Forest In Northeast China Research Articles

Abundance and diversity of soil nematode community at different altitudes in cold-temperate montane forests in northeast China

Soil nematode is an essential component of the soil micro-food web and plays an important role in soil nutrient cycling. The biogeographic distribution pattern of the soil nematode community and its main driving factors have attained greater attention in the last few decades. However, previous studies mainly focused on low and middle latitudes, and few studies were conducted in cold and temperate regions at high latitudes. The structure and diversity of soil nematodes along the altitudinal gradient are still unclear, which prevents understanding and comparing the diversity of soil nematodes at different spatial scales. In this study, six altitudes (750, 830, 950, 1100, 1300, and 1420 m) were set in the Oakley Mountain in the cold-temperate zone in China, and Illumina MiSeq sequencing method was used to explore the community composition and structural characteristics of soil nematodes in 0- to 10-cm soil. The diversity of soil nematode community in a high-latitude cold-temperate zone showed a decreasing pattern of monotonicity along the altitudinal gradient. One phylum, 3 classes, 11 orders, and 43 genera were identified; plant–parasites were the dominant trophic groups at each altitude. Altitude was the main factor affecting the diversity of soil nematodes at higher latitudes. Sobs, Chao1, Shannon index, and Faith's phylogenetic diversity all showed a downward trend with the increase in altitude. Soil bulk density, soil temperature, pH, soluble organic carbon, and soluble organic nitrogen were significant factors affecting the altitude distribution of soil nematodes. Soil pH and soluble organic nitrogen were the key factors to explain the changes in the community composition of nematodes.

Gut Bacteria Associated With Monochamus saltuarius (Coleoptera: Cerambycidae) and Their Possible Roles in Host Plant Adaptations.

Monochamus saltuarius (Coleoptera: Cerambycidae) is an important native pest in the pine forests of northeast China and a dispersing vector of an invasive species Bursaphelenchus xylophilus. To investigate the bacterial gut diversity of M. saltuarius larvae in different host species, and infer the role of symbiotic bacteria in host adaptation, we used 16S rRNA gene Illumina sequencing and liquid chromatography-mass spectrometry metabolomics processing to obtain and compare the composition of the bacterial community and metabolites in the midguts of larvae feeding on three host tree species: Pinus koraiensis, Pinus sylvestris var. mongolica, and Pinus tabuliformis. Metabolomics in xylem samples from the three aforementioned hosts were also performed. Proteobacteria and Firmicutes were the predominant bacterial phyla in the larval gut. At the genus level, Klebsiella, unclassified_f__Enterobacteriaceae, Lactococcus, and Burkholderia–Caballeronia–Paraburkholderia were most dominant in P. koraiensis and P. sylvestris var. mongolica feeders, while Burkholderia–Caballeronia–Paraburkholderia, Dyella, Pseudoxanthomonas, and Mycobacterium were most dominant in P. tabuliformis feeders. Bacterial communities were similar in diversity in P. koraiensis and P. sylvestris var. mongolica feeders, while communities were highly diverse in P. tabuliformis feeders. Compared with the other two tree species, P. tabuliformis xylems had more diverse and abundant secondary metabolites, while larvae feeding on these trees had a stronger metabolic capacity for secondary metabolites than the other two host feeders. Correlation analysis of the association of microorganisms with metabolic features showed that dominant bacterial genera in P. tabuliformis feeders were more negatively correlated with plant secondary metabolites than those of other host tree feeders.

