Adaptation of the Coniferous Forests to Natural Fire Disturbances in the Altai Mountains, Xinjiang, China

Abstract

The Altai Mountains, located in the northwesternmost part of China, have a harsh climate and little human activity, making it an excellent location to study forest ecology undisturbed by human interference. The forest is frequently struck by lightning and experiences long-term natural fire disturbances, leading to the evolution of unique fire adaptation traits in the major conifer species. To explore the role of natural fire disturbances in the Altai Mountain forest ecosystem, we conducted a study on the fire adaptation traits of Larix sibirica, Pinus sibirica, Picea obovata, and Abies sibirica, and reconstructed the fire history of the forest area over the past 100 years. We investigated three representative forest areas with varying fire disturbance conditions and habitats in the Altai Mountains. Data on fire disturbance conditions, relative air humidity, and species composition were collected in these areas. Basal diameter to diameter at breast height, relative bark thickness, and under-crown heights were measured and counted for each of the four species, and litter, bark, and wood layers were sampled and analyzed for physicochemical properties (ash, fat, and higher heating value) for each of the four species in the plots examined. We conducted a count of the four conifer species in the forest for each fire adaptation index and analyzed the differences in fire adaptation traits among the species. Larix sibirica showed fire-tolerant traits, Pinus sibirica displayed fire-embracer traits, and Picea obovata and Abies sibirica exhibited fire-avoider traits. Through the analysis of stand composition and exposure to fire disturbance in the different forest areas, we were able to correlate the fire-adaptive strategies of the four conifers with stand characteristics under varying fire disturbances and habitats. The interaction between forests and fires, and their adaptation to each other, ultimately create the current ecosystems in the Altai Mountains.

1. Introduction

Fire, as a natural factor, appeared on Earth almost at the same time as terrestrial plants [1,2]. It is an essential force driving plant succession and evolution, and it may have significantly impacted plant characteristics as early as 350 Mya [3,4,5]. Fire exerts evolutionary pressures on plants and acts as a selective force for plant evolutionary traits, driving plants to evolve fire-adapted traits in response to the fire regime of the environments in which they grow [6,7,8,9]. These plant fire-adaptive traits are closely linked to the role of the plant itself as a fuel provider that influences fire occurrence and spread [10,11]. Conifers have many fire-adapted traits, with pines evolving fire-adapted traits dating back millions of years [12,13]. Therefore, a deeper understanding of plant fire adaptations can help us understand why forests form and influence how we manage forest ecosystems [14].

Vegetation adaptations to fire can be categorized into three types: fire-tolerator, fire-embracer, and fire-avoider [4]. Fire-tolerator plants usually possess fire-resistance traits such as thick basal bark and self-pruning. On the other hand, fire-embracer plants have lower under-crown heights and are susceptible to crown fires but can seed prolifically and be rapidly recruited after a fire. Some vegetation exhibits rapid post-fire renewal traits such as serotiny and early reproduction [4,15]. Meanwhile, fire-avoider vegetation survives in environments where fire is rare. Most conifers can be categorized into one of these fire-adapted traits, and some vegetation may also exhibit a combination of traits due to the different fire regimes of the environments in which they live.

Enhanced fire resistance is a pervasive fire adaptation, and many plants have the fire-adapted traits of thick basal bark and self-pruning. Fire plays a significant role in selecting thick bark, and bark thickness may be the main plant trait influencing fire lethality [16,17,18,19]. Thick bark can effectively protect plants during fires by safeguarding the vascular formation layer and influencing heat transfer to the wood [20,21,22,23,24]. For example, Pinus radiata and Pinus muricata, which are more severely affected by fire, exhibit thicker bark [25].

Plants have developed various rapid renewal capabilities for fire adaptation, such as post-fire sprouting and serotiny [26,27]. Conversely, non-resprouters typically demonstrate greater fire tolerance and heat-stimulated germination [28]. Serotiny refers to the plant’s ability to retain seeds within long-closed cones in the plant canopy, with late-ripening cones releasing seeds only when exposed to fire or thermal shock [29,30]. The tolerance of seeds to fire is strongly associated with the frequency and severity of fires in the plant’s habitat [31]. In certain fire-prone ecosystems, plant seeds have adapted so that high temperatures significantly enhance germination rates [32,33,34]. Many Pinus spp. demonstrate the ability to withstand heat shock [35,36,37,38,39,40,41,42]. A variant of Pinus yunnanensis, known as Pinus yunnanensis (var. pygmea), was discovered to exhibit the ability to produce large numbers of cones after a fire in southwestern China [42].

