Soil particle composition, microbial communities, nutrients availability and pH are important for maintaining natural plant regeneration, and they interact to provide a healthy soil environment for plant growth and play an essential role in determining the pattern and success of forest regeneration.

Soil particle composition ranks among the most important soil physical properties, directly affecting soil moisture transport, soil erosion, accumulation, and physicochemical processes, thereby affecting the distribution pattern of vegetation renewal (Bayat et al., 2015). Soil particle size plays a crucial role in determining the type and rate of forest vegetation restoration. A study of soil properties in three forest types: mixed, pure and economic forests showed that soils with a high clay or chalk content tend to support the regeneration in mixed and pure forests, while finer sandy soils are more suitable for the development of economic forests (economic forests is a forest for the production of dried and fresh fruits, edible oils, beverages, spices, industrial raw materials and medicinal herbs, etc., and is an important part of the forest resource) (Li, 2018). Soil texture also affects regeneration dynamics differently between forest types: broadleaf forests on clay-rich soils typically show better understorey recovery, whereas coniferous forests regenerate more effectively on coarser, well-drained soils (Danilov et al., 2020).

Soil particle size can also influence essential nutrients availability for forest regeneration. For example, soils with higher clay content generally have higher carbon content and carbon-to-nitrogen ratio, both of which are positively correlated with increased soil moisture, while nitrogen and phosphorus availability may decrease (Azaryan et al., 2022). These nutrient dynamics significantly shape the recovery and growth of forest ecosystems.

Soil microbial communities play crucial ecological and physiological roles in natural forest regeneration. These microorganisms contribute to forest regeneration by improving the soil’s physical and chemical properties, thus increasing its overall suitability for plant growth (Sofo et al., 2014; Sofo et al., 2012). This process involves an increase in the modularity of the symbiotic network among soil bacterial communities and the enrichment of bacteria involved in key metabolic pathways, positively influencing forest regeneration. These effects primarily link to microbial interactions with soil nutrients, height, litter thickness, and herbaceous plant diversity (Liu J. B. et al., 2020; Massaccesi et al., 2020). For example, de Medeiros et al. (2017) demonstrated that after 17 years of natural regeneration in secondary forests, changes in plant diversity and composition promoted an increase in soil microbial biomass. In turn, and microbial communities released soil nutrients, contributing to the vegetation development and forest regeneration rate (de Medeiros et al., 2017). This highlights the important influence of plant diversity on soil microbial communities and their ecological processes. As forest communities regenerate, the complexity of soil bacterial communities tends to decrease. During the regeneration of larch (Larix gmelinii var. principis-rupprechtii), the bacterial community was found to be more active and influential on ecological processes than fungal community (Niu et al., 2022), suggesting that bacteria play a key role in forest regeneration. In addition, natural forest regeneration promotes the growth of mycorrhizal fungi. Nogueira et al. (2016) indicated that these mycorrhizal fungi significantly influenced the accumulation of soil organic carbon and carbon pools, playing an important role in the ecological succession of plant species. Further studies in Puerto Rican woodlands have also shown that natural regeneration occurring after agriculture activity leads to variations in the functional potential of soil microorganisms with stand age (Pressler et al., 2020).

Nutrient availability in soils, particularly nitrogen and phosphorus, directly impacts forest regeneration (Ricklefs et al., 1999). As forests regenerate, nitrogen and phosphorus levels typically increase with species diversity and structural complexity. Over the past several decades, research has shown that the increased nitrogen deposition, combined with global warming, has led to phosphorus limitation in subtropical forests. Under such conditions, plants tend to improve phosphorus cycling efficiency, reducing phosphorus content in litter (Kitayama et al., 2000; See et al., 2015). This leads to more phosphorus being released back into the soil through decomposition. Xiong et al. (2023) found that nitrogen limitation became more pronounced during the regeneration of Phoebe bournei forests, underscoring the importance of nutrient management for successful regeneration. In general, ammonia nitrogen and soil organic matter, among other variables related to soil properties, had a large contribution to natural regeneration (Wang et al., 2016), and Zhao et al. (2023) used SEM analysis found that TN (total nitrogen) greatly contributed to natural regeneration compared with other soil properties, TN (p < 0.05) was negatively correlated with plant regeneration, high levels of TN can lead to reduced resistance of tree seedlings, thinning of leaf thickness, etc., thus limiting the natural regeneration of plants. Though high levels TN would limit natural regeneration, but seedling growth required the consumption of a large proportion of TN (Xu et al., 2018). In sites with high fertility, nutrient input increases with stand density during natural regeneration. In sites with lower fertility, nutrient input increases with tree species richness and the proportion of nitrogen fixing tree species, indicating that the relationship between plant community renewal effect and nutrient cycling largely depends on site conditions (Machado et al., 2021).