Biotic and climatic controls on the interannual variation in canopy litterfall of a deciduous broad-leaved forest

Canopy litterfall is an important component of net primary productivity (NPP) in forests, but the climatic and biotic controls on its interannual fluctuation remain poorly understood. In this study, we quantified the interannual variation in and the influencing factors of the canopy litterfall with 45 litter traps in nine plots for 12 years (2008 − 2019) in a temperate deciduous broad-leaved forest in Northeast China. The coefficients of interannual variation in the total (7.3%) and leaf (5.6%) litterfall were substantially lower than those in the basal area (BA) increment (21.0%) and gross primary productivity (10.2%). Climatic (i.e., air temperature, thermal time, precipitation, and radiation) and biotic (i.e., BA and species richness) factors jointly controlled the annual production of litterfall for individual plots and species. Significant climate legacy effects were detected in all components of the litterfall, with the strongest in woody tissue component that contributed to 58.9% of the total litterfall variability. The most important factor driving the variability of the total litterfall and its non-leaf components was climate legacy effect (67.2% to 86.6%), while those for the leaf litterfall were BA (40.4%) and species richness (38.3%). We conclude that high species richness of the deciduous forest increases the temporal stability of canopy productivity, and leaf productivity is much more stable than wood productivity as indicated by BA increament. These findings provide insight into spatiotemporal dynamics in the litterfall and canopy NPP in temperate deciduous forests.

Effects of fire on soil respiration and its components in a Dahurian larch (Larix gmelinii) forest in northeast China: Implications for forest ecosystem carbon cycling

Fire plays a critical part in regulating soil carbon (C) budgets in forest ecosystems. However, few studies have focused on the effects of fire on soil respiration (Rs) and its heterotrophic (Rh) and autotrophic (Ra) components. In this study, we examined the response of Rs, Rh, and Ra to fire and the driving factors that affect Rs after a fire disturbance in a Dahurian larch (Larix gmelinii) forest in northeastern China. Fire disturbance significantly increased Rh by approximately 24% and significantly reduced Ra by approximately 66% in burned plots compared to unburned plots. The ratio of Rh to Rs significantly increased from 0.57 in the unburned plots to 0.79 in the burned plots. Fire disturbance also led to cumulative Rs C efflux increasing by approximately 10% and 28% during the growing season and the period of spring freeze–thaw cycles (FTC) in the burned plots. Furthermore, using structural equation modelling, we showed that T5 (soil temperature at 5 cm depth) was the main abiotic factor affecting Rs in both unburned and burned plots. Fine root biomass (FR) was the dominant biotic driver of Rs in the control plots, whereas microbial biomass carbon (MBC) was the dominant biotic driver of Rs in burned plots. Our findings demonstrated a significant discrepancy in the response of Rs components to fire disturbance. Forest fires can cause considerable losses of soil C in a cold temperate forest ecosystem, which suggests that proactive management of forest regeneration should be carried out after forest fire disturbances.

The relationship between species diversity and tree growth in natural secondary forests in Northeast China

[Objective] To understand the relationship between species diversity and tree growth in natural secondary forests in Northeast China, to determine the reasonable size of species diversity, and to carry out appropriate nurturing harvesting and artificial replanting, so as to provide a scientific and theoretical basis for secondary forest management and management. [Methods] A total of 123 sample plots were set up in the Xiaoxinganling (XXAL), Zhangguangcailing (ZGCL), Laojialing (LYL), Changbai Mountain (CBS), Hadaling (HDL) and Longgang Mountain (LGS) areas in Northeast China, they were used to investigate the species composition, importance value, diversity and tree growth in each area. [Results] A total of 48 species belonging to 17 families and 31 genera were investigated in all the sample plots, among which the sample plots in Longgang Mountain contained the largest number of families, genera and species, followed by Hada Ling, Changbai Mountain, Laoyaling, Zhangguangcai Mountain and Xiaoxinganling. The α-diversity index of species in the sample sites was the largest in Changbai Mountain and the smallest in Xiaoxinganling, and the difference between them was significant (P < 0.05), while the richness index was the largest in Longgang Mountain and the smallest in Xiaoxinganling. The difference between them was significant (P < 0.05), while the greater the difference in latitude between the regions, the more obvious the difference in β-diversity index of species in the sample sites, and the fewer species shared between the two regions. The higher the rate of community succession, the higher the average diameter at breast height and the average tree height in each region were CBS > LYL > LGS > ZGCL > HDL > XXAL. The largest breast tree species in each region was Mongolian oak in Changbai Mountain with a diameter at breast height of 64.8 cm, and the smallest breast tree species in each region was Tyrannus sylvestris in Longgang Mountain with a diameter at breast height of 4.0 cm. The highest tree species in each region was Liriodendron sylvestris in Longgang Mountain with a height of 28.9 m, and the smallest species is yellow pineapple with a height of 1.3 m in Longgang Mountain. [Conclusion] Within a certain range, species diversity has a facilitating effect on the average diameter at breast height and average tree height of species within a stand. Therefore, during the management of secondary forests, appropriate nurturing harvesting and artificial replanting should be adopted to ensure reasonable species diversity in the stands and provide optimal space for the growth of natural secondary forests.