The grass stage exhibited by some Pinus spp. at the seedling stage has also been suggested as a fire acclimation trait [43]. At this stage, the plant’s terminal buds are located near the ground surface and surrounded by dense clusters of needles, which may be destroyed by fire but protect the terminal buds from damage [44].

The high flammability of vegetation is also a fire-adapted trait. Species that exhibit post-fire seedling recruitment are relatively more flammable in the community [45]. Needles with low compaction are also favorable for understory fire ecosystems because they increase the likelihood of fire and prevent the accumulation of fuels in the understory [46]. Accelerated fire spread is also a plant self-protection mechanism [47], which may allow trees to survive fires more easily by minimizing flame residence time through rapid fire spread [11].

The Altai Mountain forest area, located in the northernmost part of China, is a representative northern taiga forest area in the country. The main distribution of tree species includes Larix sibirica Ledeb., Pinus sibirica (Loud.) Mayr, Picea obovata Ledeb., Abies sibirica Korsh., Betula pendula Roth, and Populus tremula L. Due to its remote location and minimal disturbance from humans, the forests in the Altai area are kept in a relatively primitive state. Natural fires, mainly caused by lightning, are the most significant disturbance factor for these forests, driving forest succession and contributing to the formation of an extraordinary ecological structure in the region (Figure 1).

The fire adaptations of vegetation and the fire regime of the environment in which it grows are mutually adaptive. Recent research has also demonstrated that fire regimes in some areas maintain forest species composition and that changes in the fire regime and vegetation composition respond to each other [48]. The influence of fire regimes and climate profoundly affect how forests are composed [49]. Different forests are adapted to different fire frequencies and severity levels [50]. This paper will explore the fire adaptations of plants and the fire regime characterizing the forested areas of the Altai Mountains, explaining how conifers in the region are adapted to fire and work together with fire to form the region’s ecosystems.

2. Materials and Methods

2.1. Study Site and Experimental Design

The Altai Mountain forest area is situated on the northwestern border of China, between 85°50′ and 95°25′ E and 44°50′ and 49°10′ N. Covering a total area of 2.25 million hectares, it ranges in altitude from 1600 to 2700 m. The region experiences a typical north, temperate, continental, arid, and cold climate, characterized by a short summer and a cold, long winter. The average annual temperature is 0 °C, and the average annual precipitation is 650 mm, resulting in harsh environmental conditions. Southwest winds prevail in the forest area all year round, with a maximum wind speed of up to 19 m/s and a general wind speed of less than 8 m/s. Development of the Altai Mountain forest area in China began only in the 1970s, with the establishment of nature reserves commencing in 2001 to impose strict restrictions on forest development. As a result, apart from a few herdsmen and forest rangers, human activity in the forest area is minimal. According to information obtained from rangers, human-caused fires are almost non-existent in the Altai Mountains, suggesting that the forest area is primarily affected by natural fires, particularly those caused by lightning (according to fire information from 2002 to 2018, lightning fires accounted for 96.8% of natural fires). This region is one of the primary areas in China affected by lightning fires, with forest fires occurring from May to October each year, coinciding with a high frequency of thunderstorms, dry climate, low precipitation, and easily ignitable vegetation.

The field survey for this study was conducted from May to September 2023. Due to the vast area and significant elevation gradient of the Altai Mountain forest, as well as the varying fire disturbances in different regions, we selected Habahe, Kanas, and Buerjin for the sample plot survey (Figure 2). According to information from the forestry department and local forest staff, no anthropogenic fires have occurred in these areas since the 1970s. Before 1970, the Altai Mountains were undeveloped, with minimal human activity, and the forests were almost pristine, experiencing only natural fire disturbances.

Within each study area, 20 sample plots were randomly selected, each measuring 20 m × 20 m, and the slope, canopy cover, elevation, latitude, and longitude were recorded for each plot. Individual tree surveys were conducted (those ≥ 10 cm DBH [51]), recording the tree height, basal diameter (diameter at a height of 20 cm), diameter at breast height (DBH), and under-crown height for each tree. Seed maturity and the presence or absence of openings were recorded for each conifer species in the sample plots.

