Forest soils play a critical role in the global carbon cycle, however, in boreal secondary forests, the effects of root and litter inputs on soil greenhouse gases (GHG) dynamics remain poorly understood. To address this gap, we conducted a three-year field experiment measuring soil CO2 and CH4 fluxes in birch (Betula platyphylla) forests of three stand ages (30 yr, 45 yr, and 66 yr) in the Daxing'an Mountains of China. The experiment included five treatments: a control (CK), root removal (RR), litter removal (RL), litter addition (DL) and simultaneous removal of both roots and litter (RR+RL). Root and litter removal significantly reduced soil CO2 fluxes by approximately 30 %-50 %, with the strongest reductions observed when both inputs were removed simultaneously. In contrast, litter removal uniquely enhanced soil CH4 uptake across stand ages, whereas root removal and litter addition suppressed the CH4 sink. Doubling litter inputs stimulated CO2 emissions but consistently reduced CH4 uptake, suggesting that increased litterfall may intensify GHG imbalance in boreal forests. Redundancy analysis (RDA) revealed distinct stand age and treatment dependent controls on soil CO2 and CH4 fluxes. Hierarchy partitioning analysis indicated that carbon input manipulations shifted the dominant controls on CO2 fluxes on from substrate supply toward microbial regulation, while CH4 uptake was primarily modulated by nitrogen-related variables. Our findings highlight the combined influence of root and litter inputs on soil GHG dynamics, providing new insights into carbon balance regulation in climate-sensitive boreal ecosystems.

Background and AimsThe forest fires in the Daxing'an Mountains occurred at different times, resulting in secondary birch (Betula platyphylla) forests of different stand ages. However, the soil respiration dynamics and component responses of birch forests of varying ages remain poorly understood and quantified.MethodsDuring the growing season from 2022 to 2024, we investigated and measured the soil respiration and its components, as well as the influencing factors, of birch forests in the Daxing'an Mountains, China at three age gradients (30 yr, 45 yr, and 66 yr) using the trench method.ResultsThe results showed that the average total soil respiration (Rs) in the birch forests of this region ranged from 334.79-436.80 mgm(-2 )h(-1), with the order of 45 yr > 66 yr > 30 yr. The average soil heterotrophic respiration (Rh) ranged from 211.50-303.05 mgm(-2) h(-1), with the order of 66 yr > 45 yr > 30 yr. The average soil autotrophic respiration (Ra) ranged from 100.01-166.21 mgm(-2) h(-1), with the order of 45 yr > 66 yr > 30 yr. Rh/Rs ranged from 61.79%-75.26%, and Ra/Rs ranged from 24.74%-38.21%. Soil temperature (Ts) and soil dissolved organic carbon (DOC) significantly affected both Rs and Rh, while soil volumetric water content (VWC) only significantly influenced Rs.ConclusionsOur study reveals the patterns of changes in soil respiration over stand age and its components during the succession of birch forests, providing scientific basis for assessing the carbon dynamics of forest ecosystems at different developmental stages.

The autumn freeze-thaw period plays a critical role in regulating soil carbon dynamics in permafrost region. However, the dynamics of soil carbon dioxide (CO2) and methane (CH4) fluxes during the autumn freeze-thaw period in high latitude permafrost regions of China remain poorly understood. In our study, we measured soil CO2 and CH4 fluxes using the dark chamber method in three different stand ages of birch (Betula platyphylla Sukaczev) forests (30, 45, and 66 years old) in the Daxing’an Mountains permafrost region and explored their relationships with key soil factors. Mean soil CO2 and CH4 fluxes are 19.24 to 36.44 mg m–2 h–1 and –12.22 to –25.92 μg m–2 h–1, respectively during the study period. Both soil fluxes in 30 years forest stand were significantly lower than in the older stands (45 and 66 years, P < 0.05). Across stand ages, soil CO2 fluxes were strongly driven by soil temperature and dissolved organic carbon, whereas CH4 fluxes were jointly regulated by soil temperature and volumetric water content. These findings highlight the critical but underappreciated role of soil CO2 and CH4 dynamics during the autumn freeze-thaw period in permafrost carbon feedbacks, underscoring the need to explicitly include this period for ecosystem carbon budget in permafrost regions, especially under climate warming background.

