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Monoculture practices in plantation forests reduce biodiversity and soil fertility compared with the adjacent patches of primary or secondary forests. Thus, it is urgent to convert monocultures into mixed stands to resolve those problems. In Northeast China, pure larch (Larix spp., mainly including L. olgensis, L. gmelinii, and L. kaempferi) plantations (LPs) are widely converted into mixed larch-walnut stands by introducing Juglans mandshurica (a native and light-demanding broadleaved tree species in secondary forests) into the LPs. However, the success of the natural regeneration of J. mandshurica must rely on rodent-mediated seed dispersal away from parental trees, and the dispersal processes are largely influenced by microhabitat changes during the thinning management of plantations. A field trial was conducted by releasing tagged J. mandshurica seeds in LPs with three thinning intensities (0%, 25%, and 50% thinning intensities) during two continuous growing seasons. The results indicated that among the three thinning treatments, the removal rate of seeds by rodents was the fastest, and the scatter-hoard seed (6.3 m) was most plentiful, in the 25% thinning intensity of LPs. On the contrary, the seed dispersal rate was the slowest, and the seed dispersal distance (8.3 m) was the farthest, in the stand with 50% thinning intensity in the non-mast year of 2015. This finding may be because both the changes in canopy openness and variations in microhabitats (including light incidence, soil moisture, and coverage of understory shrubs and herbs) caused by different thinning intensities have a synergetic effect on rodent-mediated seed dispersal. Inter-annual variation in seed production resulted in faster removal rates with larger cache proportion in LPs (25.93%) and shorter dispersal distance (6.4 m) in the non-mast year (2015) and vice versa in the mast year (2016). Thinning is a potentially feasible practice to promote rodent-mediated seed dispersal and the potential seed germination of J. mandshurica in LPs. We could take varying measures for different silviculture objectives to make full use of the thinning advantage in rodent-mediated seed dispersal; for instance, 25% thinning intensity (437–532 trees ha−1) provided a suitable microhabitat for rodents to disperse walnut seeds into LPs faster, and 50% thinning intensity (292–355 trees ha−1) is suggested in LPs to promote rodents to cache seeds with longer distance.

Forest gaps are important small-scale disturbances that initiate forest succession. However, natural regeneration patterns within gaps remain unclear, especially in temperate secondary forests. In this study, three-dimensional graphic systems were established to show the whole regeneration patterns of woody species with different shade tolerance in natural gaps. Furthermore, all gaps were classified into three size classes, and each gap was divided into two parts, canopy gaps (CGs) and gap peripheries (GPs). A total of 34 woody species were found in the 41 surveyed gaps, but almost no tree species had a high importance value index (≥ 0.100). Total plant density decreased from the small gaps to the large gaps, showing a density trend similar to that of shade-tolerant species. Total species richness peaked in the medium gaps, which indicated that medium gaps could provide moderate environments for most woody species. Both the species richness and plant density of shade-tolerant species were lower in CGs than in GPs, regardless of gap sizes. In contrast, both the species richness and plant density of shade-intolerant species were higher in CGs than in GPs. Interestingly, intermediate shade-tolerant species exhibited a “doughnut” regeneration pattern in the large gaps, which indicated that they could neither compete with shade-intolerant species in CGs nor invade GPs occupied by shade-tolerant species. Our findings suggest that gap formation could benefit woody species regeneration, but subsequent treatments based on gap silviculture are needed to eliminate low-value species and manage target tree species, which would help accelerate the restoration of secondary forests.

The conversion of secondary forests to pure coniferous plantations leads to a decrease in soil quality partly because of the slow decomposition rate of coniferous litter. Microbial decomposers directly regulate the decomposition process. However, little is known about how litter type and microbial communities including both bacteria and fungi interact to affect decomposition rates. A field experiment was performed consisting of seven treatments (i.e., four tree litters: Quercus mongolica, Juglans mandshurica, Acer mono, and Larix gmelinii) in which either monospecific litters or mixtures of larch litter with each broadleaved tree litter (mass ratio was 1:1) were placed into litterbags, and the microbial community composition and enzyme activities were measured. The results showed that in monospecific litters, the highest mass loss was observed in Acer litter, followed by Quercus, Juglans, and Larix litters. Furthermore, synergistic non-additive mixture effects were found in the Larix-Acer and Larix-Quercus mixtures. Most of the abundant bacterial taxa and potential functional genes encoding the hydrolytic enzymes were not significantly affected by the litter type. In contrast, each litter type was characterized by a specific fungal community, as well as different patterns of extracellular enzyme activity. The most abundant fungal decomposers in Larix litter during the first 540 days of decomposition, such as Meria, Lambertella, Venturia, and Ochroconis, were negatively correlated with most of the enzymatic activities, which possibly explains the slower decomposition rate of Larix litter. In conclusion, mixing Larix litter with broadleaved litters could change the microbial community composition, and accelerate the decomposition of hemicelluloses and lignin. Introducing broadleaved tree species with fast litter turnover rates by allowing natural regeneration and planting broadleaved tree species in pure larch plantations may be an effective way to restore soil quality in temperate forest regions.

Increased intrinsic water use efficiency enhances tree growth of native species, but not that of non-native species under warming and drying climates in Northeast China. Climate change significantly affects forest ecosystems. However, little is known about whether non-native and native tree species show similar responses to global warming. We found different trends in the basal area increment (BAI) and tree-ring stable carbon isotope ratio (δ13C) of two non-native (Pinus sylvestris var. mongolica and Populus × xiaozhuanica) and two native (Pinus tabuliformis and Ulmus pumila) tree species during the warming and drying periods from 1985 to 2014. The BAI of non-native tree species was stable, whereas that of the native tree species exhibited a significant increase. A significant increase in tree-ring δ13Ccorr (corrected for atmospheric changes in δ13C) for non-native tree species indicated increasing water stress. The intrinsic water use efficiency (iWUE, derived from tree-ring δ13C) of both non-native and native tree species increased significantly. However, the magnitude of the increase in iWUE was higher in non-native tree species than in native tree species, indicating that non-native tree species suffered stronger water stress. Increasing iWUE but no increase in BAI for non-native tree species suggested that water stress reduced stomatal conductance and, consequently, reduced carbon uptake. In contrast, increased iWUE accompanied by an enhanced BAI for native tree species indicated an increase in photosynthetic capacity induced by CO2 fertilization. These findings suggest that non-native tree species would experience greater mortality under extreme drought conditions once water stress passes a physiological threshold. However, native tree species would suffer only slightly due to benefiting from CO2 fertilization.