2.2. Fire Disturbance Year Determination

As the Altai forest area is a designated forest reserve, tree cutting is strictly prohibited, and efforts were made to minimize plant damage during the determination of fire years in each area. The study utilized the forest fire-scarred tree age analysis method to ascertain the year of fire disturbances in the sample sites. We sampled each fire-scarred tree in sample plots with a DBH > 20. A total of 53 trees were sampled in Habahe, 32 in Kanas, and 18 in Buerjin. Core samples were taken vertically from the healing tissue of the fire scars and from the intact wood of the trees using an increment borer (produced by Haglöf Sweden AB, Långsele, Sweden). The samples with obvious carbonization lines and those that recorded the complete annual rings of the trees were brought back to the laboratory for fixation, drying, and sanding. The difference between the number of annual rings at the site of fire scar formation and the total number of annual rings of the whole tree was used to determine the year of fire scar generation.

2.3. Relative Air Humidity Monitoring in Forested Areas

In 2019, meteorological observation points were established in the three study areas. These observation points were utilized for daily monitoring and recording of air relative humidity data, contributing to the assessment of the forest fire risk within the forested areas.

2.4. Bark Properties

Bark samples were collected from every tree species in the three forest areas, with 20 randomly selected trees of each species in each area. Since the basal diameter of Larix sibirica was significantly larger than the other species, bark thickness was measured starting at 20 cm. For plants affected differently by fire within the sample plot, the total bark thickness was measured at four heights (20 cm, 50 cm, 100 cm, and 150 cm), and the relative bark thickness was calculated separately [52]. To avoid severe damage to the trees during bark extraction and since the bark of Larix sibirica can be very thick (up to 20 cm), we used growth cones perpendicular to the trees to extract bark samples when measuring bark thickness. At each height, a small bark sample was taken from the east, south, west, and north of the trunk, and the total bark thickness was measured and averaged over the four thicknesses. To compare the differences in bark thickness between trees of different diameters, the relative bark thickness was calculated.

$$\mathrm{Relative}\mathrm{Bark}\mathrm{Thickness}=\frac{\mathrm{Absolute}\mathrm{Bark}\mathrm{Thickness}}{\mathrm{Tree}\mathrm{Circle}\mathrm{Diameter}}\times 100$$

(1)

2.5. Measurement of Flammability

Twenty-four conifers of each species were randomly selected (eight trees from each forest area) for needle, bark, and wood sampling. Bark sampling was performed with a hammer and chisel to protect the trees, and was limited to 2 g per tree. Wood was sampled using an increment borer, with a limit of 2 g per tree. The samples were dried, ground, and homogeneously mixed by species for the flammability analysis [53].

(1)

Determination of ash content

The ash content is the amount of mineral residue of the fuel after complete combustion. The sample was placed in a crucible and heated to 800 °C in a horse spoke furnace for 12 h. After cooling, the samples were weighed and recorded. Two parallel tests were performed for each sample. The calculation formula was as follows.

$$\mathrm{Ash}=\frac{{\mathrm{m}}_2}{{\mathrm{m}}_1}\times 100\%$$

(2)

where m₁ is the sample mass before ashing, and m₂ is the sample mass after ashing.

(2)

Determination of crude fat content

The fat content is a vital metric for measuring the flammability of the fuel. The samples were wrapped in filter paper, and the weights of the filter paper and the samples were recorded. Petroleum ether was added, and the glass containers were placed in a thermostatic water bath at 85 °C for 7 h. The samples were then dried, and the weights were recorded. After completion, the samples were dried and weighed. Two parallel tests were performed for each sample. The calculation formula was as follows.

$$\mathrm{Fat}=\frac{{\mathrm{m}}_4-{\mathrm{m}}_3}{{\mathrm{m}}_5-{\mathrm{m}}_3}\times 100\%$$

(3)

where m₃ is the weight of the filter paper (g), m₄ is the weight of the filter paper and sample before drying, and m₅ is the weight of the filter paper and sample after drying (g).

(3)

HHV determination

The (HHV) refers to the heat released by the complete combustion of a unit weight of fuel in an adiabatic state at room temperature. The XRY-1C microcomputer oxygen bomb calorimeter was used to determine the sample’s HHV, and two parallel tests were performed for each sample.

$$\mathrm{HHV}=\mathrm{K}\frac{\left[\left({\mathrm{T}}_1-{\mathrm{T}}_2\right)+\Delta \mathrm{T}\right]}{\mathrm{M}}$$

(4)

where HHV is the higher heating value, kJ/g; K denotes the water equivalent, kJ/°C; T₁ denotes the temperature of the sample before ignition, °C; T₂ denotes the temperature of the sample after ignition, °C; $\Delta \mathrm{T}$ denotes the temperature correction value, °C; and M denotes the sample mass, g.