Estimating the aboveground biomass of coniferous forest in Northeast China using spectral variables, land surface temperature and soil moisture

As a crucial indicator of forest growth and quality, estimating aboveground biomass (AGB) plays a key role in monitoring the global carbon cycle and forest health assessments. Novel methods and applications in remote sensing technology can greatly reduce the investigation time and cost and therefore have the potential to efficiently estimate AGB. Random forest (RF), combined with remote sensing images, is a popular machine learning method that has been widely used for AGB estimation. However, the accuracy of the ordinary linear variable selection method in the AGB estimation of coniferous forests is challenging due to the complexity of these forest biomes. In this study, spectral variables (spectral reflectance and vegetation index), land surface temperature (LST) and soil moisture were extracted from the operational land imager (OLI) and thermal infrared sensor (TIRS) of Landsat 8, and optimized RF regressions were established to estimate the AGB of coniferous forests in the Wangyedian forest farm, Inner Mongolia, Northeast China. We applied one linear (Pearson correlation coefficient (PC)) and four nonlinear (Kendall's τ coefficient (KC), Spearman coefficient (SC), distance correlation coefficient (DC) and the importance index) indices to select variables and establish optimized RF regressions for AGB estimation. The results showed that all the nonlinear indices provided significantly lower estimation errors than the linear index, in which the minimum root mean square error (RMSE) of 40.92 Mg/ha was obtained by the importance index in the nonlinear indices. In addition, the inclusion of LST and soil moisture significantly improved AGB estimation. The RMSE of the models constructed through the five indices decreased by 12.93%, 7.31%, 8.33%, 6.28% and 10.78%, respectively, following the application of the LST variable. In particular, when LST and soil moisture were both added into the model, the RMSE decreased by 31.47%. This study demonstrates that combining the nonlinear variable selection method with optimized RF regression can improve the efficiency of AGB estimation to support regional forest resource management and monitoring.

Use of direct seeding and seedling planting to restore Korean pine (Pinus koraiensis Sieb. Et Zucc.) in secondary forests of Northeast China

The poor natural regeneration of Korean pine represents a substantial hurdle limiting the restoration of secondary forests to mixed broad-leaved Korean pine forests (MBKPFs). Active restoration based on direct seeding or planting seedlings is accordingly considered to be a prerequisite for promoting Korean pine recruitment in secondary forests. To evaluate the most effective practices for restoring Korean pine in secondary forests, we studied the effects of microsites associated with gaps of different sizes and within-gap locations on direct seeding (rodent access/exclusion and soil/litter cover) and seedling planting (1-, 2-, and 3-year-old seedlings). We found that an average of 82.4% of sown seeds were eaten by rodents, with only 9.9% germinating and surviving after two growing seasons. Among the germinated seedlings, 37.6% were subsequently excised by rodents, whereas 6% died due to other natural causes. Furthermore, compared with the control sites (forest understorey), we detected no significant differences among the different microsites with respect to an effective reduction in rodent damage or promotion of seed germination, whereas rodent exclusion was more effective in protecting seeds from predation than the effect of microsite. With regards to planted seedlings, we found that the proportion of survival and relative root collar diameter growth were highest in the center (64.1%) and edge (60.0%) sites of large gaps, and that the survival of planted seedlings (43.7%) was considerably higher than that of seedlings derived from directly sown seeds (9.9%). Collectively, our findings indicate that, owing to seed predation and seedling excision by rodents, direct seeding with either mesh or soil cover would not represent an effective strategy for promoting Korean pine recruitment in secondary forests. Accordingly, for the purposes of restoring secondary forests to MBKPFs, we recommend planting 3-year-old Korean pine seedlings in large gaps within secondary forests.