Forest gaps can change stand structure and affect forest regeneration dynamics. The gap partitioning hypothesis claims that tree species would regenerate along gap environmental gradients owing to their different resource demands. However, many studies that examined this hypothesis in uncontrolled gap conditions have yielded inconsistent conclusions and there are limited studies testing the hypothesis taking into account biomass allocation. In this study, we examined the gap partitioning hypothesis in a temperate secondary forest ecosystem where gap formation drives forest regeneration. We created gaps with different sizes and planted two commercially important native tree species with contrasting shade tolerance in nine positions along four cardinal directions within the gaps. We found that shade tolerance was the main factor affecting seedling regeneration performance within gaps. For the light-demanding Manchurian walnut (Juglans mandshurica Maxim.), seedling growth varied significantly along the light intensity gradients, which indicated that seedlings could greatly benefit from high light areas within gaps, especially when the gap size increased. Seedling biomass in gap centers and transitions (54.0 g) were much higher than in gap edges (13.7 g) and forest understories (8.6 g). High light environments contributed to higher biomass allocation to the leaves and accelerated carbon assimilation. Low light conditions resulted in increasing proportions of stem biomass, which might promote seedling height growth, although the promotion effects were relatively limited. However, for the shade-tolerant Korean spruce (Picea koraiensis Nakai), little evidence of seedling divergence was found within gaps. Mostly, seedlings only showed growth and biomass allocation differences between gaps and forest understories. Korean spruce showed high adaptability to various gap environments and might be a generalist species rather than being limited to small gaps. The performance of Manchurian walnut strongly supported the gap partitioning hypothesis, but Korean spruce provided little evidence for gap partitioning. Therefore, planting scenarios in silvicultural practices could be suitably designed, with Manchurian walnut in high light gap areas and Korean spruce in most positions within gaps, during the early stage after competition elimination.

To improve the productivity and ecological functions, it is essential to recover secondary forests, the major forest resources in the world, by promoting the regeneration of dominant tree species. Forest gaps are a dominant form of small-scale disturbances in secondary forests, and sprout regeneration commonly occur after the gap formation from logging. Within-gap position and gap size are two key characteristics affecting tree regeneration by changing micro-environments. Promoting the sprout regeneration of dominant tree species under forest gaps with various sizes and within-gap positions is a key measure to recover secondary forests. Twelve artificial gaps were created in March 2015 and the photosynthesis and growth of stump sprouts of three dominant tree species with varying levels of shade tolerance (Quercus mongolica, Acer mono, and Tilia mandshurica) were monitored in 2016. The results showed that within-gap position and gap size had significant effects on the photosynthetic ability of stump sprouts of Q. mongolica, i.e., the moderate light condition at the center parts of large gaps was more beneficial to its photosynthesis with the maximum PNmax of 26.49 μmol m-2 s-1. Gap size significantly affected the biomass of stump sprouts of both Q. mongolica (shade intolerant tree species) and A. mono (intermediate shade tolerant tree species), e.g., the aboveground biomass of these two tree species in large gaps (178.90 g for Q. mongolica and 158.42 g for A. mono, respectively) were significantly higher than those in small gaps (50.52 g for Q. mongolica and 56.95 g for A. mono, respectively) (P

The effects of forest restoration on ecosystem services and their trade-offs are increasingly discussed by environmental managers and ecologists, but few demonstrations have analyzed ecosystem service trade-offs with a view to informing afforestation choices. Here, we examined how the Grain for Green Program (GGP), an ambitious reforestation program in China, affected ecosystem services. We quantified regulating services and provisioning service in the potential scenarios, which were developed to improve ecosystem services better. The results indicated the GGP drove 14.5% of land-use/land-cover from 2000 to 2015, and all the regulating services increased. Prioritizing reforestations in steep-sloped and riparian farmlands can promote flood mitigation, water purification, and soil retention services by 62.7%, 25.5%, and 216.1% as compared with 2015 levels, respectively, suggesting that the improvements strongly depend on afforestation locations. Driven by the new GGP policy, a high proportion of economic forest increased provisioning service (272.2%), but at the expense of decreases in soil retention (&minus, 25.1%), flood mitigation (&minus, 11.4%), water purification (&minus, 36.6%), and carbon storage (&minus, 48.5%). We identified a suitable scenario that would reduce the trade-offs, which associated with afforestation types and their spatial allocation. Identifying priority areas of afforestation types can inform the GGP policy to assure sustainable and broader benefits.

Nitrification is a crucial step in ecosystem nitrogen (N) cycling, but scaling up from plot-based measurements of gross nitrification to catchments is difficult. Here, we employed a newly developed method in which the oxygen isotope anomaly (Δ

Denitrification is a major process contributing to the removal of nitrogen (N) from ecosystems, but its rate is difficult to quantify. The natural abundance of isotopes can be used to identify the occurrence of denitrification and has recently been used to quantify denitrification rates at the ecosystem level. However, the technique requires an understanding of the isotopic enrichment factor associated with denitrification, which few studies have investigated in forest soils. Here, soils collected from two tropical and two temperate forests in China were incubated under anaerobic or aerobic laboratory conditions for two weeks to determine the N and oxygen (O) isotope enrichment factors during denitrification. We found that at room temperature (20°C), NO3- was reduced at a rate of 0.17 to 0.35μgNg-1h-1, accompanied by the isotope fractionation of N (15e) and O (18e) of 31‰ to 65‰ (48.3±2.0‰ on average) and 11‰ to 39‰ (18.9±1.7‰ on average), respectively. The N isotope effects were, unexpectedly, much higher than reported in the literature for heterotrophic denitrification (typically ranging from 5‰ to 30‰) and in other environmental settings (e.g., groundwater, marine sediments and agricultural soils). In addition, the ratios of Δδ18O:Δδ15N ranged from 0.28 to 0.60 (0.38±0.02 on average), which were lower than the canonical ratios of 0.5 to 1 for denitrification reported in other terrestrial and freshwater systems. We suggest that the isotope effects of denitrification for soils may vary greatly among regions and soil types and that gaseous N losses may have been overestimated for terrestrial ecosystems in previous studies in which lower fractionation factors were applied.

BackgroundKorean pine seeds have primary dormancy following dispersal, leading to poor seed germination and seedling establishment. Metabolic homeostasis determines whether the seeds are dormant or non-dormant. However, the specific metabolic pathways that maintain the primary dormancy of pine seeds are poorly understood.ResultsMetabolic analysis was employed on the embryos of PDRS (seeds released from primary dormancy) and PDS (primary dormant seeds) on days 0, 5 and 11 after incubation under a germination-inductive temperature. A larger metabolic switch occurred in PDRS embryos from days 0 to 11. The contents of ninety metabolites were significantly changed from days 0 to 5, 83% of which (including most sugars, organic acids and amino acids) increased, reflecting that biosynthetic metabolism processes are initiated. The contents of ninety-two metabolites showed distinct variations from days 5 to 11, 71% of which (including most organic acids and almost all amino acids) reduced substantially. Fructose 6-phosphate, inositol-3-phosphate, 3-phosphoglyceric and D-glucose-6-phosphate contents showed the most decrease with decreasing 409-, 75-, 58- and 41-fold, indicating that the glycolysis and tricarboxylic acid (TCA) cycle strongly slowed down. The contents of the most metabolites in PDS embryos also displayed a relatively larger alteration only from days 0 to 5. Although 64% of metabolites increased from days 0 to 5, their levels were still lower compared with PDRS embryos. Furthermore, most metabolites were not further accumulated from days 5 to 11. Unlike PDRS embryos, almost all amino acids in PDS embryos did not exhibit a substantial decrease from days 5 to 11. Also, there was not a major decrease in the levels of metabolites involved mainly in glycolysis and TCA cycle, while some intermediates even increased.ConclusionsThe attenuated biosynthetic metabolism processes, the lower utilization rate of amino acids and the higher operation rate of glycolysis and TCA in embryos maintain primary dormancy.