2.6. Significant Value Calculations

To investigate the position and role of the four conifers in the community in the Altai forest region, the importance value index was calculated to characterize the forest composition of the region [52].

Relative Abundance (RA):

$$\mathrm{RA}=\frac{\mathrm{Number}\mathrm{of}\mathrm{individuals}\mathrm{of}\mathrm{a}\mathrm{single}\mathrm{species}}{\mathrm{Number}\mathrm{of}\mathrm{individuals}\mathrm{of}\mathrm{all}\mathrm{species}}$$

(5)

Relative Frequency (RF):

$$\mathrm{RF}=\frac{\mathrm{Number}\mathrm{of}\mathrm{samples}\mathrm{in}\mathrm{which}\mathrm{a}\mathrm{single}\mathrm{species}\mathrm{occurs}}{\mathrm{Number}\mathrm{of}\mathrm{total}\mathrm{samples}}$$

(6)

Relative Dominance (RD):

$$\mathrm{RD}=\frac{\mathrm{Average}\mathrm{cross}-\mathrm{sectional}\mathrm{area}\mathrm{at}\mathrm{breast}\mathrm{height}\mathrm{of}\mathrm{individual}\mathrm{species}}{\mathrm{Sum}\mathrm{of}\mathrm{average}\mathrm{cross}-\mathrm{sectional}\mathrm{areas}\mathrm{at}\mathrm{breast}\mathrm{height}\mathrm{of}\mathrm{all}\mathrm{species}}$$

(7)

Importance Value (IV):

$$\mathrm{IV}=\frac{\mathrm{Relative}\mathrm{Abundance}+\mathrm{Relative}\mathrm{Frequence}+\mathrm{Relative}\mathrm{Dominance}}{3}$$

(8)

2.7. Redundancy Analysis

Redundancy analysis is a method that combines regression analysis with principal component subsystem ranking [54]. Redundancy analysis can visualize the relationship between various fire adaptation traits and the distribution of conifer species in forest areas with different natural fire disturbances. This study utilized Canoco 5.0 software for the redundancy analysis [55].

3. Results

3.1. Natural Fire Disturbance and Forest Species Composition in Altai Forests

To verify the differences in the distribution of the four conifers under different fire regimes and habitats in the Altai forest region, we selected three forest areas: Habahe, Kanas, and Buerjin. All three forest areas have a large distribution of lightning-struck trees, leading to a high incidence of lightning-strike fires. The impact of fire varies across the forest areas due to factors such as elevation and moisture conditions. To quantify the natural fire disturbance and in-story air humidity in each forest area, we reconstructed the fire history of each area using a fire-scarred tree analysis (Figure 3) and calculated the mean air humidity for each month from May to September (

Table 1. Relative air humidity in the forest area.

Forest Area	May	June	July	August	September
Habahe	30.26%	32.41%	35.78%	43.56%	40.71%
Kanas	39.67%	41.57%	44.34%	59.62%	48.87%
Buerjin	43.42%	55.78%	63.81%	67.23%	52.50%

The relative air humidity in the table is the monthly average of the relative air humidity data obtained from day-by-day measurements from 2019 to 2023.

). To quantify the species composition within the coniferous forests that experienced a fire disturbance, a detailed survey of the tree species composition in the canopy layer was conducted in the three stands (

Table 2. Vegetation composition of the tree layer.

Species	Habahe				Kanas				Buerjin
	RA	RF	RD	IV	RA	RF	RD	IV	RA	RF	RD	IV
Larix sibirica	0.459	1.000	0.638	0.699	0.327	1.000	0.688	0.672	0.181	1.000	0.448	0.543
Pinus sibirica	0.393	1.000	0.330	0.574	0.320	0.900	0.205	0.475	0.530	1.000	0.520	0.683
Picea obovata	0.107	0.500	0.029	0.212	0.349	0.750	0.091	0.397	0.229	1.000	0.030	0.420
Abies sibirica	0.041	0.100	0.003	0.048	0.004	0.150	0.016	0.057	0.060	0.200	0.002	0.087

RA denotes relative abundance, RF denotes relative frequency, RD denotes relative dominance, and IV denotes importance value.