Soil pH represents another key factor influencing forest regeneration. In tropical and temperate regions, neutral soils with pH values between 6.0 and 7.5 generally provide optimal conditions for natural regeneration, as nutrients are more readily available in this range (Horneck et al., 2011). Loeb and Mao (2021) found that higher pH levels can sometimes hinder regeneration, particularly for species such as Acer rubrum and Fraxinus americana, which are sensitive to elevated pH. In contrast, other studies, such as Kassa et al. (2020), have shown that neutral pH conditions promote richer nutrient availability and improve plant regeneration. These differences suggest that soil pH effects may vary depending on species-specific responses and the broader ecosystem context. The difference in results may due to species variations, as Robert E. Loeb focusing on a few broadleaf species, while Kassa et al. came up with generalised patterns for most tree species in the study area. Both community diversity and tree species growth were significantly and positively correlated with soil pH, indicating that soil acidity and/or alkalinity can regulate plant community diversity and the natural regeneration process in forests (Xiong et al., 2023). Soil acidity and alkalinity have different effects on the growth status across various different tree species. Soil pH closely relates to the regeneration of broad-leaved trees, but has little correlation with coniferous forests (Danilov et al., 2020). Marek Malicki and Rem et al. (2018) in Czerwona Woda River ValleyintheStołowe Mountains National Park found that soil pH affects density at all ages and heights in a Norway spruce where located in a low altitude region, and there was a negative correlation between soil pH and plant age-hight classess, that is, tree seedlings grow more under lower soil pH conditions (Dyderski et al., 2018).

In conclusion, soil properties, including particle size, microbial communities, nutrient content, and pH, play a fundamental role in shaping natural forest regeneration. Understanding how these factors interact can provide valuable insights for developing effective forest management strategies, particularly in the context of changing environmental conditions.

Geo-environmental factors such as altitude, slope, and topography are indirect ecological factors that play a key role in the natural forest regeneration by modifying climatic conditions, hydrology, soil properties and microclimatic environments. These factors play a crucial role in determining the distribution, growth, and survival of plant populations during the natural regeneration process.

Topographic heterogeneity significantly impacts early forest regeneration. This process involves the combined effects of environmental filtering and resource competition among plants. Slope orientation and gradient affect the amount of solar radiation, soil moisture, and nutrient retention, which in turn influence seedling survival and growth rates. The effect of topography on regeneration varies between species, with deciduous species being more sensitive to topographic changes than evergreen species. For instance, the mortality rate of deciduous species is estimated to be influenced by topographic factors by as much as 65%, while for evergreen species, this figure is only 30% (Zhang et al., 2023). In addition, slope gradient also affects soil depth and nutrient retention. Steeper slopes often having shallower soils that store fewer nutrients and are more prone to erosion, creating unfavorable conditions for seed storage and germination, which reduces seedling survival (Pham et al., 2022). Slope orientation plays an essential role in determining how much solar radiation the forest floor receives (Maciel-Nájera et al., 2020). Zhao et al. (2023) found that the northwest-facing slopes received more solar radiation than the west-facing slopes, leading to a higher density of seedling regeneration in the former. This suggests that slope orientation can significantly influence regeneration patterns based on the light requirements of tree species.

Terrain creates contrasting microclimates, especially between arid north and south slopes. Principe et al. (2019) found that areas with higher potential solar radiation, such as south-facing slopes, had significantly lower tree age and density due to the limiting effects of intense sunlight and higher temperatures on seedling germination and sapling development. Additionally, terrain can also partially affect soil nutrient accumulation and retention. For example, total nitrogen levels tend to increase with slope position, while phosphorus availability may decrease as slope steepness increases (Liang and Wei, 2020).

