Increasing Thinning Intensity Research Articles

Response of Fuel Characteristics, Potential Fire Behavior, and Understory Vegetation Diversity to Thinning in Platycladus orientalis Forest in Beijing, China

Objective: Active fuel management operations, such as thinning, can minimize extreme wildfire conditions while preserving ecosystem services, including maintaining understory vegetation diversity. However, the appropriate thinning intensity for balancing the above two objectives has not been sufficiently studied. Methods: This study was conducted to assess the impact of various thinning intensities (light thinning, LT, 15%; moderate thinning, MT, 35%; heavy thinning, HT, 50%; and control treatment, CK) on fuel characteristics, potential fire behavior, and understory vegetation biodiversity in Platycladus orientalis forest in Beijing using a combination of field measurements and fire behavior simulations (BehavePlus 6.0.0). Results: A significant reduction in surface and canopy fuel loads with increasing thinning intensity, notably reducing CBD to below 0.1 kg/m3 under moderate thinning, effectively prevented the occurrence of active crown fires, even under extreme weather conditions. Additionally, moderate thinning enhanced understory species diversity, yielding the highest species diversity index compared to other treatments. Conclusions: These findings suggest that moderate thinning (35%) offers an optimal balance, substantially reducing the occurrence of active crown fires while promoting biodiversity. Therefore, it is recommended to carry out moderate thinning in the study area. Forest managers can leverage this information to devise technical strategies that simultaneously meet fire prevention objectives and enhance understory vegetation species diversity in areas suitable for thinning-only treatments.

Effectiveness of thinning in mitigating post-drought legacy effects on Chinese fir in subtropical plantation forests, Southeastern China

Legacy effects following drought are widely detected across worldwide forests, significantly affecting the growth recovery and susceptibility of trees after droughts. Thinning is a common forest management practice used to alter tree growth and climate-growth relationships. Although the effects of thinning on tree response during drought have been investigated, how thinning modulates post-drought legacy effects remains largely unknown. In this study, based on tree-ring data of 140 trees, we examined the effects of thinning on post-drought legacy effects using the quantile mixed effect model for Chinese fir (Cunninghamia lanceolata) in Southeastern China. The tree-ring data were stratified sampling from a thinning experiment applied 10 years ago in 8-year-old plantations, and included four thinning intensities (20 %, 25 %, 33 %, and 50 % reduction of tree number) and an unthinned control treatment. Drought legacy effects of tree growth positively depended on tree social status with the magnitudes and variations larger in higher status classes. Dominant large trees without thinning management had the greatest drought legacy effects. Although the effects of thinning varied slightly at different quantiles, they all indicated that thinning mitigated the growth legacies after drought and the reduction effect was more pronounced with increasing thinning intensity. Thinning could also reduce post-drought climate sensitivities, but only after moderate thinning (20 % and 25 % thinning intensity). Heavier thinning (33 % and 50 % thinning intensity) instead enhanced tree growth responses to climate changes following drought. Thinning intensity needs to be carefully considered to really reap the post-drought benefits of forest thinning management. Our findings suggested that mild thinning offered an alleviation of climate dependency following drought in addition to reducing drought legacy effects on growth, benefiting tree recovery from drought. The results of this study are useful to inform management adaptive strategies for drought-vulnerable plantations under increasingly frequent droughts.

Light thinning effectively improves forest soil water replenishment in water-limited areas: Observational evidence from Robinia pseudoacacia plantations on the Loess Plateau, China

Soil water plays a key role in vegetation growth and recovery, mainly replenished by rainfall in the water-limited areas. Thinning is an important practice for forest reconstruction and soil water regulation. However, the effects of different thinning intensities on the dynamics of soil water and its replenishment by rainfall have been inadequately quantified, which could provide a guide for reasonable thinning intensity determination for in high-density forests from a hydrological perspective. In this study, one nonthinning plot and 5 plots with different thinning intensities, i.e., 35 % (light), 45 % (medium), 55 % (moderate), 65 % (heavy) and 80 % (extremely heavy) in Robinia pseudoacacia plantations, which are widely planted on the Loess Plateau, were set up. The volumetric soil water content (SWC) at 0–200 cm depths and rainfall were observed simultaneously and continuously for up to two years. The soil properties and vegetation structure in each plot were also measured regularly. The results showed that (1) the SWC in all the thinning plots was significantly greater than that in the nonthining plot (P < 0.05). (2) The replenishment increased with increasing thinning intensity, and then decreased after reaching a maximum at T35 % under most rainfall events except for light events. (3) The variations in the soil water response characteristics among the plots with different thinning intensities were mainly significantly correlated with the initial soil water content (ISWC), leaf area index (LAI) and understorey vegetation characteristics (P < 0.05). (4) Based on the relationship between soil water replenishment and retention with thinning intensity and stand density, quantitative equations were fitted to determine the reasonable thinning intensity (22–28 %) and retained stand density (1019–1091 trees/ha), respectively. Our study concluded that thinning could effectively improve soil water, especially light thinning, which could obviously enhance the replenishment of soil water by rainfall in Robinia pseudoacacia plantations. This study can provide a reference for determining forest thinning intensity or stand retention density, which is beneficial for water and reforestation management in water-limited regions.