). Broadleaf species such as Betula pendula and Populus tremula, which are typically found in hot and humid river valleys and serve as pioneer species in forests that have undergone extensive fire damage and are in the process of ecological succession, were not found in the three frequently burned forest areas selected for this study.

The Habahe forest area, being relatively dry and fire-prone, exhibited the highest number of fires among the three study sites, with a total of eight fires identified through fire-scarred tree analysis and an average fire interval of 14 years. The primary tree species in the area were Larix sibirica and Pinus sibirica, with a relatively limited distribution of Picea obovata. In contrast, the Kanas region had better in-forest moisture conditions compared to Habahe, with relatively few fire disturbances, as only four fires were detected with an average fire interval of 23 years. In this region, there was little variation in the number of Larix sibirica, Pinus sibirica, and Picea obovata in the forests. However, many Larix sibirica individuals had a diameter at breast height (DBH) of 30 cm or more, and the dominance of Larix sibirica was significantly higher than that of Pinus sibirica and Picea obovata. Pinus sibirica and Picea obovata in this region were mostly relatively young individuals that had regenerated after the fires. There were many individuals aged over 100 years in this region. Moving on, the Buerjin area had the finest moisture conditions and the fewest fires of the three forest sections, with traces from only two fire discovered and a fire interval of 59 years. Pinus sibirica was the dominant species in this area, and although Larix sibirica was numerically less abundant than Pinus sibirica and Picea obovata, the existing Larix sibirica individuals were very old and significantly more dominant than Picea obovata. Abies sibirica was only sporadically distributed in small numbers in all three forest areas.

3.2. Bark Thickness Fire Adaptation

The relative bark thickness of Larix sibirica was significantly greater than that of the other tree species, often exceeding 10 cm. Conversely, the relative bark thicknesses of Pinus sibirica and Picea obovata were relatively small and not significantly different (Figure 4).

3.3. Differences in Basal Diameter/DBH

Larix sibirica has a significantly larger base than the other conifer species. To quantify the differences in base size between plants, this study used the metric basal diameter/DBH to compare the basal diameter accretion in the different conifers (Figure 5). The basal diameter/DBH of Larix sibirica was significantly larger than that of the other three conifers, with the largest basal diameter up to 1.56 times the DBH of the other trees that we measured.

3.4. Differences in Larix sibirica Bark in Different Regions

The differences in Larix sibirica relative bark thickness and basal diameter to DBH ratios among the regions are shown in Figure 6. Specifically, the Habahe region exhibited the largest relative bark thickness and basal diameter to breast diameter ratio, followed by the Kanas region, with the Buerjin region showing the smallest values.

3.5. Differences in Under-Crown Height

The heights of the four conifers varied greatly, as well as the heights of trees of different heights, so we introduced the under-crown height/tree height to quantify the differences in height between species (Figure 7). The under-crown height and under-crown height/tree height of Larix sibirica were significantly higher than those of the other conifers, with a maximum under-crown height of 16 m and a maximum under-crown height/tree height of 0.70. In comparison, the under-crown height of Pinus sibirica was the shortest, with a maximum height of 1.76 m. The under-crown heights of Picea obovata and Abies sibirica were higher than that of Pinus sibirica.

3.6. Comparison of Flammability

The differences in fire adaptation traits were explored by sampling needles, bark, and wood from the four conifers separately and analyzing the differences in their flammability (Figure 8).

Needles are the primary fuel in forest fires. Among the four coniferous species, Pinus sibirica had the highest HHV of 22.20 kJ/g, along with a high fat content and the lowest ash content. On the other hand, Larix sibirica needles had the lowest HHV and fat content among the four conifers, and the ash content was also high. In the event of a forest fire, the bark may also be involved in the burning of the forest. The bark of Abies sibirica had the highest HHV and fat content, and the lowest ash content. Conversely, Picea obovata bark had the lowest HHV and fat, and the highest ash content. The ash content in the wood of conifers was low, and the fat content of Picea obovata and Abies sibirica was significantly higher than that of the other two conifers. The HHV of Abies sibirica was also higher, and the wood of Abies sibirica showed relatively high flammability.

3.7. Seed Characteristics of Different Conifers

At the beginning of the sample plot survey in May, it was observed that there were residual cones from the previous year on the trees, but all the cones had burst open. There were differences in the seed maturity time of each conifer species. Picea obovata had the earliest seed maturity time, starting in mid-August, with all seeds set in mid-September. Larix sibirica showed concentrated seed maturation from the beginning of September to mid-September, while Pinus sibirica and Abies sibirica had their seed maturity time in September–October. Overall, the seed maturation of the conifers occurred in September–October, coinciding with the onset of snowfall in the Altai Mountains and avoiding the fire season.