Tree water use (Q) is a key component of forest water balance. However, the diurnal (Q d) and nocturnal (Q n) water use vary with tree size in boreal forests and remain insufficiently understood. Therefore, we continuously monitored sap flow of trees across large, medium and small diameter at breast height (DBH) classes in a boreal larch (Larix gmelinii) forest in China during the growing season (May-September) from 2021 to 2024. Concurrent measurements of environmental variables were performed to quantify the effects of tree size on Q d and Q n, as well as to elucidate the underlying environmental controls. The results revealed significant tree size effects on both Q d and Q n. Large trees exhibited an earlier onset and later cessation of daily water use in comparison to medium and small trees, accompanied by greater diurnal variability in water use. Both Q d and Q n increased significantly with increasing tree size, whereas the seasonal peak water use of large and medium trees occurred later than that of small trees throughout the growing season. The contribution of Q n to Q (Q n/Q) displayed a U-shaped seasonal pattern across all tree sizes. However, Q n/Q decreased progressively with increasing tree size, with average values of 17.96%, 13.29% and 12.33% for small, medium and large trees, respectively. The environmental controls on Q d, Q n and Q n/Q differed substantially among tree sizes. Q d was regulated by net radiation (Rn), daytime vapour pressure deficit (VPDd), deep soil water content (SWC40) and daytime air temperature (Tad). In contrast, nighttime vapour pressure deficit (VPDn) emerged as the dominant driver of Q n across all tree sizes, with its relative influence increasing as tree size decreased. Notably, the drivers of Q n/Q differed from those of Q d or Q n, with nighttime wind speed (Wsn), nighttime air temperature (Tan) and shallow soil water content (SWC10) identified as the primary regulating factors. Moreover, the influence of Wsn on Q n/Q declined with decreasing tree size, whereas the effects of Tan and SWC10 increased gradually. These findings demonstrate that tree size plays a critical role in shaping the patterns of Q d and Q n and their responses to environmental controls, and provide new insights into the interaction between water use strategies and tree growth in boreal forests under future climate change by explicitly accounting for tree size effects.

Soil C pools recover very slowly following severe wildfire disturbance. It remains unclear whether forest management could enhance C accumulation in post-fire degraded soils. This study investigated changes in soil C mineralisation in Betula platyphylla forests (formed through secondary succession on intensely burned sites) following thinning operations. To compare the effects of thinning, B. platyphylla forests at 2 years (2a), 14 years (14a), and 17 years (17a) post-thinning were compared with unthinned control (CK) plots. The results indicate that thinning significantly influenced C mineralisation in post-fire degraded soils at the 0-10 cm depth, while no significant differences in the 10-20 cm depth were observed. Compared to the CK, the greatest increase in cumulative soil carbon mineralisation occurred at the 0-5 cm depth, with 14a and 17a at + 20.35 % and + 19.31 % respectively. A non-linear equation was used to fit the kinetic equation for C mineralisation. Both 14a and 17a exhibited higher potential mineralisable C than CK at the 0-10 cm depth, yet lower mineralisation rates (K). Conversely, 2a demonstrated significantly higher K values than CK at this depth. This suggests that while the long-term effects of thinning may enhance soil carbon accumulation by reducing mineralisation rates, while the short-term effects exert a stimulating effect on soil C mineralisation. RDA and VPA analyses revealed that individual site-specific environmental factors explained over 79 % of carbon mineralisation variation, with primary influences being soil C fractions, C/N, Understorey Vegetation Biomass (UVB), and soil physicochemical properties. However, when analysing different post-thinning time points collectively, environmental factors explained 81.10 % (0-10 cm) and 66.20 % (10-20 cm) of soil C mineralisation variation in the heavily burned plot. Among these, UVB (shrubs and herbaceous plants) was the key regulator of soil C mineralisation at the 0-10 cm depth, explaining 41.50 % of variance. POC & EOC exhibited similar explanatory power across the 0-20 cm profile, accounting for 23.60 % (0-10 cm) and 28.90 % (10-20 cm), respectively. Pathway analysis further revealed that UVB radiation directly positively regulates C0. It also indirectly influences carbon mineralisation processes by promoting the formation of active organic carbon fractions. Furthermore, dissolved organic carbon directly contributes positively to carbon mineralisation rates, whilst soil bulk density consistently exhibits a negative regulatory effect. This study further indicates that understorey vegetation may represent a significant regulatory pathway in soil recovery processes involving forest management interventions.