Temporal changes in size inequality and stand growth partitioning between tree sizes under various thinning intensities in subtropical Cunninghamia lanceolata plantations

Stand structural complexity and growth partitioning between different tree sizes influence trees competition and resource use efficiency, and thus are closely linked to stand productivity. The main aim of this study was to examine how the stand structure complexity (Gini coefficient Gini) and size-related partitioning of stand growth (growth dominance coefficient GDC and size-growth relationship SGR) temporally changed after different thinning intensities in Chinese fir (Cunninghamia lanceolata) forests, Southeastern China. We analyzed the data of 6 repeated measurements over a nearly 10-year period from a forest thinning experiment with progressively increasing intensity (0, 20%, 25%, 33% and 50% reduction of stand density by selection logging of releasing target trees) and assessed the effects of different thinning intensities using generalized mixed effect model. The thinning intensity of 0 was taken as a control group to identify the stand changes in unthinned stands over time. Size heterogeneity and the relative contribution of larger trees to total stand growth tended to increase as forest grew without thinning management. While the increases in Gini and SGR (excluding between 0 and 0.5) overall negatively influenced the average annual basal area increment of the remaining trees (BAI). Gini in thinned stands decreased during the short term (two years or more) after thinning and the heavy thinning had lower Gini than other treatments several years (eight years or less) after thinning. GDC reduced greater with increasing thinning intensity, and thinning had a significant negative effect on Gini and SGR and a positive effect on BAI. The thinning impacts on BAI increased with the thinning intensity, while the effect magnitudes of different thinning intensities on Gini and SGR were: T50 < T20 < T25 < T33. Overall, thinning increased the stand growth partitioning of small trees, contributing to a uniform stand structure. However, the changes in stand structural complexity and the partitioning of stand growth were related to both thinning intensity and post-thinning time, particularly to thinning intensity. The results provided some implications for designing the intensity and timing of thinning, as well as for determining the size classes of removed trees for different management goals.

Thinning promotes litter decomposition and nutrient release in poplar plantations via altering the microclimate and understory plant diversity

Thinning is widely employed in forest management to improve productivity, protect forest biodiversity and maintain ecosystem functions. Here a thinning experiment with four treatments (unthinned, CK; 30% tree removal from below, MB; 50% tree removal by interlaced thinning, HI; and 50% tree removal from below, HB ) was set up in the poplar plantation, while a followed decomposition experiment with four litter types was conducted under the poplar plantations of undergoing four thinning treatments using the litterbag technique. Thinning affected the microclimate, but only the heavy thinning (HI and HB) significantly enhanced photosynthetic photon flux density (PPFD) and soil moisture in the plantations during the growing season. Thinning promoted understory vegetation biomass and vegetation diversity via modifying the microclimate parameters. Pearson correlation analysis showed that PPFD and understory herbaceous biomass were significantly correlated to Shannon-Weiner diversity index. Both thinning intensity and litter type significantly affected the litter remaining mass over times. Overall, increasing thinning intensity and litter complexity enhanced decay rate, while reduced half live (t0.5) and t0.95 values. Correlation analysis showed that air relative humidity, soil temperature, air temperature and soil moisture significantly influenced the litter mass loss rates. Non-additive (synergistic) effects were observed when different litters were mixed, but the non-additive effect was most pronounced when more herbaceous species litter were mixed with poplar leaves and 50% thinning intensity was applied. Dynamics of nutrient release from different litter types were similar to those on the litter mass lose, depending on the litter quality and microclimatic conditions. Our results suggest that a thinning operation with 50% tree removal from below (HB) would maintain the structural and functional features of the poplar plantations at the similar sites.