Altitude is another critical geo-environmental factor influencing forest regeneration by affecting temperature, moisture, and soil nutrient composition. Trees at higher altitudes typically experience lower temperatures, which can slow down growth rates and limit regeneration success. Orman et al. (2023) found that the Norway spruce (Picea abies (L.) H. Karst.) saplings at high-altitude had a thicker, more elongated crown compared to their lower altitude samples, reflecting structural adaptations to the cooler and more challenging climate at higher elevations. With the increase of altitude, the species diversity often decreases, Sigdel et al. (2024) modelling of tree lines in the central Himalayas showed that the fir moved upward at a faster rate than the birch, while the birch replenishment was drastically reduced. Thus, fir is likely to outcompete birch at the mixed treeline and is expected to be the dominant treeline species in the Himalayan region, leading to changes in species composition near the treeline (Sigdel et al., 2024), and altitude has varying degrees of impact on the availability of soil nutrients. The availability of key soil nutrients, such as soil organic carbon, nitrogen, phosphorus, potassium, and zinc increases with altitude, while pH and available iron decrease (Wani et al., 2022). Additionally, tree seeds differed significantly across the altitudinal gradient, with seeds from lower altitudes showing higher germination rates but longer germination times. In contrast, seeds from higher altitudes exhibit lower germination rates, suggesting that seeds of Ephedra saxatilis from lower altitudes have a survival advantage over seeds from medium to high altitudes (Wardle, 1970). However, studies suggest that altitude gradients less than 300 meters may not significantly impact vegetation development or regeneration due to minimal changes in temperature and moisture conditions (Liu et al., 2012). This finding indicates that the effects of altitude on regeneration are species-specific and vary depending on the microclimatic conditions and the broader geographical context.

In summary, geo-environmental factors such as slope, altitude, and topography have a comprehensive impact on the forest regeneration. These factors alter climatic conditions, hydrological processes, and soil nutrient availability, thereby shaping the distribution, growth, and survival of tree species.

Ground layer can be divided into live forest ground layer and dead forest ground layer according to its biological activity, with live forest ground layer referring to organisms such as dwarf herbs, lichens, mosses, etc. that live in forest environments, and dead forest ground layer referring to branches, leaves, flowers and fruits that fall to the ground after the forest plants have withered and faded and can also be referred to as forest litter (Facelli and Facelli, 1993). Ground layer plays a crucial role in natural forest regeneration. It can either facilitate or inhibit the regeneration process by influencing seed germination, seedling survival, and growth. The effects of the ground layer are complex and often double-edged, depending on factors such as the thickness, composition, and moisture retention properties of the cover.

The forest surface litter layer is a key factor in regeneration process. Litter provides essential benefits such as shading, moisture retaining, insulation and protection from mechanical damage. However, it can also pose challenges through its potential to chemically inhibit seedling growth, block sunlight, or harbor pathogens and pests (Asplund et al., 2018; Calviño-Cancela et al., 2018). Increased litter thickness enhances water-holding capacity, stabilizes temperatures, and creates favorable conditions for seed germination by reducing soil moisture evaporation (Petrou and Milios, 2020). In southeastern Tibet, Wang R. H. et al. (2024) found a positive correlation between litter thickness and the presence of fir seedlings, juvenile trees, and small trees, indicating that a sufficiently thick litter layer supports regeneration. However, excessive litter can impede seedling establishment by limiting light availability, particularly in the early germination and seedling development. In some cases, seeds may perish in the litter layer as their roots can not penetrate the soil and access nutrients (Zhao et al., 2023; Lu et al., 2021). Moreover, large amounts of litter may physically block seeds from contacting the soil or create a chemical environment that is toxic to seedling growth, as decomposed litter releases compounds that inhibit germination (Willis et al., 2021). This dual role of the litter layer underscores the need for balanced management of ground layer conditions to support regeneration, for example, natural regeneration of forests is promoted through the introduction of decaying organisms such as earthworms and beetles to promote the natural decomposition of dead wood and reduce its inhibitory effect on seedling growth, and the moderate clearing of excessively thick layers of dead wood at specific times to improve light conditions and reduce anthropogenic disturbances such as the harbouring of pathogens and pests.

