Saline-alkali stress represents a major environmental constraint that severely inhibits plant growth and development. Brassinosteroids (BRs) are important phytohormones governing plant growth and abiotic stress adaptation, a process mediated by the BZR transcription factor family. However, their specific regulatory role in coordinating the response to salt stress remains unclear in Betula platyphylla (birch). This study demonstrates that overexpression (OE) lines of BpBZR1-6 exhibited enhanced salt tolerance, and BR application further alleviated damage under salt stress. In contrast, RNAi-silenced (SE) lines exhibited increased salt sensitivity, indicating that BpBZR1-6 positively regulates salt stress response via BR signaling. Mechanistically, BpBZR1-6 induced P5CSs expression, resulting in the proline content in OE lines was significantly higher than that in WT and SE lines. Furthermore, BpBZR1-6 promotes the expression of SOD and POD genes, thereby increasing SOD and POD activities, consequently, OE lines reduced H2O2 and malondialdehyde (MDA) levels. Meanwhile, exogenous BR application, the optimal BR concentration was 0.20 mg/L, enhanced antioxidant enzyme activities and decreased oxidative damage. Additionally, ChIP assays, dual-luciferase assays and RT-qPCR confirmed that BpBZR1-6 binds to the BpDWARF promoter truncation containing E-box and regulate its expression, exhibiting distinct expression patterns across different genetic backgrounds and in response to BR treatments. In summary, BpBZR1-6 is involved in regulating the salt stress response in birch, and application of an appropriate concentration of BR can effectively mitigate salt-induced damage. These findings provide a theoretical foundation for breeding salt-tolerant birch varieties and their practical application.

Projected warming and heat wave frequency may disproportionately impact growth and survival of northern tree species at their southern range limits in the boreal-temperate ecotone in North America. However, the extent to which geographic range and shade tolerance influence species' responses to warming remains uncertain. We investigated the effects of 12 levels of ex situ warming on growth, mortality, and physiology of seven tree species from the boreal-temperate ecotone, each with different southern range limits and shade tolerances. White pine (Pinus strobus), a southern temperate conifer, maintained photosynthetic capacity following prolonged warming and displayed the highest peak growth temperature. However, despite sharing a similar southern range limit, shade-tolerant eastern hemlock (Tsuga canadensis) exhibited negligible growth responses and significant foliar damage under warming. Jack pine (Pinus banksiana), a light-demanding northern boreal conifer, showed much less foliar damage but reduced photosynthetic capacity under warming. Shade-tolerant species exhibited greater foliar damage and mortality under high temperatures, while light-demanding species exhibited more tolerance regardless of range limits. We conclude that while geographic range explains some responses to warming, shade tolerance may be equally important. These findings provide empirical data to improve forest model accuracy and inform management strategies for forest regeneration and ecosystem stability.

Dalbergia odorifera T. Chen, an economically and medicinally valuable tree, suffers from heartwood shortage due to slow natural formation. Ethylene has emerged as a potent inducer of heartwood formation in D. odorifera, but long-term evidence and mechanisms remain unclear. To bridge this gap, we systematically assessed dynamic changes in morphology, anatomy, metabolomics, and antimicrobial properties of ethylene-induced heartwood zone over one, three, and five years, comparing it to 20-year-old natural heartwood. Results showed that in the ethylene-stimulated zone, heartwood resins were initially deposited at the outermost layer and progressively infiltrated inward until full saturation. The coloration of induced heartwood zone gradually intensified, eventually developing hues and grain patterns resembling natural heartwood by five years. Heartwood extracts yield and antimicrobial activity matched natural heartwood after one year. Total flavonoid content matched natural heartwood by three years, with nine flavonoid components (e.g., naringenin, formononetin) governing color development. While volatile compounds composition matched natural heartwood after five years, flavonoid profiles remained distinct, indicating volatile compounds biosynthesis preceded flavonoids. More interestingly, this study proposed a novel hypothesis for heartwood formation: during natural heartwood development, a certain factor (e.g., ethylene) induces a transition zone containing numerous "micro-zones", where cell death and heartwood substance deposition progress gradually from the outer to inner layers. This pattern leaves residual sapwood within developing heartwood, effectively resolving the theoretical conflict regarding the presence of living cells in heartwood. Thus, this study not only confirms the long-term feasibility of ethylene-induced heartwood zone formation and its underlying mechanisms, but also proposes a groundbreaking hypothesis that challenges conventional understanding of heartwood formation, laying a crucial foundation for future research on heartwood formation mechanisms and targeted cultivation of high-value tree species.