Understanding the relative contributions of transpiration (T) and evaporation (E) to evapotranspiration (ET) is critical for evaluating water use efficiency, ecosystem productivity, and soil-plant-atmosphere interactions in a changing environment. However, such partitioning and its responses to dry, normal, and wet conditions, as well as the controlling factors at multiple temporal scales, remain poorly understood in China's boreal forests, characterized by synchronization of water supply and energy demand. In this study, we used 8 years of ET data from the growing season (GS; May-September) collected via the eddy-covariance system and applied the underlying water use efficiency (uWUE) method to estimate T and E in a boreal larch forest in China. Our results revealed that E was the dominant component of ET. Specifically, T accounted for 0.44 of ET (T/ET), whereas E contributed to 0.56 of ET (E/ET) over the study period. The response of T/ET to dry conditions during the leaf defoliation stage (LDS) was more pronounced than during the leaf expansion stage (LES). Despite an increase in T/ET (reaching 0.49) during the dry season compared to the normal season (0.42), E was still the dominant contributor to ET. Furthermore, E/ET was significantly controlled by vapor pressure deficit (VPD) across daily to GS scales. Interestingly, soil water content (SWC) was not a controlling factor for regulating E/ET, indicating that atmospheric forces strongly constrained the variability of E/ET in this boreal forest. These findings highlight that E should be given greater attention in boreal forests than before. Our study suggests that effective management strategies for improving water use efficiency in such forest ecosystems are urgently needed.

Accurately estimating the net primary productivity (NPP) of forests in high-latitude, ecologically fragile regions and understanding its spatiotemporal drivers are crucial for assessing global carbon balance and achieving sustainable development under climate change. This study focuses on the Tahe River Basin in the Greater Khingan Mountains; utilising the Carnegie Ames Stanford Approach (CASA) model, we simulated the spatiotemporal dynamics of watershed NPP and investigated its driving mechanisms. Notably, this research reveals the asymmetric response mechanisms of NPP to water deficit and surplus states among different forest types in the northern forests. The results indicate that the multi-year average NPP in the Tahe River Basin from 1982 to 2022 was 483.01 g C m(-2) a(-1), exhibiting a significant increasing trend at a rate of 5.62 g C m(-2) a(-1). NPP decreased with elevation but showed no slope dependence. Precipitation dominated NPP variation, with forest-type ranking: deciduous broadleaf > deciduous coniferous > evergreen coniferous. Mild drought reduced watershed NPP by 20.39 g C m(-2) a(-1), while severe drought led to a loss of 45.27 g C m(-2) a(-1). Mild wetness slightly promoted NPP, whereas severe wetness exerted an even stronger disruptive effect on NPP than severe drought. Responses of NPP to drought varied significantly among vegetation types, with coniferous forests demonstrating stronger resistance. Severe wetness significantly reduced NPP in all three vegetation types, resulting in losses of 20.66%, 11.97% and 19.06% for deciduous broadleaf forest, evergreen coniferous forest and deciduous coniferous forest, respectively. The asymmetric response mechanism to water deficit/surplus revealed in this study, along with its dependence on forest type, addresses a critical gap in the theoretical framework of forest carbon cycle driving mechanisms in high-latitude ecologically fragile zones. It is critical for reliably evaluating the stability of forest carbon sinks and for guiding the design of effective, nature-based adaptation strategies in these regions facing climate change.

Stem CO2 efflux is an important component of forest carbon emission. The response of stem CO2 efflux rate (Es) in Betula platyphylla secondary forests at different phenological stages and environmental factors, however, has been inadequately explored. We used an LI-6800 gas analyzer to measure Es from Betula platyphylla secondary forests in northern China in four diameter classes (D1:6–10, D2:10–14, D3:14–18, and D4:18–22 cm) throughout the year and during different phenological periods. The study reveals that daily Es across all diameter classes exhibited single-peak trends per phenological stage, peaking at 13:00–19:00 (1.95–0.08 μmolm⁻2 s⁻1) and bottoming at 23:00–07:00 (0.81–0.01 μmolm⁻2 s⁻1). The Es values of D3 and D4 showed a unimodal trend throughout the year, with peaks at the full leaf period (1.34 and 1.48 μmolm⁻2 s⁻1), while D1 and D2 showed a fluctuating downward trend, peaking at the leaf spreading stage (0.87 and 0.93 μmolm⁻2 s⁻1), and then reaching a minimum at the leafless period. Air temperature (Ta) and photosynthetically active radiation (PAR) were the dominant drivers of Es across diameter classes, while soil volumetric water content (VWC) exhibited weaker effects. The responses of Es to Ta and PAR differed phenologically: in leaf spreading and leaf fall periods, Es increased with Ta and decreased with PAR; during full leaf period, Es was strongly temperature-dependent but PAR-insensitive; in leafless period, Es showed a quadratic response to PAR but minimal temperature sensitivity. These findings suggest that stem CO2 efflux in boreal Betula platyphylla forests will increase under warming climates and extended leafy periods, with secondary modulation by soil moisture dynamics.