Trees response to selective thinning and pruning in Sudanian woodland zone

IntroductionManaging the degradation and loss of Sudanian woodlands to ensure sustainable use for mainly firewood, poles, and timber is receiving increasing attention, and it requires tailored strategies to improve stand productivity. This study determined: (i) tree response (density, recruitment, and height growth) to selective stem thinning and branch pruning; and (ii) tree species that are most affected by selective stem thinning and branch pruning in the Sudanian woodland of Benin.MethodsThree homogeneous vegetation units of 80 m × 80 m each were identified, representing the three stages of woodland development (WDS): early and young WDS of 3 and 3–5 years old, respectively, and the advanced to mature secondary WDS of 6–9 years old. Three random blocks of 20 m × 20 m each, subdivided into four 10 m × 10 m treatment plots, were delineated per vegetation unit. Each of these plots was randomly assigned a treatment (T): T1—no thinning and no pruning; T2—30% thinning; T3—60% thinning; T4—100% thinning. Branch pruning was applied to all remaining stems in T2 and T3. Tree species, stem abundance and height of individuals = 1 m were recorded every 6 months in 2015 and 2016.Results and discussionThinning intensity significantly influence tree density both in WDS 2 and 3 with the 30% thinning showing the best tree density (3355.56 ± 250.19 stems/ha and 3255.56 ± 772.68 stems/ha, respectively). However, more stem recruitment was observed without thinning and pruning (883.33 ± 212.13 stems/ha), and 30% thinning and pruning (383.33 ± 164.99 stems/ha). There was a significant increase in tree height growth both in WDS 2 and 3 with an increasing thinning intensity. The two most affected tree species were Terminalia avicennioides and Pteleopsis suberosa with an average growth in height after 6 months of 105.5 ± 1.05 mm (for 30% thinning and pruning in stage 3), and 61.75 ± 0.67 mm (for 60% thinning and pruning in stage 3), respectively. Overall, moderate stem thinning (30%) at WDS 1 and more severe thinning at WDS 2 and 3 resulted in the best tree height growth and recruitment.

Forest thinning and climate interactions driving early-stage regeneration dynamics of maritime pine in Mediterranean areas

There is growing concern that changing climate and disturbance regimes are and will result in widespread forest regeneration failure, and thus, there is an increasing need for developing management and adaptation strategies to promote regeneration under climate change. Maritime pine (Pinus pinaster), an ecologically and economically relevant tree species inside and outside its natural range, will likely face such difficulties as drought is the main cause of seedling mortality in this species. Understanding the interactions between silviculture and climate conditions is therefore crucial to inform sound adaptive forest management. However, the evidence on how forest thinning may contribute to enhanced regeneration remains divided and not fully understood, especially in the context of climate change. Hence, the general goal of this study was to evaluate the effect of thinning as a forest management strategy to promote the natural regeneration of P. pinaster stands in Mediterranean areas, and to understand how climatic, edaphic and topographic factors influence the effectiveness of this silvicultural practice. This was done by comparing 13 pairs of thinned and control plots along a thinning intensity gradient (23–69 % in stand basal area) in northeastern Iberian Peninsula and by measuring early-stage regeneration dynamics, namely the density, survival and growth of seedlings for 5 consecutive years. Thinning had a strong positive effect on the establishment and survival of P. pinaster seedlings as well as a weak positive effect on early primary and secondary growth. Not only was the number of seedlings and survival in thinned plots considerably higher than in control plots (15.0 ± 2.5 vs 1.5 ± 0.5 seedlings per plot; 23 ± 4 % vs 0.7 ± 0.3 % survival) but both seedling density and survival increased with increasing thinning intensity (3-fold and 5-fold, respectively), which matches the light-demanding nature of this species. Such a positive effect of thinning was further modulated by climatic and soil conditions. Overall, the advantage conferred by thinning to seedling establishment and survival was stronger under drier and warmer conditions, which suggests that thinning is particularly beneficial in alleviating competition in water-limited forest ecosystems. Considering that the Mediterranean region is predicted to become warmer and drier under global warming, our findings indicate that thinning will likely become an increasingly important management strategy to ensure the natural regeneration, adaptation and resilience of pine forests in the face of climate change.

The effect of thinning intensity on the soil carbon pool mediated by soil microbial communities and necromass carbon in coastal zone protected forests