Mosses are another important part of the ground layer, playing a significant role in regulating the forest floor’s water and nutrient cycles, which are essential for seed germination. The impact of mosses on forest regeneration largely depends on the thickness and water content of the moss layer (Wheeler et al., 2011). Research has shown that the thickness and coverage of the moss layer are critical for water vapor exchange and can positively influence seedling emergence due to their ability to retain moisture (Wang R. H. et al., 2024). In addition, the inherent water retention capacity of the moss layer provides important hydration that promotes seed germination and seedling growth (Zamfir, 2000). However, while mosses promote the germination of seedlings, they can hinder later stages of seedling growth, particularly when the moss layer is too thick. Dense moss layers may prevent adequate water infiltration into the soil and restrict the availability of nutrients to developing saplings, resulting in high mortality rates (Ali et al., 2019). Thus, the relationship between moss thickness and regeneration is thus complex: a moderate moss cover enhances moisture conditions for germination, but excessive moss growth can create microhabitats that are too dry for seedlings, limiting their survival. The litter layer and the moss layer seem to have similar roles, and a comprehensive analysis may lead to more accurate conclusions, while it should be possible to integrate these two components in forest management, for example, by moderately controlling the thickness of the litter and moss layer, the water retention capacity of the soil, the supply of nutrients and the conditions for germination of seeds can be optimised; the appropriate removal of litter and moss to reduce the number of pathogens and pests harbouring, etc.

Shrubs also play a multifaceted role in the regeneration process. Shrub cover can positively correlate with the number of saplings and regeneration potential, as shrubs often provide a buffer against environmental extremes and protect seedlings from desiccation. For example, Wang Z. Z. et al. (2024) found that higher shrubs cover was associated with improved regeneration potential in multi-species forests, indicating that shrub cover can serve as a useful indicator of forest community regeneration potential. However, the effects of shrubs can vary depending on their dominance and diversity. In some cases, species like tamarisk (Tamarix spp.) can dominate a habitat, reducing overall regeneration success by competing with tree seedlings for resources (Acácio et al., 2024). While a diverse range of diverse shrub species may enhance seedling survival by providing a more favorable microclimate, overly dominant shrubs can block sunlight and suppress regeneration. Bannister et al. (2023) showed that in temperate rainforest ecosystems, the regeneration rate of canopy species significantly increased to more than 81% five years after cutting down understorey bamboo vegetation (e.g., moso bamboo), which was dominated by seed regeneration, and the total natural regeneration increased by 55.5% (p < 0.05; incremental increase of >1 plant/m²-yr). This restoration effect stemmed from the release of light, soil nutrients and spatial resources after the removal of understory vegetation, which accelerated the natural recovery process of degraded ecosystems (Ruggirello et al., 2023). This reflects that shrubs play dual roles in forest regeneration, and their effects are highly dependent on species characteristics, ecosystem context and management strategies: in diverse multi-species forests, native shrubs significantly enhance seedling survival and regeneration potential by moderating microclimate (reduction and resource conservation) (Parra and Moreno, 2018), while invasive dominant shrubs inhibit tree regeneration in arid zones by grabbing water via deep roots and releasing chemosensory substances, and forming a mono-dominant community (Cao et al., 2020). In contrast, Bannister et al. (2023) showed that in degraded temperate rainforests caused by anthropogenic disturbances, active removal of competing bamboo vegetation released light and nutrients, activated the soil seed bank, and increased the regeneration rate of canopy species by 81%.

Meanwhile, the interaction between shrubs and trees is often species-specific, and some shrubs can serve as ecological filters that influence the succession direction. For instance, Psychotria shrubs can buffer ecological succession in forest understories, supporting natural regeneration even in high-density conditions (da Silva et al., 2021). This suggests that the influence of shrubs on natural tree regeneration also varies among forest species. There is often overlap between shrub and tree ecological niches, which can significantly influence tree growth. Additional, ecological niche differentiation between shrubs and trees promotes diversity and allows coexistence. Under adequate light conditions, a suitable shrub layer cover is favourable to seedling growth. However, under limited light conditions, shrub cover may further inhibit light availability, hampering seedling growth (Lu et al., 2021). For example, Michael B. Walters et al. found the shrub layer of crowberry (Empetrum spp.) significantly inhibited the growth of tree seedlings below 2 meters due to light competition, but this negative effect disappeared once the seedlings grew taller (Walters et al., 2020).