Changes in watershed water resources are often linked to land use changes, but the influence of forest structure, especially the composition of tree species, plays a crucial role in hydrological processes. This impact is particularly pronounced in the Greater Khingan Mountains, where forest cover has remained consistently high over time. Understanding and quantifying how variations in tree species composition affect watershed hydrology is essential for effective management and conservation efforts in this region. This study utilized daily runoff, baseflow separation, and various remote sensing evapotranspiration data to develop an accurate SWAT model for the Tahe River Basin, a typical forest watershed in the Greater Khingan Mountains. We assessed how the proportions of the dominant tree species, Larix gmelinii and Betula platyphylla, influence streamflow components and evapotranspiration. The results show that increased proportion of Larix gmelinii significantly enhances the hydrological function. Specifically, a 10% increase in Larix gmelinii leads to a 0.64 mm decrease in evapotranspiration, and increases runoff and baseflow by 0.66 mm and 4.98 mm, respectively. The effects of the conversion between Larix gmelinii and Betula platyphylla on these hydrological components become more pronounced under wetter conditions. Simulations based on two CMIP6 future climate scenarios indicate a trend towards warmer and wetter conditions in the Tahe River Basin, leading to further increases in runoff and evapotranspiration. Larix gmelinii is crucial for maintaining future baseflow, to stabilize baseflow and manage extreme drought events, Larix gmelinii proportions of 30% under SSP245 and 90% under SSP585 are recommended. The findings reveal the sensitivity of subboreal permafrost hydrology to forest composition changes and offer critical insights for climate adaptation strategies. Additionally, they emphasize the importance of restoring Larix gmelinii forests to enhance the hydrological function.

Assessing evapotranspiration (ET) responses to droughts is of great significance for understanding the exchanges of water, carbon, and energy in a changing environment. However, the interannual variation (IAV) of ET and its environmental controls under different drought types in China’s boreal forests remain poorly understood. In this study, we integrated eight years of eddy covariance measurements and environmental observations during growing seasons to evaluate the IAV of ET and the control mechanisms of ET under no drought, atmospheric drought, soil drought, and combined drought conditions. Over the study period, the IAV of ET was relatively small with a coefficient of variation (CV) of 8.6%, whereas the evapotranspiration/precipitation (ET/P) ratio exhibited a great fluctuation with a CV of 24.6%, corresponding to the higher variability in P (CV of 31.2%). Atmospheric drought significantly increased ET by 18.05% than no drought condition, whereas soil drought significantly reduced ET by 19.02%. However, ET showed no significant difference between no drought and combined drought due to the constraint role of low soil water content (SWC) in high atmospheric demand during the combined drought. The indirect and direct driving effects of atmospheric conditions, such as net radiation (Rn), air temperature (Ta), and vapor pressure deficit (VPD) were the key to ET responses to different drought types. Furthermore, precipitation rather than energy demand or canopy greenness had a greater impact on the IAV of ET. Interestingly, the precipitation regime with larger rainfall events (> 15 mm/day) was mostly related to the IAV of ET. These findings enhance our understanding of the responses of ET to multiple drought types and the intricate relationship between ET and P, highlighting that precipitation patterns due to climate change could potentially increase the complexity of environmental control on ET variations in boreal forests.