Thinning is a common forest management measure that can effectively maintain the ecological service function of protected forests. However, the effect of thinning on the soil carbon (C) pool remains uncertain. In particular, we lack an understanding of the complete link between thinning and microbial communities, microbial necromass C, and consequently, soil C pools in coastal zone protected forests. In this study, three thinning intensities, i.e., a control treatment (CT, i.e., no thinning), light thinning (LT) and heavy thinning (HT), were established in three types of forests (Quercus acutissima Carruth, Pinus thunbergii Parl and mixed Quercus acutissima Carruth and Pinus thunbergii Parl, i.e., QAC, PTP and QP, respectively). Two years after the completion of thinning, we investigated the changes in the soil organic carbon (SOC) fractions, soil microbial community and soil microbial necromass C in the surface layer (0–20 cm) and thoroughly evaluated the relationship between the potential change in SOC and the microbial community. Compared with CT, there was no change in the SOC content under LT and HT, but thinning conducted in QAC increased the proportion of mineral-associated organic C (MAOC) in SOC. Moreover, both LT and HT reduced the soil carbon lability (CL) in the QAC and QP forests. Different thinning intensities changed the soil microbial community structure, and most of the variation was explained by thinning and the soil physicochemical properties. The proportion of soil bacterial and fungal necromass C to SOC increased with increasing thinning intensity. The content of soil bacterial and fungal necromass C was mainly controlled by the relative abundance of the core phylum (relative abundance>10 %). Thinning affected the soil C pool by affecting the content of soil bacterial and fungal necromass C, but their accumulation pathways was different. The results showed that thinning was beneficial to the stability of SOC. The microbial C pool, total organic C pool and even bacterial and fungal C pools should be distinguished when studying the soil C pool, which can effectively deepen our understanding of the mechanism by which soil microorganisms affect the soil C pool.

Response of Natural Regeneration of Pinus massoniana and Quercus variabilis Mixed Forest to Thinning Intensity and Environmental Factors

Based on target tree management, the effects of different thinning intensities and environmental factors on the natural regeneration of a Pinus massoniana and Quercus variabilis mixed forest were explored in order to provide a theoretical basis for the natural regeneration and sustainable forest management of P. massoniana and Quercus L. mixed forests. Taking the mixed forest after thinning as the research object, three average thinning intensities of WT (7.6%) for weak thinning, LT (15.3%) for light thinning, and MT (24.3%) for moderate thinning were carried out in 5 m×5 m quadrats with 7–10 replicates for each intensity level and 3 replicates for the control. Three years after the thinning, the amount of natural regeneration, growth height, regeneration density, diversity of regenerated tree species and their influencing factors at different thinning intensities were measured and analyzed. The results indicated four main features of the subsequent regeneration. (1) There were 32 species of vascular plants in the 28 quadrats 3 years after thinning, belonging to 22 families and 30 genera, and the dominant species for regeneration were arbor species. The number of regeneration species increased with increasing thinning intensity. (2) As thinning intensity increased, the number of natural regeneration plants between various height classes rose; so, the increased thinning intensity promoted the density of different height classes during regeneration. (3) As thinning intensity increased, so did species abundance S and species evenness. The degree and intrinsic diversity increased, while the Shannon-Weiner and Simpson indices showed no discernible trends. (4) Slope, aspect, and slope position, as well as thinning intensity, all had significant impacts on species richness, species evenness, and regeneration density. MT has the most appropriate promoting effect on natural regeneration and species diversity, so increased thinning intensity can promote natural regeneration and species diversity in the P. massoniana and Q. variabilis mixed forest. In addition, aspect and slope position can increase the species richness S and diversity of natural regeneration, whereas slope has a clear inhibitory effect on the species richness S and diversity during natural regeneration.

A Comparison of Soil C, N, and P Stoichiometry Characteristics under Different Thinning Intensities in a Subtropical Moso bamboo (Phyllostachys edulis) Forest of China

Land management using suitable thinning intensities can promote the yield of Moso bamboo (Phyllostachys edulis) forests and alter the nutrient content and stoichiometric characteristics within the soil. However, the effects of different thinning intensities on soil carbon (C), nitrogen (N), and phosphorus (P) stoichiometry of P. edulis forests have not been comprehensively studied. Here, we evaluated the effects of three thinning intensities—no thinning control (NT), moderate thinning (annual removal of 15% of mature bamboo, MT), and heavy thinning (annual removal of 33% of mature bamboo, IT)—on the soil organic C (SOC), total N (TN), and total P (TP) stocks and their stoichiometry characteristics of a P. edulis forest located in the subtropical zone. The results showed that SOC, TN, and TP stocks decreased with increasing soil depth after three years of thinning. The SOC, TN, and TP stocks and the ecological stoichiometry ratios were varied with the change of thinning intensity and SOC stocks declining with the increased thinning intensity, but TN and TP stocks for the MT treatment were enhanced. The order of TN and TP stocks was MT &gt; NT &gt; IT. In comparison to CK, TN and TP stocks for MT increased significantly by 18.8 and 37.3%, while SOC, TN, and TP stocks for IT decreased by 31.0%, 7.2%, and 21.4%, respectively. The C:N and C:P ratios of MT decreased by 32.8% and 39.0%, and those of IT decreased by 26.5% and 15.6%, respectively. In summary, we conclude that the MT is an effective management strategy to promote soil nutrient cycling and provides a reference for formulating management strategies in subtropical Moso bamboo forests.