Reliable spatial information on growing stock volume (GSV) and biomass is critical for creating management strategies for plantation forests. This study developed empirical models to map th...

In the headwater catchments of the Hun River, Northeast China, secondary forests (SF) have been replaced by plantations since the 1960s. Concern has been growing over this loss and the decline in water quality caused by the plantations. To test the effects of plantations on water quality, we selected two separate catchments covered by SF and Pinus koraiensis plantations (KP) to monitor physical and chemical properties of various hydrological variables including throughfall, stemflow, through-litterfall and runoff (flowing out of outlets of the catchments). The physical properties of water declined after water flowed through the two catchments as compared with rainwater. The pH of runoff in both catchments also dramatically decreased. The concentrations of Cl−, NO3− and NH4+ in the runoff from the two catchments were similar (concentrations of Cl− and NH4+ in both catchments were similar to those in rainwater). Total P concentration in runoff of the SF catchment was higher than that of the KP catchment (P concentrations in both catchments were also higher than in rainwater) because P concentrations in litter and soil of the SF catchment were higher than those in the KP catchment. In summary, the rainwater became acidic in both catchments, but the responses of most water quality variables were similar in the two catchments, suggesting that appropriate ratios of KP in SF are feasible for secondary forest recovery and for preserving water quality (KP did not cause a decline in quality) in the headstream regions in Northeast of China.

The long-term stressful utilization of forests and grasslands has led to ecosystem degradation and C loss. Since the late 1970s China has launched six key national ecological restoration projects to protect its environment and restore degraded ecosystems. Here, we conducted a large-scale field investigation and a literature survey of biomass and soil C in China's forest, shrubland, and grassland ecosystems across the regions where the six projects were implemented (∼16% of the country's land area). We investigated the changes in the C stocks of these ecosystems to evaluate the contributions of the projects to the country's C sink between 2001 and 2010. Over this decade, we estimated that the total annual C sink in the project region was 132 Tg C per y (1 Tg = 1012 g), over half of which (74 Tg C per y, 56%) was attributed to the implementation of the projects. Our results demonstrate that these restoration projects have substantially contributed to CO2 mitigation in China.

Natural Mongolian pine trees of different ages consistently use shallow water throughout the main growing season; therefore, water stored in the shallow soil layer is vital for maintaining their viability. Mongolian pine (Pinus sylvestris var. mongolica) plantations in sandy regions often experience dieback after 30–35 years of growth due to water deficiency, whereas natural Mongolian pine forests remain healthy during the same growth stage. However, the water use strategies of natural Mongolian pines remains unclear. Therefore, δ2H and δ18O in twig xylem water, soil water and groundwater were analyzed in 10–20, 20–30 and 30–50-year-old natural Mongolian pine trees to identify their water sources. In addition, needle δ13C was measured simultaneously to assess water use efficiency. Results showed that pine trees of different ages utilized soil water from the same depth. During the growing season (June–August), all pine trees utilized water from 0 to 20 cm soil depth, regardless of the soil water condition. During the end of growing season (September and October), even though soil moisture content in the 0–20 cm depth was higher, pine trees of different ages utilized water from the 0–60 cm soil depth in September and switched to utilize water from the 20–80 cm soil depth in October. There were no significant differences in needle δ13C among the sampling dates for trees in each age group, indicating that pine trees can absorb sufficient water to satisfy their water requirements regardless of age. These findings suggest that water stored in the shallow soil layer (0–20 cm) plays an important role in supporting tree transpiration during the growing season (June–August). Therefore, the stability of shallow soil is vital for maintaining the viability of natural Mongolian pine forests.

The frequently observed forest decline in water-limited regions may be associated with impaired tree hydraulics, but the precise physiological mechanisms remain poorly understood. We compared hydraulic architecture of Mongolian pine (Pinus sylvestris var. mongolica) trees of different size classes from a plantation and a natural forest site to test whether greater hydraulic limitation with increasing size plays an important role in tree decline observed in the more water-limited plantation site. We found that trees from plantations overall showed significantly lower stem hydraulic efficiency. More importantly, plantation-grown trees showed significant declines in stem hydraulic conductivity and hydraulic safety margins as well as syndromes of stronger drought stress with increasing size, whereas no such trends were observed at the natural forest site. Most notably, the leaf to sapwood area ratio (LA/SA) showed a strong linear decline with increasing tree size at the plantation site. Although compensatory adjustments in LA/SA may mitigate the effect of increased water stress in larger trees, they may result in greater risk of carbon imbalance, eventually limiting tree growth at the plantation site. Our results provide a potential mechanistic explanation for the widespread decline of Mongolian pine trees in plantations of Northern China.

In a semiarid sandy ecosystem, water is the most important factor in determining survival and growth of Mongolian pine (Pinus sylvestris var. mongolica) in a sparse wood grassland. However, little is known about canopy transpiration of the Mongolian pine in the sparse wood grassland and its influence on the ecosystem water cycling. In this study, we quantified canopy transpiration of Mongolian pine in a sparse wood grassland by using sap flow techniques in combination with observations of climatic factors and soil moisture during two consecutive growing seasons in 2011 and 2012. Results showed that daily canopy transpiration ranged from 0.02 to 0.27 mm day−1 and 0.01 to 0.29 mm day−1, with mean values of 0.14 and 0.15 mm day−1 in 2011 and 2012, respectively. Daily canopy transpiration increased significantly with solar radiation and vapor pressure deficit, and tended to level off at vapor pressure deficit ≥1.5 kPa in both observation years. Although precipitation and soil moisture in August and September were lower in 2011 than in 2012, monthly canopy transpiration in these two months of 2011 were no less than that in 2012, suggesting that trees were able to tap groundwater in August and September of 2011. Throughout the growing seasons, the accumulated canopy transpiration was 20.9 and 22.9 mm in 2011 and 2012, respectively, occupying 6.6 and 4.5% of precipitation (316.7 and 510.3 mm) and 8.0 and 6.1% of evapotranspiration (260.5 and 372.7 mm), indicating that canopy transpiration accounted for a small proportion of the ecosystem water budget. Thus, water budget was balanced in both years. These findings suggest that Mongolian pine in a sparse wood grassland could use groundwater to maintain the canopy transpiration, and the water supply could satisfy the transpiration requirements for growth, therefore, Mongolian pine in a sparse wood grassland could maintain stable under current water conditions.