3.8. Association between the Fire Adaptation Traits of Coniferous Trees and the Distribution of Vegetation in Different Forest Areas

We examined the relationships between various fire adaptation traits and the distribution of conifers in the different forest areas affected by natural fire disturbances (Figure 9). The explanatory variables included relative bark thickness, basal diameter/DBH, under-crown height, and needle flammability of each species, while the response variables were the IV of each species in Habahe, Kanas, and Buerjin. The relative bark thickness, basal diameter/DBH, and under-crown height significantly influenced the distribution of species in the forest areas affected by natural fires, particularly in the Habahe region, where the fire disturbance was most pronounced. The plant flammability indicators HHV and fat content exhibited negative effects on the vegetation distribution, with their impact being relatively reduced in the less fire-disturbed Buerjin area.

3.9. Association between Natural Fire Disturbance Scenarios and the Distribution of Different Conifer Species

We examined the relationship between natural fire disturbance scenarios and the distribution of different conifer species (Figure 10). The explanatory variables included the number of fires and average relative air humidity in each forest area, while the response variables were the IV of the four conifer species. An increase in the number of fires significantly influenced the distribution of Larix sibirica. The relative air humidity exhibited a significant positive effect on the distribution of Picea obovata and Abies sibirica, while its impact on the distribution of Pinus sibirica was relatively weak.

4. Discussion

4.1. Characterization of Fire Adaptation of Four Conifer Species within the Altai Forests

Larix sibirica demonstrates fire-tolerant traits, including thick bark, large basal diameters, and high under-crown heights. The thickness of the bark is the primary factor determining its heat tolerance [24]. In comparison with other coniferous species, Larix sibirica’s bark and large basal diameter effectively protect its wood during a fire. The average under-crown height of Larix sibirica can reach up to 12.29 m, effectively preventing flames from spreading along the branches to the crown and avoiding the occurrence of crown fires. Larix sibirica shows a much stronger resistance to fires compared to other dominant species. This is evidenced by the presence of only fire scars on Larix sibirica in the sample plot survey. This suggests that Larix sibirica has a higher likelihood of survival in fires compared to other species.

In contrast, Pinus sibirica has relatively thin bark, averaging only 1.52, and the basal diameter does not differ significantly from the DBH, indicating a lower fire tolerance. However, the pine cones of Pinus sibirica are tough and thick. Unlike the other three conifer species that release their seeds with feathery wings after maturity, Pinus sibirica cones naturally split open in hot and humid environments, exposing the seeds with their hard pericarp, effectively avoiding the risk of their seeds being exposed and burned in a fire. The thick pine towers and hard seed coat also contribute to the species’ survival during fires. The larger seeds also store abundant nutrients, ensuring their ability to germinate for many years after being planted. Pinus sibirica seeds remain viable for 2–3 years after falling to the ground. Germination is more likely to occur when the seeds are stimulated by high temperatures [56]. Pinus sibirica needles, bark, and wood exhibit higher flammability compared to other coniferous species. This heightened flammability may represent a fire-adapted ability to kill the competing plants, as suggested by [57], thereby enhancing the survival advantage of the species. Pinus sibirica demonstrates fire-embracer traits.

In contrast, Picea obovata and Abies sibirica prefer shady and moist conditions within forest stands, making it challenging for their trees and seeds to survive fires. Abies sibirica, in particular, requires high air humidity and sufficient heat [58], which contributes to its rare distribution in the Altai Mountains. Compared to other coniferous species, both Picea obovata and Abies sibirica exhibit fire-avoider traits. However, Picea obovata seedlings can thrive in cool and moist environments, allowing them to grow in the lower layers of the forest within Larix sibirica and Pinus sibirica communities that are renewed after fires. Over time, Picea obovata can gradually establish a certain degree of dominance in the forest.

The substantial amount of snow in the Altai Mountains before May and after September plays a significant role in suppressing fires [59]. The concentration of the four conifer species in September–October effectively mitigates the risk of plant seed death due to forest fires.