Changes in size inequality and growth dominance in thinned and unthinned uneven-aged interior Douglas-fir dominated stands

This study sought to gain insight into the impact of thinning treatments on stand structure dynamics in uneven-aged interior Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco) dominated stands in central British Columbia (BC), Canada. We applied the Gini coefficient (GC) and the growth dominance coefficient (GDc) to determine how size inequality and growth dominance changed through time in both pre-commercially thinned (PCT) and unthinned stands across a moisture gradient. We used data from 24 plots in three blocks of a long-term study on the response to PCT, comprising five measurements over a 21-year period. The stand structures of each of the thinned plots were not identical before thinning. Therefore, the GC of the post-thinning measurements was expressed as a ratio relative to the pre-thinning treatment GC, referred to as the Gini coefficient ratio (GCr), to compare changes in size inequality through time across the PCT treatments. Three PCT treatments were applied, which differed in intensity and spatial pattern. All PCT treatments were applied in such a way that size inequality was reduced immediately after thinning. Size inequality did not decrease with increasing thinning intensity. There was a non-constant decrease in GCr in the unthinned plots, but an increase in the more heavily thinned plots over time. GDc decreased with increasing thinning intensity, where smaller trees were relatively more productive as the thinning intensity increased. Over time, thinned plots on the site with limited soil moisture and the moistest site shifted from negative to zero to positive growth dominance, while the GDc in the thinned plots with moderate soil moisture remained negative. These results suggest that site characteristics impact the response to PCT in these stands, and consequently should be considered in the design of the thinning treatments.

Forest Management under Climate Change: A Decision Analysis of Thinning Interventions for Water Services and Biomass in a Norway Spruce Stand in South Germany

Climate change is producing threats to forests’ capacity of regulating water regimes. Therefore, thinning strategies can be applied to mitigate climate change impacts more efficiently by providing more spaces for trees to utilize resources e.g., water and nutrients. This study examined the effects of different thinning intensities and intervals on water characteristics and biomass growth of a 75-year-old Norway spruce (Picea abies) stand in the Black Forest, Germany. Here we used a water and management sensitive update of the process-based forest growth model 3PG, 3PG-Hydro. We applied light (10%), moderate (30%), and heavy thinning (50% intensity) in the interval of 10, 25, and 50 years of the management period. We simulated growth with climate change scenario RCP 8.5 data from 1995 to 2065. We analyzed the effects of the different thinning regimens on biomass, evapotranspiration as well as water yield. Thinning intensity and interval as well as their interaction have significant influence on production of stand biomass and water yield for all thinning regimes applied (p &lt; 0.05). However, there is no significant difference (p &gt; 0.05) in accumulated biomass (thinned biomass added to the stand biomass) between the applied thinning regimes. Light thinning in a long interval (50 years) produced highest stand biomass among the applied thinning regimes. Furthermore, the prediction showed that accumulated water yield increased with increasing thinning intensity. Our study concludes that repeated moderate thinning at intermediate intervals results in a high water yield without losing biomass production.

The effects of crop tree thinning intensity on the ability of dominant tree species to sequester carbon in a temperate deciduous mixed forest, northeastern China

Forest management is one of the important nature-based solutions for climate mitigation. Thinning can indirectly influence tree physiology by changing the microclimate and directly change the stand biomass, which can impact forest carbon sequestration. However, previous results about how thinning might influence carbon stocks remain inconsistent regarding post-thinning carbon accretion. In this study, crop tree release (CTR) thinning in four intensities (CK: 0% of basal area removal, LT: 17.25%, MT: 34.73%, and HT: 51.87%) were conducted in a temperate deciduous forest in Jiaohe, northeastern China in 2011. Plot inventories in 2011, 2013, 2015, 2018 and 2021 and tree cores collected in 2017 and 2018 offered the opportunity to examine how are the interannual carbon sequestration ability of Korean pine and Manchurian ash responded to CTR thinning in four intensities. We quantify the carbon sequestration ability of trees by calculating individual stem carbon stock and annual carbon stock rate to examine whether the previous inconsistency was attributed to different responses of species, and the ignorance of frozen carbon content. The results show: (1) after thinning, the underestimation of carbon stocks of Manchurian ash decreased with the increasing thinning intensity. The greatest underestimation of Manchurian ash reaches 2922 kg ha−1, while that of Korean pine only reaches 283 kg ha−1. Compared with Manchurian ash, the conventional carbon fraction of 0.5 for Korean pine is more appropriate, and the misestimation of Korean pine didn't show an obvious pattern with the intensity of thinning. (2) Under light thinning, both species maintained a stable carbon stock growth, and the frozen carbon content of Korean pine was significantly increased. During the 10 years after light thinning, the individual stem carbon of Korean pine increased from 57 kg to 81 kg, and Manchurian ash increased from 201 kg to 268 kg. The average rate of increase of individual stem carbon is positively related to tree size. Removing such large-diameter trees from the stand is likely to decrease carbon stock rate. Therefore, it is essential to design carbon-friendly silviculture prescriptions worldwide under the consideration of species, sizes, and intensities.