Forest plant and macrofungal differences in the Greater and Lesser Khingan Mountains in Northeast China: A regional-historical comparison and its implications

Forests in Northeast China in the Greater and Lesser Khingan Mountains (GKM and LKM) account for nearly 1/3 of the total state-owned forests in the country. Regional and historical comparisons of forest plants and macrofungi will favor biological conservation, forest management and economic development. A total of 1067 sampling plots were surveyed on forest composition and structure, with a macrofungi survey at Liangshui and Huzhong Nature Reserves in the center of two regions. Regional and historical differences of these parameters were analyzed with a redundancy ordination of their complex associations. There were 61–76 families, 189–196 genera, and 369–384 species, which was only 1/3 of the historical records. The same dominant species were larch and birch with Korean pine (a climax species) less as expected from past surveys in the LKM. Shrub and herb species were different in the two regions, as expected from historical records. There was 10–50% lower species diversity (except for herb evenness), but 1.8- to 4-time higher macrofungi diversity in the GKM. Compared with the LKM, both tree heights and macrofungi density were higher. Nevertheless, current heights averaging 10 m are half of historical records (> 20 m in the 1960s). Edible macrofungi were the highest proportion in both regions, about twice that of other fungal groups, having important roles in the local economy. A major factor explaining plant diversity variations in both regions was herb cover, followed by shrubs in the GKM and herb-dominant species in the LKM. Factors responsible for macrofungi variations were tree density and shrub height. Vaccinium vitis-idaea and Larix gmelinii in the GKM but tree size and diversity were important factors in the LKM. Our findings highlighted large spatial and historical differences between the GKM and LKM in plant-macrofungal composition, forest structure, and their complex associations, which will favor precise conservation and management of forest resources in two region in the future.

An optimal firebreak design for the boreal forest of China

Wildfire often causes the loss of lives, damage to property, and alteration of the environment. Great efforts have been made in the world to reduce the impacts of fire including fuel reduction and modification, prescribe burning, and firebreak construction. Firebreaks have been used in forest fire management in China for a long time; however, no assessments of their effectiveness in fire risk reduction due to density, width, and spatial pattern have been conducted. In this study, we used burn probability (BP) and construction cost as measures to determine the optimal firebreak design for the boreal forests in Northeast China. Using the current landscape without artificial firebreaks as the baseline, we assessed the BP changes for 49 simulated firebreak scenarios that included combinations of eleven densities, three widths, and two spatial patterns. The Burn-P3 model was used to simulate BP for each scenario based on weather conditions from fire seasons during 2005–2018. Our results showed that: 1) BP decreased with increasing firebreak density when width and spatial pattern remained constant; 2) BP decreased significantly within 2000 m of firebreaks when width increased from 30 m to 60/90 m and remained at a constant density; 3) areas of high and very high BP can effectively be reduced through the use of spatially optimized firebreak designs; and 4) an optimal firebreak design was determined that is 4.1 m/ha in density, 30 m wide in grasslands and 60 m wide in forests, and in the optimized BP-based spatial distribution pattern. The optimal firebreak design may reduce BP by 25.2% in comparison to that under current scenario, with further reductions in high- and very-high-BP areas.