To investigate the impacts of phenological changes in cold-temperate zones on the concentrations of non-structural carbohydrates (NSCs) and their components in leaves, we selected dominant tree species from different forest communities (Larix gmelinii forest, Betula platyphylla forest, and Populus davidiana forest). Leaf samples were collected based on phenological rhythms to explore the influence of phenological changes on leaf NSCs. The results showed that the contents of soluble sugar, starch, and NSC in the leaves of the three tree species exhibited significant phenological changes. Soluble sugar content was significantly higher than starch content, accounting for 73.7% to 96.3% of leaf NSCs. As phenological rhythms changed, foliar NSC and soluble sugar contents in the three tree species showed a unimodal pattern, while starch content fluctuated. Foliar NSC content of the three tree species was the lowest at the early stage of leaf expansion. The NSC content of P. davidiana was the lowest among the three tree species. From full leaf expansion to the leaf discoloration phase, the NSC content in the leaves of B. platyphylla and P. davidiana was higher than that of L. gmelinii. The peak NSC content occurred during the leaf discoloration phase, with P. davidiana (151.36 g·kg-1) > B. platyphylla (146.64 g·kg-1) > L. gmelinii (132.20 g·kg-1). During the leaf-fall period, the NSC content in the leaves decreased, with P. davidiana showing the largest decrease at 61.8%. Redundancy analysis indicated that soil temperature and available nitrogen content had significant effects on the NSC and its components. There were certain time differences in the phenological periods of the dominant species in the cold-temperate forest. The impact of phenological changes on leaf NSC content showed similar trends among different species, with the effect being more pronounced in B. platyphylla and P. davidiana than in L. gmelinii. Such a result indicated that different tree species in cold-temperate zones respond differently to climate change.

Despite decades of recovery, soil carbon in heavily burned areas has failed to reach pre-fire levels. It is unclear whether stand management practices can promote soil organic carbon accumulation at such sites. This study evaluated the changes in soil labile organic carbon (LOC) fractions (including dissolved organic carbon (DOC), microbial biomass carbon (MBC), and easily oxidizable organic carbon (EOC)) and the carbon pool management index (CPMI) after the thinning of a heavily burned area in the Daxing'an Mountains and selected sample plots. This study compared thinned birch secondary forests (17 years (17a-S), 14 years (14a-U), 2 years (2a-S) postthinning, where 17a-S and 2a-S were strip thinned and 14a-U was uniform thinned) with unthinned control (CK) plots. The contents of soil LOC and CPMI at a depth of 0-10 cm were found to increase with thinning, indicating that thinning promoted the accumulation of soil organic carbon in secondary forests in heavily burned areas. The two-way ANOVA showed that the differences in C fractions and CPMI at different times after thinning were significant, whereas the differences between thinning methods were not significant. In comparison to CK, only the DOC content was found to be significantly elevated at 2a-S. However, at both 14a-U and 17a-S, the elevation of the LOC fraction content reached a significant level. Among them, 14a-U demonstrated the most pronounced improvement (DOC (+11.37%), MBC (+42.80%), and EOC (+19.51%)). The CPMI at the 0-10 cm depth also increased significantly (18.20% similar to 27.77%) at 14a-U. The study revealed that soil bulk density and understorey vegetation biomass were the main influences on the changes in soil LOC fractions and CPMI postthinning. This finding also indicates that greater attention should be given not only to the soil itself but also to the understorey vegetation during forest soil carbon restoration under conservation management.

[Objective]Due to the large differences in understorey vegetation and soil physicochemical properties of Larix gmelinii forest in the cold temperate zone of the Rreater Khingan Mountains,the characteristics and availability of soil phosphorus morphological changes of Larix gmelinii forest under different forests were studied,so as to provide scientific data for scientific evaluation of soil phosphorus supply level and phosphorus cycle in the cold temperate zone forest.[Methods]Four kinds of understory vegetation in the cold temperate zone of the Greater Khingan Mountains,namely,Larix gmelinii forest(Rhododendron simsii-Larix gmelinii forest,Rhododendron tomentosum-Larix gmelinii forest,Carex schmidtii-Larix gmelinii forest and Moss-Larix gmelinii forest)were selected as the research objects.Soil samples from the 0-5 cm,5-10 cm and 10-20 cm soil layer were collected from June,August and October 2022.Hedley phosphorus classification method was used to determine the phosphorus content of different forms in soil.[Results](1)During the observation period,soil active phosphorus(H2O—Pi,NaHCO3—Pi,NaHCO3—Po)and medium active phosphorus(NaOH—Pi,NaOH—Po)of the four understorey vegetation Larix gmelinii forest showed a decrease trend from June to October,and the content of medium active phosphorus accounted for 30.08％to 52.80％of total phosphorus.The content of inorganic phosphorus was higher than that of organic phosphorus.The content of active phosphorus and medium active phosphorus in 0-20 cm soil layer of Rhododendron simsii-Larix gmelinii forest and Rhododendron tomentosum-Larix gmelinii forest were higher than those of the Carex schmidtii-Larix gmelinii forest and Moss-Larix gmelinii forest.The soil stability phosphorus(HCl—Pi,HCl—Po,and residual—P)in the Carex schmidtii-Larix gmelinii forest was significantly higher than that in the other three larix forests,and the stable state phosphorus accounted for 58.86％～65.81％of the total phosphorus.(2)Soil total phosphorus(TP),available phosphorus(AP)and phosphorus activation coefficient(PAC)in Larix gmelinii forest were 391.81 to 1 081.02,7.34 to 83.90 mg/kg,and 1.62％to 7.76％from June to October,respectively.The content of AP and PAC in the soil of the Rhododendron simsii-Larix gmelinii forest and Rhododendron tomentosum-Larix gmelinii forest were higher than those of the other two larix forests,showing higher phosphorus availability.On the other hand,there was insufficient phosphorus supply in the soil layer of the Carex schmidtii-Larix gmelinii forest and Moss-Larix gmelinii forest from 5-10 cm and 10-20 cm in August and October.(3)Soil moisture contentwas the main influencing factor of soil phosphorus in the Rhododendron simsii-Larix gmelinii forest and the Rhododendron tomentosum-Larix gmelinii forest,soluble carbon was the main influencing factor of the Carex schmidtii-Larix gmelinii forest,and ammonium nitrogen had a greater influence on the Moss-Larix gmelinii forest.[Conclusion]Understory vegetation had a significant effect on soil phosphorus morphological form and phosphorus availability in Larix gmelinii forest.The content of active phosphorus,medium active phosphorus and available phosphorus in the soil of Rhododendron simsii-Larix gmelinii forest and Rhododendron tomentosum-Larix gmelinii forest was higher,and the activation coefficients of P were all above 2.00％;However,the content of stable phosphorus in soil of the Carex schmidtii-Larix gmelinii forest was higher,and the availability of soil phosphorus was lower.This results are of great significance for the study of ecological function of understory vegetation and soil nutrient cycle in the cold temperate forest ecosystem.