Variation in fine root traits with thinning intensity in a Chinese fir plantation insights from branching order and functional groups

Thinning is a widely used practice in forest management, but the acclimation mechanisms of fine roots to forest thinning are still unclear. We examined the variations in fine root traits of different branching orders and functional groups along a thinning intensity gradient in a 26-year-old Chinese fir (Cunninghamia lanceolata) plantation. With increasing thinning intensity, the root C concentration (RCC), root N concentration (RNC), specific root area (SRA), and specific root length (SRL) of the absorptive roots (the first two orders) significantly decreased, while root abundance (root biomass and root length density) and root tissue density (RTD) significantly increased. Fifty-four percent of the variation in the absorptive root traits could be explained by the soil N concentration and the biomass and diversity of the understorey vegetation. Conversely, transport root (third- and higher-order) traits did not vary significantly among different thinning intensities. The covariation of absorptive root traits across thinning intensities regarding two dimensions was as follows: the first dimension (46% of the total variation) represented changes in root abundance and chemical traits (related to RCC, RNC), belonging to an extensive foraging strategy; the second dimension (41% of the total variation) represented variations in root morphological traits (related to RTD, SRL and SRA), which is an intensive foraging strategy (i.e., root economic spectrum). These results suggested that the absorptive roots of Chinese fir adopt two-dimensional strategies to acclimate to the altered surroundings after thinning.

Hardwood mixture increases stand productivity through increasing the amount of leaf nitrogen and modifying biomass allocation in a conifer plantation

Recent studies have demonstrated positive effects of hardwood mixtures on stand productivity in conifer plantations. However, the extent to which hardwood mixture increases leaf nitrogen levels and improves stand productivity (in terms of nutrient cycling) remains poorly understood. In this study, we estimated leaf nitrogen per stand (leaf Nstand) by multiplying mass-based leaf nitrogen (leaf Nmass) and leaf mass per stand (leaf Mstand) in conifers, hardwoods, and understory plants in a thinning experiment. The experiment, performed in 2018, was conducted under unthinned (Control) with few hardwoods, 33% thinned (Weak) with a small amount of hardwoods, and 67% thinned (Intensive) with abundant hardwoods in a conifer Cryptomeria japonica (C. japonica) plantation. We also investigated the stand volume of conifers and hardwoods during the 12-year period (2008–2020) after the second thinning (2008), in all three groups. Of all plants—conifers (C. japonica), hardwoods, and understory plants—stand volume, leaf Mstand, and leaf Nstand were all highest in Control and lowest in Intensive. The ratio between the values of Intensive and Control was 27.5% for stand volume, 58.9% for leaf Mstand, and 72.8% for leaf Nstand. This is due to the fact that the leaf Mstand of hardwoods—which had approximately 2–3 times the leaf Nmass of C. japonica—increased rapidly with increasing thinning intensity, and the leaf Mstand of the understory plants—which had approximately twice the leaf Nmass of C. japonica—increased monotonically with increasing thinning intensity, although both the leaf Mstand and leaf Nstand decreased with increasing thinning intensity in both current-year and old C. japonica leaves. In conifers, the relative increment in stand volume was positively correlated with the leaf mass ratio (ratio between the leaf Mstand and stand volume of conifer) and leaf N ratio of conifer (ratio between the leaf Nstand and stand volume), suggesting that disproportionate biomass investment in leaves enhances productivity in conifers. The study suggests that hardwood mixture can significantly increase forest productivity, mainly due to a disproportionate increase in leaf nitrogen in hardwoods and an increase in biomass allocation to leaves in conifers in a conifer, C. japonica plantation.