Tree species diversity enhances plant-soil interactions in a temperate forest in northeast China

The plant-soil interactions may drive the diversity and functioning of forests, but we do not fully understand how interrelationships between plant and soil compartments are underlined by multiple ecological mechanisms. Here, we hypothesize that positive plant-soil interactions enhance biodiversity and functioning in a temperate forest. To do so, we tested the relationships between plant diversity (i.e., tree and herb species richness) and functions (i.e., coarse woody productivity and litterfall productivity), and soil diversity (i.e. bacterial, fungal and nematode) and functions (i.e. soil nutrient and carbon stock), and their interrelationships in a temperate forest in northeast China. The positive relationship between diversity and functioning was predominant within plant and soil compartments, and hence, provide support to the niche complementarity effect. As such, the positive interrelationships between the diversity of soil and plant compartments provide support to the positive plant-soil interactions. Tree species diversity was positively related with herb species diversity and coarse-woody productivity. Importantly, tree species diversity had pronounced positive effect on soil biodiversity resulting in increased soil carbon stocks, indicating that tree species diversity effect matters for linking positive interrelationships between plant and soil compartments of a temperate forest. This study shows that tree diversity effect is the main regulating biotic mechanism for linking the positive connections between plant and soil compartments of a temperate forest, and hence, the niche complementarity effect can enhance forest functioning through positive interactions on resource supply. We argue that linking the multiple key functions and diversity indices of forests can enhance our knowledge on the main influential factors and underlying ecological mechanisms.

Variations of the effects of reduced precipitation and N addition on microbial diversity among different seasons in a temperate forest

Changes in microbial diversity affect biogeochemical cycles and ecosystem functioning. Increasing nitrogen (N) deposition and reducing precipitation significantly affect microbial diversity during the growing season. Furthermore, the variations of the effects of reduced precipitation and N addition on microbial diversity among different periods is still unclear. In order to reveal these effects, an 8-year field experiment was set up in a temperate old growth forest of northeast China, including four treatments: control, reduced precipitation (−30% of through-fall), N addition (50 kg N ha−1 yr−1), and their combined effect (−30% of through-fall and 50 kg N ha−1 yr−1), respectively. To reach a better understanding, microbial diversity among different periods was measured. The results showed that bacterial and fungal diversity significantly decreased in the growing season with N addition but significantly increased in winter. These positive or negative effects of reduced precipitation and N addition on bacterial and fungal diversity were attributed to changes in diversity of the main phyla of bacteria and fungi. Moreover, the positive impact of reduced precipitation on fungal diversity increased with N addition in the winter, while it decreased with N addition in the growing season. Our structural equation modeling showed that the negative impacts of reduced precipitation on soil moisture led to increased bacterial and fungal diversity. Our results illustrate that the direction of effect of reduced precipitation and N addition on microbial diversity may be shifted with seasonal changes and microbial taxonomic level, which provides new insight and framework for understanding and modeling responses of microbial diversity to N deposition and changes in precipitation pattern during different seasons.

Effects of season and food on the scatter-hoarding behavior of rodents in temperate forests of Northeast China