Larch (Larix spp.) plantations are important for timber production in Northeast China, but this monoculture practice has led to problems such as decreased soil fertility and water-holding capacity. To examine the possibility of gradually converting pure larch plantations to mixed stands by small-scale canopy regulation, we planted seedlings of two species with contrasting shade tolerance, light-demanding Manchurian walnut (Juglans mandshurica Maxim.) and shade-tolerant Korean spruce (Picea koraiensis Nakai), in larch plantations with four different canopy retention intensities (larger gap, 160 m2; smaller gap, 45 m2; thinning, 25% intensity based on basal area; and control, forest understory). After two growing seasons, we found that both species had higher survival rates and growth rates in larger gaps than in forest understories, but the detailed responses to treatments differed between species. Manchurian walnut responded strongly to larger gaps but insensitively to other treatments, especially with respect to biomass accumulation. In contrast, Korean spruce responded gradually with increasing canopy openness. However, canopy treatments had almost no effect on non-structural carbohydrate (NSC) concentration, biomass allocation, and NSC pool allocation, which only differed between species. Our findings indicated that the two species of contrasting shade-tolerance were able to survive and grow in larch plantations, and a small-scale canopy treatment, especially creating gaps of ∼160 m2 in size, could significantly improve seedling survival and growth during the first two years. Therefore, enrichment planting in conjunction with a low-intensity canopy regulation may play an effective role in converting larch plantations to mixed stands while maintaining continuous stand functions during the conversion process.

Nutrient resorption before abscission is an important nutrient conservation mechanism regulated by climatic conditions and soil nutrients. However, our current understanding of leaf nutrient resorption is primarily derived from site-specific studies or from the use of green-leaf nutrient concentrations to represent those in soils. It remains unknown how nutrient resorption responds to natural soil-nutrient concentrations at a global scale. The effects of plant functional groups, climatic conditions, and soil nutrients and their interactions on leaf nutrient resorption are also unknown. In this study, we established a global database derived from 85 published papers, including 547 reports of nitrogen and phosphorus resorption efficiency (NRE and PRE), climatic factors (LAT, latitude; MAT, mean annual temperature; MAP, mean annual precipitation) and soil-nutrient data (STN, soil total nitrogen; STP, soil total phosphorus) across 111 research sites. The results demonstrated that mean NRE and PRE were 48.4 and 53.3%, respectively. NRE of trees was lower than those of shrubs. NRE and PRE of coniferous species were both higher than those of broad-leaved species. Evergreen species had higher PRE than did deciduous species. NRE was negatively related to STN, but PRE and STP were not related. Both NRE and PRE decreased with increasing MAT and MAP but increased with increasing LAT. Plant functional groups, climate and soil nutrients jointly explained 22 and 32% of the variations in NRE and PRE, respectively. It is important to note that climate (especially MAT) explained 12 and 29% of the variations in NRE and PRE, respectively, implying that continuing global warming will exert an increasingly profound influence on plant nutrient cycles.

Due to increasing timber demands, large areas of secondary forests have been converted to larch plantations (LPs) in Northeast China because the secondary forests could not produce timber as much as LPs. However, there are a series of ecological problems such as lower soil fertility, reduced water-holding capacity and acidification of surface runoff water occurring in LPs because of the single-species composition of LPs. Therefore, a guidance on how to transform LPs into larch-broadleaf mixed forests (LBMFs) at a large spatial scale is needed for local foresters. First, Landsat time series data set and SPOT-5 images were combined to map the spatial–temporal distribution of LPs in Northeast China. Then, the topographical characteristics of LPs in 2010s were determined. Furthermore, three sub-regions of LPs were divided closely linking to their ecosystem services and forest management aims. Finally, detailed information on how to transform the LPs into LBMFs was given according to the three sub-regions. The results showed that the area of LPs increased during 1980s and 2010s, and reached 2.61 million ha in 2010s. Of which, 0.72 million ha (27.6%) and 1.89 million ha (72.4%) LPs distributed in slopes less than 5° and greater than 5°, respectively. Of the LPs (72.4%) in slopes greater than 5°, 48.7 and 23.7% located in downslope (LPs locating at the down slope of adjacent secondary forests in the same aspect) and upslope (LPs locating at the up slope of adjacent secondary forests in the same aspect), respectively; 0.10 million ha (3.8%) located in slopes greater than 25°. The LPs were divided into Sub-Regions-I, II, III according to slopes. For Sub-region-I (top role is to produce timber), self-fertilizing shrub species can be introduced after clean cutting for young-aged LPs, and release thinning can be applied for middle-aged LPs, fast growth thinning for near-matured LPs, and clear cutting for matured LPs. For Sub-Region-II (the priority is to provide water conservation combined with timber production), the LPs should be induced into LBMFs. The LPs located the downslope positions in Sub-Region-II can be transformed to LBMFs by natural regeneration of broadleaved tree species after thinning because of enough seed sources of broadleaved tree species from the secondary forests locating the upslope positions. The LPs located the upslope positions or the region where seed sources of broadleaved tree species are unavailable in Sub-Region-II must be induced to LBMFs by artificial regeneration. For Sub-Region-III (the preference is only for water conservation because the slope is greater than 25°), the LPs should be particularly protected from intensive disturbances, and induced into LBMFs by natural regeneration.

The changes of nutrient limitation status for tree growth across a plantation chronosequence have great implications for plantation management. The underlying mechanisms for such a shift, however, have seldom been addressed. While plant nutrient use strategies would change in response to soil nutrient alteration, they may also create feedback on soil nutrient dynamics and thus plant nutrient limitation status. We examined soil and foliar nutrients of larch (Larix kaempferi), the dominant timber species in Northeast China, across a plantation chronosequence. Total soil N increased but total soil P decreased across the chronosequence. Similarly, N concentrations in the green leaves were positively correlated, and P concentrations were negatively correlated with stand age. Foliar N:P ratios, N and P resorption efficiencies and PRE:NRE were positively correlated with stand age, indicating the shift from N-limitation to P-limitation across the chronosequence. P concentration in senesced leaves decreased and N:P ratios increased across the chronosequence, which has implications for decomposition and nutrient release. Nutrient resorption, soil pH, biomass P sequestration and imbalanced inputs of N and P would contribute to the occurrence of P-limitation with increased stand age. Furthermore, adaptive fertilization management strategies should consider the shift of nutrient limitation patterns across the chronosequence.

Larch (Larix spp.) is planted extensively after clear-cut of secondary forests in Northeast China to satisfy the enormous timber demands. However, some serious problems arise in larch plantations (LPs) due to mono-species composition, such as a decline in soil fertility and acidification of surface runoff. Converting pure LPs into larch-broadleaf mixed forests would resolve these issues. Since larch-walnut mixed forests are mutually beneficial, it is crucial to promote the regeneration of Juglans mandshurica in LPs. Rodent-mediated seed dispersal of J. mandshurica determines the regeneration success or failure and is closely related to stand structure. Forest gaps are formed by an uneven thinning that changes stand structure in plantation management; the gap size and within-gap position are two key characteristics that affect tree regeneration by changing the micro-environment. We hypothesized that rodent-mediated walnut seed dispersal were strongly influenced by these two key factors. The field trial was conducted by releasing tagged J. mandshurica seeds in two sizes of artificial gaps in LPs (small and medium gaps) and at four positions (south, north, east and west) of the gap edges. The seed releases were conducted at three stages of seed rain (early, middle and final stages) during two continuous growing seasons. Our results showed that only the seed removal rate was significantly affected by gap size and within-gap position; the rate in medium gaps was significantly higher than that in small gaps (Wald = 24.57, df = 1, P

Larch (Larix spp.) is widely distributed in the boreal and temperate areas. Nitrogen (N) is considered as the major limiting element for these areas. Thinning is a common forest management practice. Thus, it is imperative to obtain a better understanding on how thinning could affect N status of larch plantations, and thus optimize the thinning intensity for sustainable forest management. In this study, we measured N concentrations and 15N natural abundance (δ15N) of needles and surface soil (0–10 cm) in a larch plantation with T0, T25 and T50 treatments (0%, 25% and 50% thinning intensities, respectively) in Northeast China. We found that needle and soil δ15N in T25 was the lowest, and the highest in T50. No significant differences were observed for needle and soil N concentrations among the thinning intensities. T25 exhibited the highest N resorption efficiency, indicating highest N use efficiency. Overall, N cycling in T25 was more closed than the control, and with lower soil N availability, whi...