Effect of thinning intensity on understory herbaceous diversity and biomass in mixed coniferous and broad-leaved forests of Changbai Mountain

BackgroundHerbs are an important part of the forest ecosystem, and their diversity and biomass can reflect the restoration of vegetation after forest thinning disturbances. Based on the near-mature secondary coniferous and broad-leaved mixed forest in Jilin Province Forestry Experimental Zone, this study analyzed seasonal changes of species diversity and biomass of the understory herb layer after different intensities of thinning.ResultsThe results showed that although the composition of herbaceous species and the ranking of importance values were affected by thinning intensity, they were mainly determined by seasonal changes. Across the entire growing season, the species with the highest importance values in thinning treatments included Carex pilosa, Aegopodium alpestre, Meehania urticifolia, and Filipendula palmata, which dominated the herb layer of the coniferous and broad-leaved mixed forest. The number of species, Margalef index, Shannon-Wiener index and Simpson index all had their highest values in May, and gradually decreased with months. Pielou index was roughly inverted “N” throughout the growing season. Thinning did not increase the species diversity. Thinning can promote the total biomass, above- and below-ground biomass. The number of plants per unit area and coverage were related to the total biomass, above- and below-ground biomass. The average height had a significantly positive correlation with herb biomass in May but not in July. However, it exerted a significantly negative correlation with herb biomass in September. The biomass in the same month increased with increasing thinning intensity. Total herb biomass, above- and below-ground biomass showed positive correlations with Shannon-Winner index, Simpson index and Pielou evenness index in May.ConclusionsThinning mainly changed the light environment in the forest, which would improve the plant diversity and biomass of herb layer in a short time. And different thinning intensity had different effects on the diversity of understory herb layer. The findings provide theoretical basis and reference for reasonable thinning and tending in coniferous and broad-leaved mixed forests.

Effects of thinning intensity on the understory water-holding capacity of Pinus sylvestris var. mongolica plantation in the Bashang area of north China.

The complex terrain and poor climatic conditions in Bashang area of Hebei Province result in water and soil loss and geological disasters, which pose a serious threat to ecological safety in North China. In order to improve local environmental quality, barren-resistant and fast-growing tree species such as Pinus sylvestris var. mongolica and Larix gmelinii are planted with large areas. However, unreasonable plantation density will lead to inefficient utilization of rainfall and intensify the conflict between forest and water. In this study, we analyzed the effects of five thinning intensities (0, 20%, 40%, 60%, 80%) of P. sylvestris var. mongolica plantation on herbs, litter, soil and overall water-holding capacity, with the aim to provide scientific basis for management of P. sylvestris var. mongolica. The results showed that water-holding rate of herb varied from 47.7% to 90.7%, and that the water-holding capacity of herb decreased with increasing thinning intensity. When the thinning intensity was less than 40%, water-holding capacity decreased slowly, and then decreased rapidly. With the increase of thinning intensity, natural water-holding rate and maximum water-holding rate of undecomposed layer and semi-decomposed layer decreased gradually, with the effective water-holding rate being 60%>40%>20%>80%>0, and the water-holding capacity of semi-decomposed layer being better than that of undecomposed layer. The water-holding capacity of soil decreased gradually with the increases of thinning intensity. Thinning intensity less than 40% promoted water holding capacity. Under different thinning intensities, the total water-holding rate of understory was 8.3%-14.3%, with an order of 20%>0>40%>60%>80%. In view of understory all layers and overall changes, the thinning intensity at 20% in the study area could effectively improve the understory water-holding capacity and achieve better ecological benefits.

Short-Term Thinning Influences the Rhizosphere Fungal Community Assembly of Pinus massoniana by Altering the Understory Vegetation Diversity.

Thinning can significantly promote forest productivity and ecological function. Rhizosphere fungi play an indispensable role in regulating nutrient cycling between plants and the environment, and their community composition can positively respond to anthropogenic disturbance. However, the initial effects of thinning on rhizosphere fungal community assembly have seldom been reported. In this research, we studied the alterations in the rhizosphere fungal communities of 29-year-old Pinus massoniana in East Sichuan 2 years after three different thinning intensity treatments. In addition, the responses of fungal community and functional group composition to alterations in understory vegetation and soil physiochemical properties were analyzed. Three thinning intensities were set, which were 0 (CK), 25% (LIT), and 50% (HIT), respectively. The results suggested that the richness index and Shannon index of understory vegetation increased significantly with increasing thinning intensity. The alpha diversity indices of rhizosphere fungal community and soil physiochemical properties did not show significant differences among the three treatments. The relative abundances of 17 fungal indicator species varied regularly with increasing thinning intensity, and most of them belong to Hypocreales and Eurotiales, indicating that these two orders were potential indicators for different thinning treatments. Rhizosphere fungal community assembly was determined by deterministic process, and it was driven by the diversity of understory vegetation in the initial stage of thinning. The Simpson index and Pielou index of herbs were useful measures of the main environmental factors driving the differentiation of fungal functional group composition. Based on network analysis, thinning resulted in distinct co-occurrence patterns of rhizosphere fungal functional groups. This research elucidates the initial role of thinning in rhizosphere fungal community assembly of P. massoniana and has practical significance for the functional restoration and protection of local forest ecosystem.