Abstract The autumn freeze-thaw period is an important period to influence soil carbon dynamics. However, the activities of soil CO2 and CH4 in different stand ages of birch forests in high latitude perennial permafrost regions of China during this period are not clear. Therefore, in this study, we investigated soil CO2 and CH4 fluxes in 30a, 45a, and 66a Brich forests in the Daxing'an Mountains permafrost zone, and established the relationships between soil CO2 and CH4 fluxes and soil temperature and humidity, and soil active carbon and nitrogen. The results showed that soil CO2 and CH4 fluxes of all three stand ages showed a gradual decrease during the monitoring period. Soil CO2 and CH4 fluxes showed a trend of increasing and then decreasing with stand age. The average soil CO2 flux of 30a (19.24±4.03) mg·m-2 h-1 was the lowest throughout the monitoring period and was significantly smaller than that of the other two stand ages (P<0.05). 45a and 66a had average soil CO2 fluxes of 36.44±5.20 mg·m-2 h-1 and 35.49±4.22 mg·m-2 h-1, respectively, which were not significantly different from each other (P>0.05). The average uptake fluxes of 45a (-25.92±3.55) μg·m-2 h-1 > 66a (-24.73±4.01) μg·m-2 h-1 > 30a (-12.22±2.33) μg·m-2 h-1, and 30a was highly significantly smaller than 45a and 66a (P<0.01), and the difference between 45a and 66a was not significant (P>0.05). Soil temperature and moisture were both highly significant on soil CO2 and CH4 fluxes. NO3--N and MBC/N had greater effects on soil CO2 and CH4. The global warming potentials ranged from 2171.84—4132.93 kg/hm-2.

Sap flow dynamics are critical for understanding how vegetation consumes water and adapts to environmental stress. The response of sap flow in boreal birch secondary forests to rainfall variations during the rainy season, however, has been inadequately explored. Our study indicated that photosynthetically active radiation (PAR) and vapour pressure deficit (VPD) are the primary drivers of sap flow density in birch trees across different diameter classes (Fds: small trees, Fdm: medium-sized trees, Fdl: large trees). Soil water content (SWC) significantly reduces sap flow when it falls below the 0.18 cm3/cm3. Sap flow density increased with PAR and initially with VPD but plateaued at higher VPD levels due to saturation. A hierarchy of sap flow density was observed, with Fdl > Fdm > Fds, each responding differently to PAR, VPD and SWC. With decreasing rainfall across rainy seasons, the influence of PAR on Fds and Fdm weakened, while the influence of VPD strengthened. For Fdl, the impact of VPD peaked and then declined, while the influence of PAR showed an inverse pattern. In the dry season, Fdl was primarily driven by PAR and influenced by VPD and SWC, whereas Fds was mainly controlled by VPD, with minimal effects from PAR and SWC. The response of Fdm to SWC was similar to that of Fdl, but it mirrored the response of Fds to PAR and VPD. These findings suggest that sap flow in boreal birch forests may become increasingly susceptible to SWC stress as global climate change intensifies.