Previous studies suggest that when using natural regeneration of Korean pine (Pinus koraiensis Sieb. et Zucc.) many seeds do not germinate until the second Spring after dispersal and hence are subject to significant predation pressures. This could be an important factor affecting the restoration of Mixed-broadleaved Korean pine forests in northeast China. This study examined changes of dormancy status under laboratory and field conditions as well as physiological mechanisms involving abscisic acid (ABA), gibberellin acids (GA₁₊₃, GA₄₊₇) and the storage reserves of seeds. Our results suggest that seeds of Korean pine have primary dormancy that can be classified as morphophysiological dormancy, i.e. when dispersed they have an underdeveloped embryo with a physiological component of dormancy. A proportion of seeds germinate in the first year after dispersal as the winter chill has broken the physiological dormancy but only when the embryo has had time to elongate in April–June. The remaining seeds germinate in the second Spring after dispersal when embryos are fully developed and the physiological dormancy imposed in the Summer after dispersal has been broken by winter chilling. Thus the seed strategy of Korean pine attempts to reduce risks by providing opportunities for germination in 2 years following dispersal.

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 gaps play an important role in tree regeneration and forest restoration in modern silviculture. Many previous studies examined gap effects on seedling survival, growth, or both but lacked a systemic assessment incorporating morphological and physiological responses to varying environmental gradients along gap size and within-gap position, which limits foresters to understand the general gap impacts on the early stage of regeneration. Here, we systemically evaluated gap impacts on over 20 regeneration-related variables from two dominant tree species (Manchurian walnut [Juglans mandshurica Maxim.] and Korean spruce [Picea koraiensis Nakai]) planted along the gap-understory gradients based on our previous studies of survival and growth of these two species. The factor analysis of mixed data indicated that seedling specific leaf area, biomass allocation, chlorophyll content, and NSC content in gaps were largely determined by interspecific differences, but seedling survival, growth, and biomass accumulation were mainly related to gap size and within-gap position. Specifically, the net photosynthetic rates of Manchurian walnut were higher in large and medium gaps (or gap centers and transitions) than those in small gaps and forest understory (or gap edges). The root starch content of seedlings in gaps was almost twice as much as that in the forest understory, which also reflected species divergence in gaps. By contrast, Korean spruce exhibited less variation in net photosynthesis and root starch content and fewer differences for most other examined variables, which indicated that it had broad ecological niches and a relatively small gap would promote its regeneration during the seedling stage. Our findings provide evidence of gap partitioning on explaining regeneration patterns of light-demanding species. Therefore, gap size and within-gap position can compensate for each other during gap-based silviculture practices. Timely selective logging can extend an existing gap in a specific direction to change the relative position of target tree species for better light conditions. Appropriate selection and match of tree species to gap size and within-gap position during enrichment planting can improve gap space utilization.

Larch (Larix spp.) is a dominant timber species in northeast China. However, compared with the adjacent secondary forests, the soil nutrient conditions in the 40-year-old larch plantations have significantly deteriorated. Moreover, large quantities of nutrients are removed from sites when larch plantations are harvested, leading to further depletion of soil nutrients. Therefore, it is essential to assess nutrient removal under different harvesting scenarios to improve nutrient management. In this study, we quantified biomass and nutrient (including macro-nutrients: nitrogen (N), phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg), and micro-nutrients: iron (Fe), manganese (Mn), copper (Cu) and zinc (Zn)) accumulation in above- and below-ground components along a chronosequence (10-, 21-, 34- and 55-year-old, respectively) of larch plantations in northeast China. Our results showed that the root/shoot ratio was 0.23, 0.12, 0.18 and 0.24 in the 10-, 21-, 34- and 55-year-old stands, respectively, with an average of 0.19. For each larch tree thinned in the 10- and 21-year-old stands or harvested in the 34- and 55-year-old stands, 173.6, 1197.3, 1878.1 and 4230.7 g of nutrient elements were removed from the sites, respectively. If the leaves, branches, bark and roots remained at the sites, nutrient removal was reduced by 89, 69, 59 and 44% in the 10-, 21-, 34-, and 55-year-old stands, respectively. Branches and leaves contained the largest proportion of nutrients in the 10- and 21-year-old stands, and should thus remain at the sites to avoid further nutrient removal during early thinning. In contrast, debarking stems was a feasible practice when clear-cutting the 34- and 55-year-old stands because bark contained large amounts of nutrients (especially N, P and K). Substantial proportions of the macro-nutrient N and the micro-nutrient Zn were accumulated in the stems, and thus, these two elements would become depleted after harvesting. Except for the stems, macro-nutrients (N, P and K) were mainly stored in the leaves of the 10- and 21-year-old stands and the bark and roots of the 34- and 55-year-old stands, By contrast, macro-nutrients (Ca and Mg) and micro-nutrients (Fe, Mn, Cu and Zn) were primarily concentrated in the branches and roots of all tested stands. Therefore, nutrient loss may be avoided by leaving the harvested components containing specific nutrients at the sites.

Snow/wind damage is one of the important natural disturbances in forest ecosystems, especially in a montane secondary forest. However, the effects of snow/wind damage remain unclear which affects the management of these forests. Therefore, we investigated the responses of species, individual tree traits and stand structure to snow/wind damage in a montane secondary forest. Results show that, amongst the canopy trees, Betula costata exhibited the most uprooting, bending and overall damage ratio (the number of damaged stems to the total number of stems in a plot); Quercus mongolica showed the highest breakage ratio and Fraxinus mandshurica and Juglans mandshurica the least overall damage ratios. Among the subcanopy trees, Carpinus cordata, Acer mono, Acer tegmentosum and Acer pseudo-sieboldianum showed the least uprooting and breakage, and the most bending damage. A. pseudo-sieboldianum demonstrated the lowest breakage and highest bending damage ratios. These findings indicate that different species have various sensitivities to snow/wind damage. Larger trees (taller, wider crowns) tend to break and become uprooted, while smaller trees are bent or remain undamaged, suggesting that tree characteristics significantly influence the types of damage from snow and wind. Stands of Q. mongolica and B. costata had the highest damage ratios, whereas A. pseudo-sieboldianum had the lowest snapping ratio. In summary, the severity and type of snow/wind damage are related to individual tree attributes and stand-level characteristics. Therefore, selection of suitable species (e.g., shorter, smaller with deep root systems, hard wood, bending resistance and compression resistance) and appropriate thinning are recommended for planting in the montane secondary forests.