To explore the differences in hoarding strategies of rodents for different seeds in various seasons, we labeled and released the seeds of Pinus koraiensis, Corylus mandshurica, Quercus mongolica and Prunus sibirica in temperate forests of Northeast China and investigated the fate of the seeds in spring and autumn. The analysis showed that the hoarding strategies of the rodents varied substantially between seasons. The seeds were consumed faster in the spring than in the autumn. More than 50% of the seeds in the two seasons were consumed by the 16th day. It took 36 days to consume 75% of the seeds in the spring and 44 days in the autumn. The rate of consumption of the seeds in the spring was greater than in the autumn, and the rate of spread of the seeds was greater in the autumn. The distances of removal for the consumption and dispersal of seeds in the spring (3.26 ± 3.21 m and 4.15 ± 3.52 m, respectively) were both shorter than those in the autumn (3.74 ± 3.41 m and 4.87 ± 3.94 m, respectively). In addition, the fate of different seeds varied significantly owing to differences in hoarding strategies. The seeds of the three preferred species, P. koraiensis, C. mandshurica, and Q. mongolica, were quickly consumed. More than 90% of the seeds of these species were consumed. Only 21% of Pr. sibirica seeds were slowly consumed, and the two seasons had the same seed consumption rate patterns: the consumption rate of P. koraiensis seeds was the highest, followed by C. mandshurica, then Q. mongolica, and finally Pr. sibirica. The median removal times of the two seasons were different, but the rules were the same: P. koraiensis was the shortest, followed by C. mandshurica, and the third was Q. mongolica. In both seasons, the most predated in situ seeds were those of P. koraiensis; the most hoarded seeds were those of C. mandshurica, and the most unconsumed seeds were those of Pr. sibirica.

Modelling leaf phenology of some trees with accumulated temperature in a temperate forest in northeast China

Global warming is causing substantially earlier spring leaf phenology in temperate‐zone trees. Understanding the drivers and mechanisms behind the observed plant phenological changes is important for predicting future phenological dynamics. Here, leaf phenological dynamic data (including number and area) observed in situ for three tree species (namely, Tilia amurensis, Pinus koraiensis and Betula platyphylla) on Changbai Mountain from 2017 to 2020 were analyzed. We found that none of the tree species showed active accumulated temperature (AAT) differences in phenological events and phase; however, differences in time (i.e., photoperiod) were observed and could be explained by AAT being the primary controller of plant phenology. Moreover, we developed process-based spring leaf phenology models that were fitted and validated using the leaf development process and time series. Compared with the time-based models, the phenology model with AAT as the independent variable is more robust in fitting and predicting leaf area and leaf number. Our novel findings provide evidence of AAT effects on leaf unfolding, whereby when the AAT reaches a certain threshold, the corresponding phenology will be triggered. Therefore, we used AAT-based models and plot survey data to simulate the leaf area index (LAI) dynamics of the tree species studied, which provides a feasible method to understand the complex processes scaling up from the plant and forest‐levels. Our study has provided a new answer to how temperature affects spring leaf phenology in temperate forests, and significantly improved the predictability for leaf development.

Community composition and structure along environmental gradients of Larix gmelinii forest in northeast China

Community composition and structure along environmental gradients of Larix gmelinii forest in northeast China

The population status and threats of Taxus cuspidata, a plant species with extremely small populations in China

Taxus cuspidata Siebold et Zuccarini, one of the plant species with extremely small populations in China, is listed as a first-class national key protected wild plant that urgently needs to be rescued. However, there are different viewpoints on the population status of T. cuspidata. To elucidate the current actual population status of T. cuspidata, we conducted a systematic field survey in the distribution area of T. cuspidata in China, and analyzed the size structure, sex ratio, dynamics, and threat factors of T. cuspidata populations. Our results are as follows. (1) T. cuspidata was mainly distributed in Wangqing County, Helong City, Hunchun City of Jilin Province, and Muling City of Heilongjiang Province, while seldom distributed in Liaoning Province. T. cuspidata was distributed in the broad-leaved-conifer-Korean pine mixed forest in Northeast China, mostly as sporadic single individuals or scattered small populations. The highest density of adult individuals of T. cuspidata was 20 individuals/0.1 ha in DHZ, and the highest density of regeneration individuals was 28 individuals/0.1 ha in JGL. There was a significant exponential relationship (p < 0.05) and a serious polarization between the number of adult and regeneration individuals. (2) The T. cuspidata population showed an inverted J-shaped size structure, and its survival curve belonged to the Deevey Ⅲ, in which the number of surviving individuals decreased sharply from the sapling to the seventh size class. The ratio of females to males in the T. cuspidata population was 1:2.30, which was significantly different from 1:1 (p < 0.001). (3) Disturbance intensity was the primary environmental variable affecting the regeneration and adult individuals of T. cuspidata. Interestingly, disturbance factors together with habitat factors explained 41.6% and 59.8% of the variation in the number of regeneration and adult individuals respectively, and habitat factors (29.6%) had a greater independent effect than disturbance factors (17.3%) on regeneration individuals, whereas disturbance factors (16.0%) had a greater independent effect than habitat factors (12.5%) on adult individuals. Taken together, we highlighted that excessive disturbance directly resulted in the sharp decline in the population size and habitat area of T. cuspidata in China. Currently, T. cuspidata is endangered and faced with a high risk of local extinction in China. Therefore, we should continue to pay more attention to this species and we suggest that the International Union for Conservation of Nature (IUCN) could reconsider the threatened category of T. cuspidata.