Partitioning forest ecosystem evapotranspiration (ET) into transpiration (T) and evaporation (E) has been widely discussed, as it is crucial for understanding hydrological processes and functions. However, quantifying the hydrological importance of the understory ecosystem is limited, particularly in China’s boreal forests. In this study, we first employed a framework with concurrent eddy covariance (EC) systems and the underlying water use efficiency (uWUE) method to partition ET in a natural boreal larch forest (Larix gmelinii) of China during the growing seasons of 2020 to 2022. The results showed that understory transpiration (Tu) and overstory transpiration (Tos) contributed 21.8 and 78.2% to T, respectively, while the contributions of understory evaporation (Eu) and overstory evaporation (Eos) to E were 41.2 and 58.8%, respectively. Understory evapotranspiration (ETu) contributed 32.6% to the total ET, suggesting the considerable importance of understory vegetation to water flux in the boreal larch forest. Interestingly, Tos and Eos had a similar contribution to ET. While the controlling mechanisms in Tu/T, Eu/E, and ETu/ET were more complex than those in Tu, Eu, and ETu, the leaf area index (LAI) of the overstory and solar radiation (SR) played greater roles in regulating each component and its respective ratios. Those results highlight the non-negligible role of understory vegetation in hydrological processes in boreal forests, which must be considered when managing or modelling forest-water relationships. Our study also demonstrates the effective utility of concurrent EC systems in partitioning ET in natural boreal forest ecosystems.

Investigating the soil water dynamics in relation to rainfall events is of great importance for enhancing our understanding of eco-hydrological processes and improving soil hydrological models. However, the lack of long-term, in-situ observational data and the neglect of soil moisture decoupling have hindered the study of dynamic responses in soil water regimes to rainfall events in China’s boreal forests with permafrost. This study utilized soil water content monitoring and rainfall observational data from the Xing’an larch (Larix gmenilii) forest in Northeast China during the growing seasons of 2015–2021 to evaluate the dynamic response of soil water regimes to rainfall events. The results show a significant increase in the decoupling strength between soil water and rainfall with increasing soil depth. The thresholds of rainfall amount (RA), duration (RD), and intensity (RI) for triggering soil wetting events also increased. Soil water dynamic processes exhibited inconsistent and complex patterns in response to the hierarchical effects of RA, RD, and RI along the vertical soil profile. However, the dominant controls on soil water dynamic responses to RA, RD, and RI varied depending on specific response metrics, such as the accumulated soil water content increments (ASMI), duration time (DT), and the mean slope of the soil wetting curve (Sm). Interestingly, the dominant factor for affecting response time (RT) changed from RI to RD as soil depths increased. Importantly, the decoupling of soil moisture led to a substantial reduction in the responsiveness of these metrics of subsurface soil water to rainfall, and the role of the decoupling effect diminished with increasing soil depths. These findings highlight the complexity of the dynamic response of soil water to rainfall events and provide new insights into the relationship between soil water dynamics and rainfall features through the introduction of the decoupling of soil moisture in boreal forests.

With over 700 million km 2 Siberia is the largest expanse of the northern boreal forest—deciduous‐needleleaf larch. Temperatures are increasing across this region, but the consequences to carbon balances are not well understood for larch forests. We present flux measurements from a larch forest near the southern edge of Central‐Siberia where permafrost degradation and ecosystem shifts are already observed. Results indicate net carbon exchanges are influenced by the seasonality of permafrost active layers, temperature and humidity, and soil water availability. During periods when surface soils are fully thawed, larch forest is a significant carbon sink. During the spring‐thaw and fall‐freeze transition, there is a weak signal of carbon uptake at mid‐day. Net carbon exchanges are near‐zero when the soil is fully frozen from the surface down to the permafrost. We fit an empirical ecosystem functional model to quantify the dependence of larch‐forest carbon balance on climatic drivers. The model provides a basis for ecosystem carbon budgets over time and space. Larch differs from boreal evergreens by having higher maximum productivity and lower respiration, leading to an increased carbon sink. Comparison to previous measurements from another northern larch site suggests climate change will result in an increased forest carbon sink if the southern larch subtype replaces the northern subtype. Observations of carbon fluxes in Siberian larch are still too sparse to adequately determine age dependence, inter‐annual variability, and spatial heterogeneity though they suggest that boreal larch accounts for a larger fraction of global carbon uptake than has been previously recognized.