Continuous increases in anthropogenic nitrogen (N) deposition are likely to change soil microbial properties, and ultimately to affect soil carbon (C) storage. Temperate plantation forests play key roles in C sequestration, yet mechanisms underlying the influences of N deposition on soil organic matter accumulation are poorly understood. This study assessed the effect of N addition on soil microbial properties and soil organic matter distribution in a larch (Larix gmelinii) plantation. In a 9-year experiment in the plantation, N was applied at 100 kg N ha−1 a−1 to study the effects on soil C and N mineralization, microbial biomass, enzyme activity, and C and N in soil organic matter density fractions, and organic matter chemistry. The results showed that N addition had no influence on C and N contents in whole soil. However, soil C in different fractions responded to N addition differently. Soil C in light fractions did not change with N addition, while soil C in heavy fractions increased significantly. These results suggested that more soil C in heavy fractions was stabilized in the N-treated soils. However, microbial biomass C and N and phenol oxidase activity decreased in the N-treated soils and thus soil C increased in heavy fractions. Although N addition reduced microbial biomass and phenol oxidase activity, it had little effect on soil C mineralization, hydrolytic enzyme activities, δ13C value in soil and C–H stretch, carboxylates and amides, and C–O stretch in soil organic matter chemistry measured by Fourier transform infrared spectra. We conclude that N addition (1) altered microbial biomass and activity without affecting soil C in light fractions and (2) resulted in an increase in soil C in heavy fractions and that this increase was controlled by phenol oxidase activity and soil N availability.

In response to increasing timber demands, extensive areas of secondary forests have been converted to larch (Larix gmelinii) plantations in northeast China. It has been reported that the conversions have led to reduction in soil organic matter and nutrient availability. Soil microbes play crucial roles in nutrient cycling. However, the effects of forest conversion on soil microbes are not yet well understood in temperate forest ecosystems. In this study, the structure and function of bacterial and fungal communities were compared between larch plantations and adjacent secondary forests relative to surface litter and two soil layers (0–10 cm and 10–20 cm) using Illumina MiSeq sequencing. We found that the impacts of the forest conversion on soil microbes mainly occurred in the top (0–10 cm) soil layer. Soils in secondary forests were associated with a higher proportion of copiotrophic bacteria (Proteobacteria and Actinobacteria). In contrast, larch plantation soil was enriched with Acidobacteria, which are oligotrophs that appear to be well suited to acidic conditions. These differences in soil bacterial communities confirm that the forest conversion resulted in soil acidification and a large decline in available soil nutrient in larch plantations. Furthermore, the results of bacterial functional genes, inferred by PICRUSt, and the results of extracellular enzyme assays indicate that fungi likely dominated soil organic carbon decomposition in forest soils. Sordariomycetes was the most abundant fungal class in soils of secondary forests. The abundance of Sordariomycetes fungi was related to higher levels of almost all enzymes measured, except for cellobiohydrolase and β-1,4-N-acetyl-glucosamidase. These two enzymes were positively associated with Agaricomycetes fungi in soils of secondary forests. The fungal community in the soils of larch plantations was dominated by the class Agaricomycetes, which was positively associated with activities of both hydrolytic and oxidative enzymes. These results indicate that fungal classes, Sordariomycetes and Agaricomycetes, are most likely responsible for the decomposition of soil organic matter in secondary forests, while Agaricomycetes fungi might be more important in controlling decomposition in larch plantations.

To reveal the effects of water stress on the decline in introduced Mongolian pine (Pinus sylvestris var. mongolica) plantations, radial growth (basal area increment, BAI) and tree-ring cellulose δ13C (carbon isotope composition) were compared in an introduced Mongolian pine plantation and a natural Mongolian pine forest during 1965–2009. Results showed that the BAI of plantation trees increased until 1990, followed by an abrupt decline from 1990 to 1996 and maintained a stable level thereafter. In contrast, no sign of decreased growth was found for natural trees. Tree-ring δ13Ccorr of plantation trees, corrected for atmospheric changes in δ13C, significantly increased before and after 1990. However, no significant variations in tree-ring δ13Ccorr for natural trees before and after 1990 indicated no obviously water stress. Intrinsic water-use efficiency (iWUE, calculated from tree-ring δ13C) significantly increased before and after 1990 for both forests. Significant negative relationship between BA...

The impacts of anthropogenic nitrogen (N) deposition on forest ecosystems depend in large part on its fate. However, our understanding of the fates of different forms of deposited N as well as the redistribution over time within different ecosystems is limited. In this study, we used the 15 N-tracer method to investigate both the short-term (1 week to 3 months) and long-term (1-3 yr) fates of deposited NH4 + or NO3 - by following the recovery of the 15 N in different ecosystem compartments in a larch plantation forest and a mixed forest located in northeastern China. The results showed similar total ecosystem retention for deposited NH4 + and NO3 - , but their distribution within the ecosystems (plants vs. soil) differed distinctly particularly in the short-term, with higher 15 NO3 - recoveries in plants (while lower recoveries in organic layer) than found for 15 NH4 + . The different short-term fate was likely related to the higher mobility of 15 NO3 - than 15 NH4 + in soils instead of plant uptake preferences for NO3 - over NH4 + . In the long-term, differences between N forms became less prevalent but higher recoveries in trees (particularly in the larch forest) of 15 NO3 - than 15 NH4 + tracer persisted, suggesting that incoming NO3 - may contribute more to plant biomass increment and forest carbon sequestration than incoming NH4 + . Differences between the two forests in recoveries were largely driven by a higher 15 N recovery in the organic layer (both N forms) and in trees (for 15 NO3 - ) in the larch forest compared to the mixed forest. This was due to a more abundant organic layer and possibly higher tree N demand in the larch forest than in the mixed forest. Leachate 15 N loss was minor (

International audience; AbstractKey messageThe gap dynamics in the studied secondary forest were comparable to those of other temperate forests; large openings were filled within 30 years by afforestation; large and medium gaps closed 30–40 years after they formed.ContextGaps have important roles in forest regeneration and plant succession. However, it is difficult to determine gap dynamics over long time periods at regional scales.AimsWe studied how the dynamics of gaps and large openings (oversized “gaps”) changed in a secondary temperate forest over 50 years.MethodsWe computed the dynamic indices of gaps (16–3257 m2) and large openings (>3257 m2) using remote sensing techniques applied to six satellite images that were taken approximately every 10 years. Additionally, number-based gap closure ratios were calculated at each interval.ResultsGap dynamics were comparable in magnitude to those calculated for other temperate forests, and 60% and 53% of the large and medium gaps had closed within 30–40 and 20–30 years, respectively. The small gaps closed within 10 years, based on ground-level surveys, and 79.2% of large openings that existed in 1964 were covered by artificial forests in 1994.ConclusionGaps of different sizes closed within 40 years due to natural regeneration. Large openings had closed within 30 years via afforestation. These findings can be used for evaluating recovery status and for predicting succession times in secondary forest structures driven by gap formation.