Previous Atlantic Multidecadal Oscillation (AMO) modulates the lightning-ignited fire regime in the boreal forest of Northeast China

Lightning-ignited fire is sensitive to climatic change and responsible for large fires in boreal forests. In addition to global-warming caused fire increase, large-scale climate oscillations have significantly contributed to fire variability. However, the leading climate oscillation driving lightning-ignited fire and the mechanisms connecting regional and large-scale climate in the boreal forest of Northeast China, the most fire-prone biome of China, are still unclear. By compositing fire, climate, and atmospheric data, we found that the previous Atlantic Multidecadal Oscillation (AMO) was significantly coherent with the May to August temperature–evapotranspiration variability and lightning-ignited fire occurrence. These connections were valid at both the interannual and multidecadal time scales. Different from previous viewpoints, we found no connection of fire occurrence with the El Niño-Southern Oscillation and Pacific Decadal Oscillation. A warm AMO was followed by high sea level pressure and geopotential height over the study region. We assume these atmospheric anomalies are associated with descending atmospheric motion, producing adiabatic warming and less precipitation on the land surface, both of which favour high fuel aridity and lightning ignition. Therefore, we believe that the winter AMO could be a promising predictor for lightning-ignited fire occurrences in the following summer.

Regeneration and succession: A 50-year gap dynamic in temperate secondary forests, Northeast China

Forest gaps are essential small-scale disturbances in forest succession. However, little attention has been paid to the long-term effects of gap dynamics on woody species regeneration and succession. We selected 20 medium and 25 large gaps representing a range of age classes (0–10, 10–20, 20–30, 30–40, and 40–50 years) from remote-sensing images obtained in 1964, 1976, 1986, 1993, 2003, and 2014 to examine long-term gap regeneration processes in a secondary forest in Northeast China. We conducted field surveys to determine the regeneration status, density index (DI), and richness index (RI) of selected gaps. The importance value (IV) of each woody species was calculated, and all species were classified by shade tolerance. The results showed that in gaps of 0–10 years, the DI and RI were mainly dominated by shade-intolerant species in large gaps and intermediate species in medium gaps. When gap age increased to 10–20 years, the greatest RI was observed in large gaps due to an increase in intermediate species, and the dominant species (IV > 0.1) changed from shrubs to trees (Acer mono). The DI and RI decreased over time in medium gaps, but A. mono remained the dominant species in these gaps. By 20–30 years after gap formation, large gaps showed decreased DI and RI, associated with the exclusion of shade-intolerant and intermediate species, and Tilia amurensis joined the dominant species class. In medium gaps, a decrease in DI was associated with self-thinning among intermediate shade-tolerant species. Once gap age exceeded 30 years, the DI, RI, and dominant species became stable in both large and medium gaps. Our findings indicated that natural gaps can improve the regeneration of late-successional species in secondary forests, especially at the first 30 years. However, there was a barrier in succession of the secondary forests to climax forests relying on natural gap regeneration due to the absence of key species such as Korean pine (Pinus koraiensis Sieb. et Zucc.). These results can provide a significant reference for close-to-nature management of temperate secondary forests in practice.