[目的]研究寒温带落叶树种和常绿树种针叶生态化学计量特征的差异及随龄级增加的变化规律,揭示不同生活型针叶树种生态化学计量特征与生长阶段之间的关系,为寒温带植物生长与养分供给的研究提供依据.[方法]以大兴安岭北部兴安落叶松和樟子松为对象,研究5-9月针叶C、N、P含量及化学计量特征,利用内稳性指数和Pearson相关系数分析C、N、P之间及与环境因子的相关性.[结果](1)不同生长阶段兴安落叶松和樟子松针叶N、P月平均含量表现为随龄级的增长而降低,C含量则是随龄级的增加而增加,且5-8月兴安落叶松针叶N、P含量高于樟子松,C含量则低于樟子松.(2)兴安落叶松针叶C∶N、C∶P和N∶P月平均值均高于樟子松,4个龄级兴安落叶松针叶N∶P均小于14,而樟子松在5-7月N∶P小于14,8-9月N∶P大于16.兴安落叶松针叶N、P以及N∶P稳定性大于樟子松.(3)土壤含水率与2树种针叶生态化学计量特征呈极显著相关,樟子松针叶N含量与C、N、P化学计量比呈显著相关,兴安落叶松针叶C含量与N∶P、土壤铵态氮和硝态氮显著相关.[结论]大兴安岭北部不同生活型的2种针叶树种,叶片生态化学计量对寒冷气候和冻土生境的适应策略不同,兴安落叶松是本区的顶级群落优势种,但其生长受到N的限制,而樟子松生长的限制因子因季节变化而不同.

以大兴安岭北部 3 种典型森林(白桦(Betula platyphylla)林、樟子松(Pinus sylvestris var.mongolica)林和兴安落叶松(Larix gmelinii)林)为研究对象,通过对对照、去除凋落物、去除草毡 6 层以及去除凋落物和草毡层 4 种处理下不同土层(0～10 cm和＞10～20 cm)的土壤全氮、有效氮、微生物量氮及其影响因子的研究,探究凋落物和草毡层对寒温带典型森林土壤氮素的影响.结果表明,1)在 0～10 cm土层,与对照相比,去除凋落物后土壤全氮、铵态氮和硝态氮含量在白桦林和兴安落叶松林中无显著变化,樟子松林土壤全氮和铵态氮含量无显著变化,而硝态氮含量升高了53.76%(P＜0.05);土壤微生物量氮含量在白桦林和兴安落叶松林中分别降低了 14.34%和 25.94%(P＜0.05),在樟子松林中无显著变化.去除草毡层后土壤全氮含量在樟子松林和兴安落叶松林中变化不显著,在白桦林中降低了 41.25%(P＜0.05);土壤铵态氮含量在兴安落叶松林中无显著变化,在白桦林中降低了 52.84%(P＜0.05),而在樟子松林中升高了 69.09%(P＜0.05);土壤硝态氮含量在白桦林和兴安落叶松林中无显著变化,在樟子松林中升高了 44.09%(P＜0.05);土壤微生物量氮含量在 3 种林型中降低了 16.95%～47.20%(P＜0.05).去除凋落物和草毡层后,土壤全氮和微生物量氮含量在 3 种林型中均显著降低(P＜0.05);土壤铵态氮含量在兴安落叶松中无显著变化,在白桦林中显著降低(P＜0.05),而在樟子松中显著升高(P＜0.05);土壤硝态氮含量在白桦林和兴安落叶松林中无显著变化,而在樟子松林中升高了 44.09%(P＜0.05).2)在＞10～20 cm土层,经过不同去除处理,土壤全氮和微生物量氮含量在 3 种林型中无显著变化;土壤铵态氮和硝态氮含量在白桦林和兴安落叶松林中无显著变化,而在樟子松林中有所升高,且在去除凋落物以及去除草毡层后达显著水平(P＜0.05).3)由相关性分析可知,土壤含水率、pH、总有机碳和可溶性有机碳等是土壤氮素的重要影响因子.由此可见,凋落物和草毡层的存在与否对土壤氮素具有重要影响,但影响程度会因林型和土层的不同而异.研究结果为该地区森林土壤氮循环的研究提供理论基础.