Temperate deciduous trees depend on stored carbon for their growth and metabolic processes in early spring. Despite their importance, annual changes in the contribution of stored carbon to the development of new organs are not adequately understood. We conducted 13C labelling experiments on apple (Malus domestica) saplings and a mature tree grown under normal conditions, and investigated 13C concentration in new aboveground organs in early spring for several years after labelling. In the saplings, assimilated 13C during the growing season was detected in the terminal buds around the bud break from the first to the fourth year after labelling. Similar results were found in the flower buds, leaves, and annual shoots of mature trees pink bud stage, in which the concentration of assimilated 13C was detected from the first to the third year after labelling. The concentration of assimilated 13C in the new organs of saplings and mature trees decreased exponentially over time after labelling. The reduction patterns of the assimilated 13C in the new organs of the saplings and mature trees fit better with the two-pool than the one-pool exponential models, indicating that while stored carbon in fast and slow pools was included in the new organs, almost all stored carbon used was from the fast pool. Our findings also suggested that the use of stored carbon in the development of new organs in early spring would not differ between saplings and mature trees.

Savannas cover a significant portion of the earth's land surface, yet how they will respond to increases in rainfall variability and drought frequency and intensity expected with climate change, remains poorly understood. Studies of hydraulic-related traits of savanna trees are rare with most existing research focusing on temperate and tropical forest species. We measured growth, photosynthetic rates, monthly predawn and midday xylem pressure potentials and eight traits relevant to xylem, leaf safety and water storage capacity in six co-occurring Southern African semi-arid savanna species. The six species adopted different hydraulic strategies, ranging from drought tolerance (e.g. high WD, low xylem vulnerability to cavitation and ${\mathcal{\psi}}_{\mathcal{tlp}}$) in Dichrostachys cinerea to drought avoidance (e.g. high capacitance and shoot saturated water content) in Terminalia sericea. Drought avoiding species with high capacitance had higher growth and photosynthetic rates while drought tolerant species had slow growth and low photosynthetic rates, when soil water was not limiting. The different hydraulic strategies found in the six study species suggest that savanna tree species exploit different ecohydrological niches, likely contributing to their co-existence in an environment where rainfall and soil water availability is highly variable. All of the strategies allowed for survival during shorter growing season droughts. Previous studies have shown that both drought avoiders and tolerators may be vulnerable to mortality during more extensive droughts in savannas. We suggest that access to deeper soil water combined with higher capacitance, as found in Sclerocarya birrea, appears to be the most successful strategy to survive extensive drought.

Interpreting tree-ring oxygen isotope composition (δ18Oring) is complicated by the combined influences of source water (δ18Osw) and relative humidity (RH). This study investigates intra- and interannual δ18Oring signals in Scots pine stands in southern and northern Finland over a ten-year period (2010-2019). We applied correlation analysis and process-based intra-annual δ18O and δ13C modeling to disentangle RH and δ18Osw signals in δ18Oring. Growth models for xylogenesis were used to date the analyzed tree-ring subsections. Dual isotope modeling provided additional constraint to evaluate the uncertainties caused by xylogenesis. Our results show that δ18Oring signals were dominated by RH, due to its much higher relative variability compared to that of δ18Osw. Generally, correlations were stronger at inter-annual than intra-annual resolution. Modeling indicated that additional factors complicate the interpretation of intra-annual δ18Oring signals beyond the combined effects of RH and δ18Osw. We show that seasonal variations in the proportion of oxygen exchange with source water during the during pathway to tree-ring cellulose may explain the lower RH signal at intra-annual resolution. Incorporating variable oxygen exchange improved model performance and aligned modeled δ18Oring more closely with observations. Despite the encouraging modeling results using growth models for dating tree-ring subsections, we recognize that time integration and alignment will continue to challenge the interpretation of intra-annual isotope signals. Our study demonstrates that combining empirical data analysis with mechanistic modeling is essential for resolving the environmental drivers of δ18Oring and for extending interpretations beyond site-specific conditions. Our findings are particularly relevant as intra-annual δ18O analysis becomes more common, underscoring the importance of time integration and dating tree-ring subsections, highlighting future research needs (e.g., varying oxygen exchange), and advancing their use for climate reconstruction.