4.2. Stress Effects of Larix sibirica Bark from Fire Disturbances

Our investigations into the stress effects of Larix sibirica bark from fire disturbances revealed significant findings. We observed that trees with visible burn marks exhibited significantly greater relative bark thickness and basal diameter to DBH ratios in comparison to unaffected trees. Drawing from the fire-adapted trait of increased relative bark thickness to enhance fire resistance in Quercus spp. after frequent fire stimulation, as observed by [52], we hypothesized that Larix sibirica also responds with increased bark thickness and basal diameter to bolster its fire resistance when exposed to high-severity fires.

It is important to note that the study was unable to conclusively rule out the impact of other factors, such as topography, drought, and snowfall, on the thicker bark and larger basal diameter observed in Larix sibirica.

4.3. Ecological Significance of Natural Fire Disturbances in a Coniferous Forest in the Altai Mountains

The primary source of fire in the Altai Mountains is lightning fires that occur during dry thunderstorms in the summer, with a low probability of occurrence. This study selected three areas with apparent traces of fire for sample plot investigation. The local forestry management department confirmed that these areas have not experienced human-made fires in the past 50 years or so, indicating that these fires were natural.

All four conifer species were distributed in the three study areas, but the species composition of the stands differed due to varying fire disturbance and habitat conditions. The different fire-adaptive strategies of the four conifer species are evident in their respective stand characteristics under varying fire disturbances and habitats. Larix sibirica demonstrates fire-tolerant traits, Pinus sibirica exhibits fire-embracing traits, and Picea obovata and Abies sibirica exhibit fire-avoiding traits. Specifically, Larix sibirica is resilient and capable of surviving forest fires, while Pinus sibirica and Picea obovata struggle to survive fires, although Pinus sibirica is more likely to experience substantial regeneration post-fire (Figure 11).

In the driest and most fire-disturbed areas of Habahe, Larix sibirica is the dominant species, with a widespread presence of Pinus sibirica and relatively few Picea obovata. This could be attributed to the shorter intervals between fires, lower fuel accumulation, and reduced fire intensity, making it more conducive for Larix sibirica to thrive. Conversely, other species unable to withstand fires are more likely to perish, further reinforcing the dominant position of Larix sibirica in the forested areas. In contrast, in the Kanas region, which experiences fewer fire disturbances relative to the Habahe region, the dominance of Larix sibirica is significantly diminished. The growth dominance of Pinus sibirica and Picea obovata increases, with the relative abundance of Picea obovata even surpassing that of Larix sibirica. The Picea obovata in the Habahe and Kanas regions are small trees. However, in the Kanas region, their total number has exceeded that of Larix sibirica and Pinus sibirica. In the wettest area of Buerjin, Pinus sibirica has become the dominant species, favored by the relatively wet environment for the maturation of pine cones and the release of seeds. Simultaneously, Picea obovata seedlings can also grow in large quantities. The dominance of Pinus sibirica over Picea obovata is likely due to its highly flammable pine needles, which can suppress competing plants during fires, while its seeds, protected by thick pine cones, can survive and establish populations after fires. Larix sibirica trees in the Buerjin area are not numerous but are large, resulting in high relative dominance (RD) and a high importance value (IV). However, due to the light-loving nature of Larix sibirica seedlings, it is challenging for them to grow when the upper layers of the stand are shaded. If there is a prolonged lack of fire disturbances leading to a reduction in the number of trees in the upper layers, Larix sibirica will struggle to regenerate and lose out in competition with Pinus sibirica and Picea obovata.

In the sample plot survey, it was observed that all the trees affected by lightning strikes were Larix sibirica. This suggests that Larix sibirica may attract lightning more readily compared to the other species, and there may also be a higher lightning strike rate in stands with a dense distribution of Larix sibirica. If this mechanism exists, Larix sibirica may indirectly reinforce its dominance in the forest by increasing the probability of lightning fires. This hypothesis will be further investigated in the future.

In addition to fire disturbance and habitat, vegetation distribution is influenced by several difficult-to-quantify factors. These include the presence or absence of mother trees (which provide seeds) after a fire, the occurrence of extreme weather (e.g., continuous high temperatures and low rainfall), and predation by forest animals. However, it is undeniable that fire disturbances, as a crucial ecological factor, profoundly affect the vegetation distribution and forest structure in Altai Mountain forests. The prominent conifers in the forests have fire-adapted traits suited to the region’s fire regime. Based on the fire-adaptive ability of the different conifers, various species show different distributions in forest stands with differing fire disturbance conditions.