Effects of Thinning Practice, High Pruning and Slash Management on Crop Tree and Stand Growth in Young Even-Aged Stands of Planted Silver Birch (Betula pendula Roth)

The objective was to quantify the influence of thinning, high pruning and slash management on crop tree and stand growth in young even-aged stands of planted silver birch (Betula pendula Roth). This study was based on two field experiments, aged six and eleven years at initiation and re-measured after six and eight years, respectively. Treatments included the unthinned control, moderate thinning mainly from below (removing 28–33% of standing volume), point thinning to favor 300 trees per ha and with no thinning elsewhere in the plot (removing 16–25%), and heavy thinning leaving 600 evenly distributed potential future crop trees per ha (removing 64–75%). Slash management (extraction or retention) was applied to heavily thinned plots. High pruning removing 30–70% of the green crown was carried out in some plots with point or heavy thinning on 300 or 600 trees per ha, respectively. Stand volume growth increased with increasing pre-treatment mean annual volume increment and decreased with increasing thinning intensity as compared to the unthinned control. LS-means estimates indicated a reduction for moderate thinning by 14%, for point thinning by 12% and for heavy thinning (combined with pruning) by 62%. However, in the youngest experiment, heavy thinning (without pruning) reduced growth by 54%. Combining these results with results from a similar experiment in Canada, the reduction in stand volume growth (RedIv%) depending on thinning removal (RemV%), both expressed as a percentage of the unthinned control, was quantified as RedIv% = −23.67 + 1.16·RemV% (calibration range: 30–83%). For heavy thinning (large quantities of slash), slash extraction resulted in no reduction in stand volume growth as compared to slash retention. The instantaneous numeric reduction in the average stem diameter of the 300 thickest trees per ha (D300) due to thinning was 3.5, 15–21% and 955–11% with moderate, point and heavy thinning, respectively. The subsequent average annual increase in D300 during the observation period was 8.5%, 25 and 18%, respectively. In the youngest experiment, pruning in unthinned plots led to a reduction in the annual increase of D300 by 14%, and heavy thinning in unpruned plots led to an increase by 30%. The growth of pre-selected potential future crop trees increased with increasing thinning intensity. In heavily thinned plots, pruning reduced growth increasingly with increasing pruning severity; LS-means estimates indicated 21% larger growth on stem diameter for unpruned trees and 3% for pruned trees. As an adverse side effect, heavily thinned plots with only 600 trees per ha were at increased risk of windthrow for some years after the thinning intervention. In the oldest experiment, 95–21% of the trees in these plots were damaged by wind.

Effects of Thinning on Growth and Regeneration of Cyclobalanopsis glauca Secondary Forest

Based on the problems of poor productivity and natural regeneration of Cyclobalanopsis glauca secondary forest in hilly areas in southeast Hunan Province, experiments of thinning were carried out to analyze the effects on stand growth and regeneration of different thinning intensities and to provide guidance for managing secondary forests. Cyclobalanopsis glauca secondary forest was selected for research object, then experimental design was arranged with a randomized block by thinning intensity such as control(un-thinning), light thinning (10%), moderate thinning (20%) and heavy thinning (30%). Tree measuring factor and number of naturally regenerated seedlings were investigated for researching the responses to thinning treatment. Results showed that there were significant differences of mean DBH and current annual increment of DBH in terms of the four thinning treatments, and the larger the thinning intensity, the higher the DHB increment. The maximum mean height (10.1m) happened in stand of heavy thinning. However, significant differences existed in the current annual increment of height among the four thinning treatments. Stand volume was 93.27, 91.49, and 91.90 m3 hm−2 respectively in stand of heavy thinning, moderate thinning, light thinning, and were all lower than that of the control, but current annual increment of volume was 5.65, 4.43 and 3.75 m3 hm−2 yr−1, respectively and higher than that of the control. The Simpson index of shrub layer and herb layer of heavy thinning stand was 0.45 and 0.80, Shannon-Wiener index was 1.03 and 2.03, respectively. There were significant differences of naturally regenerated seedlings according to four thinning treatments only for seedlings of 5∼30 cm level, but there were no significant differences for seedlings of 30∼50 and 50∼150 cm level. The growing space and nutritional condition of retained trees after thinning were effectively improved, stand DBH and height increased with the increasing thinning intensity. However, stand volume was not significantly different among four thinning treatments. Besides, thinning at appropriate intensities could increase the biodiversity in shrub layer and herb layer, quantities of naturally regenerated seedlings also increased with the increasing thinning intensity. Heavy thinning (30%) was the best one among four thinning treatments for silvicultural interventions on Cyclobalanopsis glauca secondary forest.