Leaf minimum conductance (gmin) is important in determining plant responses to drought. However, we do not understand how gmin is related to drought tolerance, and how important it is in determining the time to reach physiological breakpoints during dehydration. In 18 coexisting tropical species we quantified gmin to test relationships with mild, moderate and severe dehydration thresholds associated with tolerance to turgor loss, structural integrity breakdown, and disruption of Photosystem-II function. We tested the relative importance of gmin and thresholds in determining time to reach physiological breakpoints, quantified other hydraulic and functional traits, and used principal component analyses to identify the major axes of trait variation. The significant variation observed in gmin across species was unrelated to thresholds for mild and severe dehydration, and consistently explained variation in time to reach critical levels of dehydration. gmin was negatively related to maximum stomatal conductance, but unrelated to most other functional and hydraulic traits. These results highlight the importance of avoiding dehydration via minimizing gmin, and suggest that avoidance and tolerance to dehydration represent independent strategies for leaves to cope with drought. Thus, integrating gmin with contemporary threshold-based metrics is essential for a more comprehensive understanding of species vulnerability to drought.

Litter decomposition is a key process regulating terrestrial carbon and nutrient cycling. Mycorrhizal association is a fundamental trait shaping both litter chemistry and root-microbe interactions, yet its role in regulating litter decomposition remains unclear in species-rich subtropical forests. We conducted two in-situ litter decomposition experiments to disentangle the effects of mycorrhizal type and stand ECM dominance on litter decomposition in subtropical forest. A one-year field litter decomposition experiment of 21 AM and 6 ECM tree species indicated that AM litter initially decomposed faster than ECM litter, primarily due to lower leaf dry matter content (LDMC), but this difference disappeared over time. Across the entire year, litter nutrient concentrations were the primary determinants of mass loss. AM and ECM litter decomposition in plots with a natural gradient of ECM tree dominance showed that decomposition rates of both AM and ECM litter decreased with increasing ECM dominance at the stand level. This pattern was linked to lower soil N availability, higher fine-root biomass, and reduced saprotrophic fungal abundance, suggesting that intensified competition between ECM fungi and saprotrophs constrained litter decay. Our findings highlight that tree mycorrhizal strategy, especially ECM dominance at the community scale, regulates decomposition by coupling above- and belowground traits and processes. This emphasizes the importance of forest mycorrhizal dominance as a determinant of litter turnover and nutrient cycling in subtropical mountainous forests.

In forests of the southwestern US, the seasonality - or phenology - of tree growth is affected by a combination of limiting temperatures and water availability. But, in this topographically diverse area, temperature and precipitation vary by elevation and therefore may have differing effects on tree phenology across the landscape. Our over-arching research question was: how does variation in temperature and water availability drive differences in the timing of tree growth and water use across species and sites in northern Arizona? We analyze three years of high-frequency measurements of stem growth and sap flow velocity collected across five sites from 1400 to 2600 meters elevation. We supplemented these data with lower-frequency measurements of xylogenesis, budburst, and potential photosynthetic quantum efficiency. Our study species - Pinus ponderosa, Pinus edulis, Juniperus osteosperma, Juniperus scopulorum and Quercus gambelii - represent the dominant tree species across northern Arizona. We found that all metrics of tree phenology tended to be limited by water availability at lower elevations. Lower elevations had bimodal patterns of growth and sap flow velocity which reflected precipitation inputs. Higher elevations had more consistent unimodal patterns which aligned with daylength, and growth was rarely limited by water availability. Microcore data supported growth rates from dendrometers, but microcores were able to capture xylogenesis when dendrometers were unable to - surprisingly, even when stem diameter was shrinking due to water limitation. A machine learning model showed soil volumetric water content was the best predictor of radial growth and sap flow velocity at lower elevations but was only marginally better than daylength and temperature at higher elevations. Thus, tree activity in trailing-edge, low elevation forests was more impacted by moisture, and less affected by temperature, compared to forests at higher elevations. Results, which show that arid woodlands and forests in the American Southwest are adapted to grow opportunistically when neither water availability nor air temperature is limiting, are synthesized in a new conceptual model.