Soil phosphorus fractions and their availability over natural succession from clear-cut of a mixed broadleaved and Korean pine forest in northeast China

To assess phosphorus (P) status of forest soil under naturally restored vegetation, P fractions in the 10-cm soil layer were quantified at different successional stages on the clear-cut site of mixed broadleaved and Korean pine forest. Four communities of shrub, softwood broad-leaved forest, softwood and hardwood broad-leaved forest, and hardwood broad-leaved forest represented different successional stages. A soil sample from a primary broad-leaved and Korean pine stand was the control. A sequential P fractionation scheme extracted empirically defined pools of P and path analysis used to partition the direct and indirect contribution of soil P fractions to available P. The results show that available P increased significantly with long-term succession, while both sodium bicarbonate-extractable P (NaHCO3-P) and sodium hydroxide-extractable P (NaOH-P) fractions were reduced in early successional stages and increased in late stages. Compared to the primary forest, concentrations of P fractions in the four stages significantly decreased except for HCl-P, indicating that soil P supplements over the long-term did not return to primary forest levels. The results of related analysis also showed that NaHCO3-Pi levels were significantly related to available phosphorus. According to the path analysis coefficient, NaHCO3-Pi exhibited the highest effect on available P among eight P fractions; the indirect effects of other P fractions via NaHCO3-Pi were larger than those with other P fractions. Overall, this study suggests that soil P bioavailability gradually improved during natural vegetation restoration on clear-cut sites mainly through the increase of NaHCO3-P, where phosphorous is immediately available, and subsequently available phosphorus NaOH-P.

Long-term (42 years) effect of thinning on soil CO2 emission in a mixed broadleaved-Korean pine (Pinus koraiensis) forest in Northeast China

Thinning is an important forest management practice that has great potential to influence regional soil carbon storage and dynamics. The present study measured soil respiration (RS, the efflux of CO2 emitted) and its two components (heterotrophic (RH) and autotrophic (RA) respiration) from soil 42 years after thinning in comparison to un-thinning (control). Autotrophic respiration was significantly greater in the thinning plot, approximately 44% higher compared to the control, while both Rs and RH were slightly, but not significantly, higher in the thinning plot. Higher fine root biomass might have contributed to the higher Ra in the thinning plot. Both Rs and RH showed clear soil temperature-dependent seasonal patterns, whereas Ra was less responsive to changes in temperature, especially within one specific season. The annual and season-specific temperature sensitivities of RS and RH were lower in the thinning plot, specifically during the mid-growing season. Furthermore, variations in the season-specific temperature sensitivity of Rs and RH were less intense in the thinning plot. We conclude that forest thinning can reduce the temperature sensitivity of RS and RH during the mid-growing season and increase soil CO2 emission in the long term.

Relative contributions of different substrates to soil N2O emission and their responses to N addition in a temperate forest

With increasing nitrogen (N) deposition, soil nitrous oxide (N2O) emission is expected to increase, causing positive feedback to global warming. However, the substrates of soil N2O emission, especially their responses to N addition, are still unclear. Here, we conducted an in situ 15N tracing experiment to study the substrates of N2O (i.e., ammonium-derived, nitrate-derived and organic N-derived N2O emission) under N addition treatment in a temperate forest in northeast China. Nitrate derived N2O through denitrification contributed most to the total N2O emission, pointing to the importance of denitrification under ambient N deposition. NH4NO3 addition of 50 kg N ha−1 yr−1 significantly increased organic N derived N2O on the 6th day after N addition, which suggests that heterotrophic nitrification may be the dominating process with higher N deposition rate. However, because soil pH and the examined functional genes did not change after N addition, future studies should be carried out to understand if the increase of heterotrophic nitrification is transient. Our study emphasizes the role of organic N pool in soil N2O emissions, highlighting the importance of considering the heterotrophic nitrification process while studying soil N cycling or modeling soil N2O emission.