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Key message The accuracy of gap age estimation can be improved from 5–6 to 2 years by analyzing the annual width and carbon isotope discrimination of the rings of gap-surrounding trees. Context Gap age has a direct link to the composition and structure of regeneration. However, the accuracy of gap age estimation is still limited. Aim We aim to improve the accuracy of gap age estimation by analyzing the width in combination with carbon isotope discrimination (∆13C) of the rings of gap-surrounding trees. Methods Twenty-four gap-surrounding trees (nine Ulmus laciniata, eight Fraxinus rhynchophylla, and seven Juglans mandshurica) were selected from eight artificial gaps created in December 2004. First, the growth release (i.e., peak time of percent growth change) for sample trees was measured based on the tree-ring width to identify the rough time range of gap formation. Then, the ∆13C of rings during the time range were analyzed for determining the precise year of gap formation. Results The peak time of percent growth change occurred from 2005 to 2010 for U. laciniata, 2004 to 2008 for F. rhynchophylla, and 2002 to 2007 for J. mandshurica. Within the range of 2002 to 2010, the ∆13C of rings for all sample trees significantly reduced in 2005–2006 (p < 0.05), which was the estimated year of gap formation. Conclusion The introduction of ∆13C analysis could effectively reduce the estimating deviations of gap age by only considering tree-ring width analyses and finally improve the accuracy of gap age estimation within 2 years, which can provide reliable information for gap management.

Understanding the natural regeneration of Korean pine (Pinus koraiensis Sieb. et Zucc.) in mixed broadleaved–Korean pine (MBK) forests is crucial for MBK forest conservation and secondary deciduous broadleaved forest restoration. We hypothesized the ratio of Korean pine basal area (RKp) in MBK stands affected its natural regeneration. Regeneration censuses, including the height, root collar diameter, age, and growth stages (younger seedling, older seedling, smaller sapling, and taller sapling) of Korean pine, were conducted in northeast China. Results indicated the stem density and age composition of younger seedlings were positively correlated with RKp, whereas those of the saplings were negatively correlated with RKp. In the stands with lower RKp (

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.

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.

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.

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.

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.

Non-structural carbohydrates (NSC) are critical to plant signaling, gene transcription, physiological processes, survival and growth. Previous studies have demonstrated that species differing in NSC contents have various responses to stressful environments. However, effects of intra- and interspecific competition or N fertilization on NSC have not been previously studied. We employed two coniferous tree species, Larix kaempferi and Larix olgensis, to investigate the effects of intra- and interspecific competition on NSC under two N supply levels, with and without N fertilization. N fertilization significantly promoted the growth rates, biomass accumulation, carbon isotope composition and N concentration of leaves of the two species. Competition patterns significantly affected the average root diameter. Interspecific competition tended to increase the specific root length and specific root tip density. Also, interspecific competition significantly increased the N concentration of leaves but declined their C:N ratio under no N fertilization (N) treatments. L. kaempferi greatly benefited from the presence of L. olgensis, particularly under N fertilization (N+) treatments. Competition patterns, N fertilization and their interactions promoted N and carbohydrate storage. Our results demonstrated that N was a crucial factor to drive and regulate carbon balance. Differences between species in carbohydrate metabolism may contribute to their coexistence or niche differentiation. (C) 2016 Elsevier B.V. All rights reserved.

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.

The unique terrain, geography, and climate patterns of the eastern United States encourage periodic occurrences of catastrophic ice storms capable of large-scale damage or destruction of forests. However, the pervasive and persistent effects of these glaze events on regional forest distribution and composition have rarely been studied. In the southeastern US, ice storm frequency and intensity increase with increasing latitude and along the complex gradients from the coast (low, flat, sediment controlled and temperature moderated near the ocean) to the interior (high, rugged, bedrock controlled, distant from warming ocean). To investigate the potential influence of this disturbance gradient on regional forest composition, we studied the differential responses of trees (canopy position, lifeform group, and species group) to a particularly severe ice storm. Our results indicated that tree mortality and damage (canopy damage, bent bole, snapped bole, and uprooted) varied significantly between overstory and understory trees, and among species and lifeform groups. Overstory trees were more prone to glaze damage than understory trees, and evergreen broadleaf species were the most susceptible to glaze damage, while deciduous species were the least susceptible. Among the pine species studied, slash pine (Pinus elliottii Engelm.) and longleaf pine (P. palustris Mill.) suffered more severe damage and mortality than loblolly pine (P. taeda L.). Further, ice damage was correlated with distribution-based differences in injury susceptibility among pine and deciduous tree species. The most ice storm-tolerant pine species, loblolly pine, had the most northerly distribution (39.51°N), while the least resistant species were those with more southerly distribution (e.g., 33.29°N for slash pine). These results support hypotheses that the distributions of evergreen tree species are regulated by periodic catastrophic ice storms. Therefore, predicting future distributions of tree species in response to climate change should consider the role of ice storms in shaping the forest composition.

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.