Fire disturbances can give Larix sibirica a competitive advantage due to its higher likelihood of surviving fires. Pinus sibirica struggles to survive fires, but its highly flammable pine needles enable it to kill other plants. Its seeds can be preserved in forest fires and dominate post-fire. In wetter environments like Buerjin, Pinus sibirica can more easily establish dominant positions. Picea obovata exhibits fire-avoidance traits, but its seedlings can thrive in shady conditions, leading to an abundance of young Picea obovata trees in all three study areas. Abies sibirica is rarely found due to its demanding habitat requirements. As forest stands mature, the sunlight-demanding Larix sibirica is likely to become old-growth or gradually replaced by Pinus sibirica and Picea obovata in the absence of fire disturbances. The understory shrub layer and forest microclimate may undergo unforeseen shifts.

It is worth noting that due to the lack of shrubs in Altai forests, not every fire develops into a crown fire. For example, studies in the Kanas region have shown that low-intensity fires can also occur in forested areas [60,61]. Only a small number of trees burn in low-intensity fires, which explains the presence of a small number of Picea obovata and Abies sibirica in forested areas.

Although the four species of conifers have different fire adaptation strategies, they have all adapted to the wildfire regime in the Altai Mountains under the long-term disturbance from natural fires, becoming an essential part of the Altai Mountains ecosystem. It is not only the vegetation that has adapted to fire but the entire forest itself, which, along with fire, constitutes the entire Altai Mountain ecosystem. Forests and fires together form the entire Altai ecosystem, constantly renewing and maintaining a dynamic and stable state in the face of wildfire disturbances.

4.4. Discussion on Forest Fire Management Measures in the Altai Forest Area

As a result of the increasing ability to monitor and prevent forest fires, although trees in the Altai Mountains are struck by lightning and catch fire every year, most of these fires are extinguished by firefighters within a short period of time and do not become forest fires. The absence of fire-scarred trees produced after 1994 in the sample plots (Figure 3) supports this observation. While fire prevention is crucial, the prolonged lack of fire disturbances can have a significant impact on forest turnover and population structure maintenance. For instance, species such as Larix sibirica and the fire-tolerant Pinus sibirica may lose their competitive edge against Picea obovata in the absence of fire, leading to changes in stand structure and function throughout the Altai Mountains.

Plants are not inherently adapted to fire, but rather to specific fire regimes. The prolonged absence of fire disturbances in the Altai forests may ultimately alter the fire regime and ecological structure of the entire forest. Several studies have demonstrated that long-term fire bans have already led to significant changes in forest structures and ecological functions [62,63,64]. It remains uncertain whether similar effects will occur in the Altai Mountains. Therefore, future forest fire management in the Altai Mountains should consider the potential need to allow natural fires to occur under controlled conditions. Government-led forest management practices, such as selective deforestation based on forest stand conditions, can serve as an alternative to fire, controlling tree density and promoting the growth of new seedlings, thereby facilitating forest stand renewal. When properly integrated and utilized, fire can provide various benefits to humans in most ecosystems [65]. The judicious use of fire can be an effective means of forest management and ecological restoration [51]. However, the effective use of fire while preventing uncontrolled wildfires in the Altai Mountains necessitates further research.

Fire is an integral part of ecosystems, coexisting with other life forms such as plants. The interaction and adaptation between plants and fire regimes have shaped the forest ecosystems we observe today. In future discussions on fire prevention and forest management, it is essential to focus on understanding the mechanisms of fire–plant interactions and utilizing fire in a manner that supports the sustainability of plant communities.

5. Conclusions

We conducted a comprehensive analysis of fire-adapted traits such as bark thickness, height below branches, basal diameter, and burning capacity of four conifers: Larix sibirica, Pinus sibirica, Picea obovata, and Abies sibirica in the Altai Mountains. Larix sibirica exhibits fire-tolerant traits, Pinus sibirica exhibits fire-embracing traits, and Picea obovata and Abies sibirica exhibit fire-avoiding traits. To explore the ecological significance of natural fires in the Altai Mountains, we selected three representative forest areas, analyzed their fire-scarred trees to reconstruct their fire history, and investigated the structural and habitat characteristics of the forests. The dominance of Larix sibirica in the more fire-prone forests was obvious, while the dominance of Pinus sibirica was apparent in the relatively less fire-disturbed forests. Picea obovata also exhibited some dominance in the less fire-disturbed forests, and the fire-adapted strategies of the different species were consistent with their distributions in the different forest stands. Fire coexists not only with specific vegetation but also with the entire Altai forest ecosystem.