Populus trees play an important role in windbreak construction in northern China, but widespread decline and mortality of Populus windbreaks have occurred in recent years. The phenomenon of die-backs from the top of Populus trees downwards is commonly found in declining windbreaks preceding tree deaths, particularly in water-limited areas. To provide a mechanistic explanation for such top-down diebacks, we analyzed the variation in xylem hydraulics, leaf water relations, and tissue non-structural carbohydrate contents (NSC) along the vertical axes of Populus pseudo-simonii trees, i.e. one of the most commonly used tree species for creating windbreaks in northern China. Consistent changes were observed along the height gradient of the tree crown, i.e. with the height increase branches showed a clear trend of decrease in hydraulic efficiency and leaves showed symptoms of greater drought stress. The stem hydraulic conductivity standardized using whole cross-sectional wood area decreased from 4.66 at the bottom to 1.69 kg m−1 s−1 MPa−1 at the top of the tree crown. Such a change in hydraulic efficiency was accompanied by significant increase in the degree of xylem embolism (i.e. from

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.

Mongolian pine (Pinus sylvestris var. mongolica), the most important tree species for sand fixation in semi-arid sandy lands, experiences dieback in plantations after approximately 35–40 years since planting. Water limitation was considered as an important cause for the dieback. The spatial distribution of root systems plays a key role in determining water resources used by trees. However, it is still unknown for the spatial distribution of Mongolian pine trees with different ages, which constrains our understanding of the mechanism underlying the dieback and management of Mongolian pine plantations. Horizontal and vertical distributions of coarse roots [root diameter (RD) ≥ 5 mm] of Mongolian pine trees with 10, 20, 30, 40, 50 years old were estimated by ground-penetrating radar. Meanwhile, the distributions of fine roots (RD ≤ 2 mm) were simultaneously measured using soil coring method. Results showed that the horizontal distance of coarse roots from trees increased with tree ages up to 30–40 years, i.e., 3.0–4.0 m for 10- and 20-year-old trees, ca. 6.1 m for 30- and 40-year-old trees; but merely 4.85 m for 50-year-old trees. The fine root densities kept relatively even within 4.0 m from the trees for 10- and 20- year-old trees (ca. 0.02 mg cm−3), whereas, they decreased sharply with increasing of distances from the trees for 30-, 40- and 50-year-old trees (0.015–0.025 mg cm−3 at 1.0 m from trees to ca. 0.00 mg cm−3 at 5.0 or 6.0 m from trees). In the vertical direction, the rooting depth, at which 95% of roots located, was 83.5, 99.9, 123.6, 105.2 and 125.9 cm for the coarse roots of 10-, 20-, 30-, 40- and 50-year-old trees, respectively, whereas it was 140.6, 152.2, 135.5, 144.2 and 127.0 cm for fine roots, respectively. These findings indicated that the spatial distribution of roots of Mongolian pine trees shrank for the 50-year old trees, possibly due to being close to mature stage in plantation sites. Based on spatial distribution of roots, we suggested that the stand densities should be determined according to the area occupied by tree root systems in order to avoid the dieback or mortality due to water competition, i.e., 403–515, 321–402, 217–290, 139–179 trees ha−1 for 10-, 20-, 30- and 40-year-old trees, respectively (none for 50-year-old plantations as they were close to mature stage). Our findings provide new insights into stand density optimization of plantations from tree belowground competition for water in semi-arid and arid sandy regions.

Poplar (Populus × xiaozhuanica) plantations play an important role in controlling desertification in semiarid sandy regions of Northeast China, but their dieback occurs frequently in extreme drought years due to greater water loss by transpiration than water uptake. However, little is known about dynamics of canopy transpiration in poplar plantations, which limits our understanding of dieback mechanisms and proper management of these poplar plantations. Here, canopy transpiration and canopy conductance in 18-year-old poplar plantations were quantified by sap flow measurements in combination with monitoring of concurrent environmental variables during two consecutive growing seasons in normal and wet years (2018 and 2019). Results showed that daily canopy transpiration averaged 1.2 mm d−1 (between 0.3 and 1.9 mm d−1) and 1.5 mm d−1 (between 0.2 and 2.4 mm d−1) in 2018 and 2019, respectively. Solar radiation explained more variability of daily canopy transpiration than vapor pressure deficit (VPD) in both years, indicating that canopy transpiration was more controlled by radiation than by VPD. Total canopy transpiration during the growing seasons in 2018 and 2019 was 184.1 mm and 235.9 mm, respectively, accounting for 47.1 % and 44.2 % of precipitation over the same period. Sum of canopy transpiration, soil evaporation and change in soil water storage was higher than precipitation in most months, indicating that trees took up water from deep soil layer (>1.0 m) and groundwater. Additionally, canopy conductance averaged 1.4 m s−1 and 1.9 m s−1 in 2018 and 2019, respectively, and decreased significantly with increasing VPD. However, sensitivity of canopy conductance to VPD decreased from 0.62 in 2018 to 0.27 in 2019, indicating a shift from more to less strict stomatal regulation. These findings indicate that poplar plantations are vulnerable to dieback during extreme drought years with decline in groundwater level due to utilization of groundwater for higher canopy transpiration rate.

Gap formation and closure play important roles in forest succession. Most studies focused on regeneration within gaps, but less is known about the growth dynamics of canopy trees surrounding the gaps (i.e., gap border trees), which limits our understanding of the gap-filling process. In this study, terrestrial laser scanning (TLS) was used to quantify the crown asymmetry of six canopy tree species with different wood densities among young gaps (15a), old gaps (32a), and closed forest stands (CK). The size of expanded gaps ranged from 166.8 to 408.1 m2 with an average of 271.5 m2. Each gap border tree was separated into two parts, i.e., one part facing the gap (FG) and the other facing the forest interior (FF). The ratios (FG/FF) of crown length, crown projected area, and crown volume of these two parts were calculated to represent one-, two-, and three-dimensional crown asymmetry, respectively. We found that 90% gap border trees had asymmetric crown toward gaps with an average ratio of 1.58. For trees in the closed forest stands, although crown asymmetry occurred randomly in different directions and had a large variation, the average ratio was close to 1.00. However, the gap age and the location of gap border tree (i.e., north, east, south, and west of the gap) did not show significant influences on the crown asymmetry because differences of crown asymmetry probably disappeared after the long period of gap formation. The crown asymmetry exhibited inter-specific variations and was positively correlated with wood density, which indicated that tree species with a higher wood density such as Acer mono (0.61 g cm−3) could support a more asymmetric crown and have a higher asymmetric crown threshold than others such as Juglans mandshurica (0.45 g cm−3). Crown asymmetry had similar results among different dimensions. Our findings indicated that the general crown asymmetry of gap border trees might increase snapping and uprooting risks during wind and snow disturbances. Although gap-based silviculture is widely applied in forest management, additional practices such as thinning or selective harvesting should be considered to create more space and alleviate an asymmetric crown induced by gap formation.