Gap size is critically important to ecological processes that drive forest dynamics within the gap, yet its threshold has never been explicitly defined. Consequently, gap sizes reported in the literature ranged from 4 m 2 to 2 ha, which makes comparisons among and synthesis of the published gap studies difficult. We suggested that the lower size limit be defined by the mean shadow length (SL) of canopy trees surrounding the gap (CTSG) at local 12:00 during growing season (GS), while the upper size limit be defined by considering the farthest impact of CTSG on growth of shade intolerant tree species, which was determined by the mean of SL at the initial and the final times when 30-min photosynthetic active radiation (PAR) is more than the light saturation point for shade intolerant tree species each day during GS. The lower and upper limits of expanded gaps (the canopy gap plus the area extending to the bases of the canopy trees surrounding the gap) represented by gap diameter: CTSG height ( R D/H ) were 0.49 and 3.49, respectively, for temperate forest areas. The lower limit of gap size is determined only by the location and the height of CTSG, which should be applicable worldwide. We also tried to provide a universal method for determining the upper limit of gap size without applying the observed PAR data, and using only sunshine duration, an easily obtained variable from meteorological stations worldwide. We suggest that expanded gaps may be classified as: small gap, 0.49 D/H ≤ 1.0, medium gap, 1.0 D/H ≤ 2.0; large gap, 2.0 D/H

Forest gaps, openings in the canopy caused by death of one or more trees, have a profound effect on forest regeneration and drive the forest growth cycle. It is therefore necessary to understand the effects of forest gaps on regeneration for modern forest management. In order to provide a quantitative assessment of the effects of forest gaps on regeneration of woody plants, we conducted this review of gap effects on woody plant regeneration on the basis of 527 observations from 42 individual papers, and reported the results of these data in a meta-analysis. Overall, densities of regenerated woody plants were significantly greater (359%) in forest gaps than on the closed-canopy forest floor. The regeneration density in gaps of plantation forests was significantly greater (P < 0.05) than that of natural forest because the regeneration in gaps of plantation forests was improved by both gap effects and experimental measures. Similarly, in comparison to natural gaps, regeneration was better enhanced in artificial gaps. Regeneration density exhibited a significantly positive correlation with gap size, but a negative correlation with gap age because the gap size decreased with increasing gap age. Shade tolerance of woody plants affected regeneration density in gaps and understory. Average regeneration density of shade-tolerant species exhibited a significantly positive response to gaps but densities remained lower in total than those of intermediate and shade-intolerant species. Gap effects on regeneration decreased in response to increasing temperature and precipitation because of the limiting effects of lower temperature and moisture on woody plant regeneration. In summary, forest gaps enhance woody plant regeneration, and the effects of gaps varied by forest type, gap characteristics, environmental factors and plant traits. The results of this meta-analysis are useful for better understanding the effects and roles of gaps on forest regeneration and forest management.

Canopy gap is the driving force of forest succession. Due to the uncontrollability, however, the influences of natural disturbances on gap formation and gap distribution pattern have been rarely understood in temperate secondary forest ecosystems. We monitored the gap formation and gap distribution pattern using high-resolution remote sensing images before and after two disturbances (wind/snowstorm in 2003 and flood in 2013). The results showed that after wind/snowstorm, the gap nearest neighbor index (GNNI) decreased, the vacant land area did not obviously change while the gap fraction and gaps density (especially medium size) increased. After the flood, GNNI decreased, the number of small gaps increased but larger gaps were in many cases extended to vacant land areas leading to a smaller total number of medium and large gaps but considerable increase in vacant land area. We also found that the gap densities increased with slope and altitude for wind/snowstorm-formed gaps, but they increased with increasing slope and decreasing altitude for flood-formed gaps. These results indicated that gaps were aggregated in steep slope and high altitude areas after wind/snowstorm, but in steep slope and low altitude areas after the flood. Medium gaps were mainly created by the wind/snowstorm due to the individual-level death of dominant tree with the continuous fall of surrounding trees. While, vacant lands were obviously created during the flood because of integral sweeping. Besides, smaller trees were easily damaged by runoff of flood, which induced small gaps. In summary, forest managers may pay more attention to use gaps to accelerate forest succession after wind/snowstorms and to restore vegetation in vacant lands after floods.

Larch (Larix spp.) is planted after a clear-cut of secondary forests (SFs) to meet the timber demand of Northeast China. However, the declination of soil fertility and the acidification of surface runoff in larch plantations (LPs) resulting from mono-species composition have threatened the sustainable development of LPs. Converting pure LPs into larch-walnut mixed forests can solve those problems, in which it is crucial to promote the seed regeneration of Juglans mandshurica in LPs. The success of walnut seed regeneration is dependent on rodent removing seed away from mother trees and the dispersal processes rely on the stand structure. The spatial distribution pattern between LPs and SFs is a type of stand structure that might affect seed dispersal. There are two typical spatial patterns in Northeast China due to different topographical conditions, that is, contour type (C-T, LPs and SFs located at the same slope position and aspect) and downslope type (D-T, LPs located at the down slope of the adjacent SF in the same aspect). The objectives of our study were to verify the effects of the spatial distribution pattern of LPs on rodent-mediated seed dispersal and to determine the optimal spatial distribution type for seed dispersal. The field trial was conducted by releasing tagged J. mandshurica seeds at three stages of seed rain during two continuous growing seasons in two types of spatial distribution patterns for LPs. We found that contour type had a substantial advantage in the rodent-mediated seed dispersal, that is, the dispersal rate and the proportion of scatter-hoarded seeds in LPs of C-T stands were much higher than in D-T stands, respectively. These differences may be caused by the variation in the topographic factor for these two types of spatial distribution pattern of LPs. In the C-T stands, it was a translational motion for rodents to remove walnuts from SFs to LPs, which would be an energy efficient and favorable method of seed dispersal. Fluctuation of seed abundance had an effect on rodent behaviors and seed fates, that is, there was a faster dispersal rate and lower proportion of seeds in situ in the non-masting year of 2015 than in the masting year of 2016 and proportion of scatter-hoarded seeds reached the highest during the early stage and lowest at the middle stage of seed rain. Thus, the contour type of LPs with a favorable terrain advantage is potentially conducive to dispersing walnut seeds by rodents during early stage of seed rain and converting larch plantations into larch-walnut mixed forests.