In conclusion, the ground layer is a critical component of the natural forest regeneration, with both positive and negative effects on seedling germination and growth. While the litter and moss layers promote moisture retention and protection, they may also create physical or chemical barriers to seedling development. Similarly, shrub cover can either facilitate or hinder regeneration, depending on species composition and light availability. Effective management of the ground cover layer is essential to balance its benefits and challenges, ensuring successful forest regeneration.

In recent years, due to climate change, anthropogenic activities and biological invasions, insect pest have been frequent in forests, especially in fir (Abies fabri) and Chinese pine (Pinus tabuliformis), serious impediments to forest regeneration and restoration (Fiala and Holusa, 2022; Xu et al., 2023). Insect pests are a type of animal disturbance that can cause a series of abnormal morphological, organisational or physiological-ecological changes in a forest tree, leading to stunted growth, reduced yield and quality, or even causing the death of the tree or the entire stand and ecological degradation (Xu et al., 2002). Since planted forests are mostly concentrated pure forests, they can provide a large number of consistent food sources and ideal habitats for pests and diseases; moreover, the low biodiversity, simple food chain and food web structure of planted forests are not conducive to the habitation of natural enemies, and their low systemic resistance makes them highly susceptible to large-scale pests and diseases, therefore, natural regeneration of planted forests will be subject to more adverse impacts than that of natural forests. In northern China, large areas of pure plantation poplar forests are commonly attacked by stem-boring and leaf-feeding pests due to their monoculture and low biodiversity (Zhang and Zhang, 2004). However, insect pest also occurs in natural forests, thus affecting the regeneration of forest trees. For example, Lieffers et al. (2024) through an in-depth survey of 33 small plots heavily infested with the insect in Alberta, Canada. In forest regeneration after infestation by mountain pine beetle (MPB, Dendroctonus ponderosae), the quality of tree regeneration was poor in the last 6–9 years, and the original dominance of the number and quality of pine seedlings were low (only 42% of the plots were found with seedlings). The dominant species of the forest may be gradually transformed during the regeneration (Lieffers et al., 2024). There are also deciduous insects (caterpillars, leaf wasp larvae, etc.) that can significantly reduce acorn production, resulting in constraints on the natural regeneration of oaks in the forests (Eötvös et al., 2023).

Some natural disasters such as earthquakes, tsunamis, volcanic eruptions, and snowstorms, although their frequency is low, can have serious impacts on regeneration when they occur. The density of young regenerated trees is negatively correlated with the basal area of surviving trees after a storm and positively correlated with the mortality of canopy trees. Canopy trees that survive storm disturbances protect understory seedlings and therefore play an important role in delaying the regeneration process (Cerioni et al., 2022). In addition, Cao et al. (2023)’s study on the renewal of Chinese gugertree (Schima superba Gardner & Champ.) after freezing disaster in Jianglangshan Nature Reserve, China, found that the mortality rate of tree sprouts increased every year for six years after being affected by freezing disaster, and the mortality rate of sprouts that sprouted earlier was relatively low, which may mean that the renewal of new sprouts that sprouted earlier. This may imply that early bud regeneration is more effective and that forest regeneration can be facilitated in forest management by retaining early vigorous buds and pruning late buds (Cao et al., 2023). And in areas with frequent snowstorms, trees also have different mechanisms to promote renewal and recovery from forest damage. A study by Nakano and Sakio (2018) in an area of heavy snowfall on Sado Island, Japan, found that young Japanese wingnut (Pterocarya rhoifolia) trees that had become established were often damaged by snow pressure, but also produced some newsprouting stems. These results suggest that in areas of heavy snowfall, water huckleberries can maintain their population size through sprouting, thus allowing the trees to recover from snow damage (Nakano and Sakio, 2018).