Mongolian pine ( Pinus sylvestris var. mongolica ) plantations in semiarid sandy lands play an important role in the prevention and control of desertification in northern China, but plantation dieback often occurs approximately 30–35 years after planting due to water deficiency. However, the seasonal changes in the water sources used by Mongolian pine trees of different ages remain unclear, and these changes influence the management of the plantations. During two consecutive years, 2012 and 2013, stable isotopes of hydrogen and oxygen from twig xylem water, soil water at different depths (0–20, 20–40, 40–60 and 60–100 cm), precipitation and groundwater were analyzed in 10-, 22-, 32- and 42-year-old Mongolian pine plantations to identify the sources of water used by the trees during different seasons (spring, summer and autumn). The soil water content, groundwater level and δ 13 C of the needles were measured concurrently, and the results showed that the 10- and 22-year-old trees only used soil water during the two years of observation, but the 32- and 42-year-old trees utilized both soil water and groundwater. In spring (late May), when soil water content was low, the 10- and 22-year-old trees used water from the 20–60 cm soil layer, but the 32- and 42-year-old trees utilized water from the 40–100 cm soil layer and groundwater. In summer (mid or late July), when soil water content was high, the trees of different ages used water from the 0–60 cm soil layer, and in autumn (mid or late September), when the soil water content was relatively high (2012), the trees of different ages used water from the 20–100 cm soil layer. However, when the soil water content was relatively low (2013), the 10- and 22-year-old trees continued to use water from the 20–100 cm soil layer, but the 32- and 42-year-old trees absorbed water from both the 60–100 cm soil layer and the groundwater. The contribution of groundwater to tree transpiration increased with decreasing soil moisture for the 32- and 42-year-old trees, and groundwater was the dominant water source (e.g., 60.2% of tree transpiration in the 42-year-old trees) when the soil moisture was approximately 20% of field capacity (3.5%). In addition, the significantly higher δ 13 C in the needles of the 22-year-old trees relative to the four ages of pine trees suggested higher water use efficiency in the 22-year-old trees. These findings suggest that the old Mongolian pine plantations (e.g., 30–35 years old) will experience greater mortality in the future if the groundwater level suddenly declines under drought conditions, but the young Mongolian pine plantations would suffer only slightly because their roots are unable to reach the saturated zone.

In Northeast China, larch (Larix spp.) plantations exchanged from broadleaved secondary forests to meet the demand of timber consumption are facing some serious problems, including soil fertility decline, acidification of surface runoff and low levels of biodiversity. How to promote the regeneration of broadleaved tree species in larch plantations and convert the pure larch plantations into mixed larch-broadleaved forests would be a top priority to solve some of these issues. The success of this conversion mainly depends on the seedling performances of the selected broadleaved tree species. Seedlings of four dominant broadleaved tree species (intermediate shade tolerant species: Fraxinus mandshurica and Acer mono, shade intolerant species: Juglans mandshurica and Quercus mongolica) from the secondary forests were planted in the thinned pure larch stands (0%, 25%, 50% and 100% thinning) to explore the adaptive strategies (biomass accumulation, leaf structural, nutrient and physiological traits) of two shade-tolerant types of broadleaved tree species responding to the thinning in larch plantations. We found that compared with the intermediate shade tolerant group, the shade intolerant species had higher leaf construction cost (CC), carbon:phosphorus (C:P) ratio, nitrogen:phosphorus (N:P) ratio, and nonstructural carbohydrate concentration (NSC) in stems and roots at all thinning treatments, and had higher leaf ash and specific leaf area (SLA) only in the larch plantations with a canopy openness more than 25 ± 1% (i.e., the 50% and 100% thinning intensity). Across all the thinning treatments, higher survival rates were found for the shade intolerant species relative to the intermediate shade tolerant species, but relative height growth (RHG) did not differ significantly between the groups. When the canopy openness of larch plantations was less than 21% (i.e., 0% and 25% thinning intensity), the intermediate shade tolerant group could utilize essential growth resources more efficiently by investing less energy per unit of RHG, and had a greater photosynthetic capacity at minimal costs relative to the shade intolerant species. However, regression analyses indicated that across all the thinning treatments (and hence all the light irradiance in this study), at a given NSC, there was a higher RHG for the shade intolerant group; also at a common SLA, there were a lower CC and lower area-based N for the shade intolerant group. Ordination using 15 eco-physiological and chemical traits of plants indicated that the major axis of shade intolerant-intermediate shade tolerant dichotomy was primarily driven by leaf NSC, CC and C:P ratio, and was significantly linked with RHG. These results demonstrated that it is a potentially feasible practice for building uneven-aged larch-broadleaved mixed forests by thinning and planting seedlings of native broadleaved tree species in pure larch plantations. Furthermore, the regeneration of the intermediate shade tolerant group and the shade intolerant species could be promoted in larch plantations with the canopy openness less than 21% and more than 25%, respectively. This study may provide an insight into uneven-aged and mixed silviculture for larch plantation forests.

The mixed-broadleaved Korean pine ( Pinus koraiensis Sieb. et Zucc.) forest (MBKPF) is a zonal climax ranging from Northeast China to the Far East region of Russia. However, since the 1950s, most of the MBKPFs in Northeast China have become the mixed broadleaved secondary forests due to the heavy forest exploitation by timber harvesting. The natural regeneration of Korean pine is generally poor because of the light limitation in Northeast China. This study was conducted to understand how Korean pine seedlings adapt to different canopy openness (CO) and what would be the minimum CO for its establishment and growth in the natural forest stands. In order to test the hypothesis that Korean pine seedlings have acclimated to low light regimes, the responses of photosynthetic, morphological and growth parameters to light environments were examined in Korean pine seedlings that were field-grown in secondary forests. The results can provide the references for the restoration of MBKPFs in Northeast China. Three-year-old Korean pine seedlings were under-planted in secondary forest stands with three levels of CO at 30%, 15% and 7%, which were created by modifying the branches of the secondary forest stands. After one year of acclimation under the secondary forest stands, the variations in the physiological and morphological characteristics of the needles as well as the growth parameters were measured. The maximum net photosynthesis ( A max ), dark respiration ( R d ), needle chlorophyll (Chl) contents and total needle nitrogen (N) contents all responded positively to the increased CO. The total non-structural carbohydrate (TNC), A max , R d , and N contents in the 7% stand were significantly lower than those in the 15% and 30% stands ( p F v ′/ F m ′) in the 7% stand was the lowest and was significantly different from those in the 15% and 30% stands ( p p > 0.05). These results indicated that 7% CO seedlings were experiencing some degree of low light stress. The leaf mass area (LMA) and height growth decreased as the CO decreased from 30% to 15% to 7%, while the lateral shoot growth increased. Therefore, needle morphology, tree height and crown growth of Korean pine seedlings displayed a large plasticity in response to variations in light intensities. These findings suggest that Korean pine seedlings can strongly acclimate in the forest canopy by adjusting physiological traits that are associated with light energy distribution in the PSII and by adjusting the morphological features of their needles. These results suggest that the minimum canopy openness required for the survival of P. koraiensis seedlings (4 years old) grown under a canopy should be more than 7%.