N addition and supplemental precipitation also suggested that increasing N deposition and precipitation in the future might potentially affect carbon sequestration of Q. acutissima during the early growing season in central China. The effects of N addition and supplemental precipitation on tree growth are complex and might vary depending on the growth period and local climatic conditions. Therefore, future models of tree growth need to consider multiple-time scales and local climatic conditions when simulating and projecting global change.