Key leaf traits objectively reflect plant adaptation to environmental changes and significantly impact the structure and function of forest ecosystems (Wei et al., 2016). Common key leaf traits include Specific Leaf Area (SLA), Leaf Dry Matter Content (LDMC), Leaf Nitrogen (LN) content, and Leaf Phosphorus (LP) content (Wright et al., 2004). SLA and LDMC are commonly used to reflect trees’ resource utilization and allocation strategies (Kuppler et al., 2020; Wang et al., 2022a; Liu et al., 2023a). Leaf Nitrogen and Phosphorus contents are typically associated with trees’ growth rate and nutritional status (Tang et al., 2018). Studies have shown that the potential process of photosynthesis depends on leaf nitrogen (N) and phosphorus (P) concentrations, with trees having higher leaf N and P contents often exhibiting faster growth rates (Ellsworth et al., 2022).

Elevational gradients provide a unique natural laboratory for revealing variations in key leaf traits and their adaptations to environmental changes because they encapsulate a broad range of environmental conditions within a relatively confined geographical space (Gao and Liu, 2018). Changes in elevation are associated with systematic variations in temperature, moisture availability, and solar radiation, each of which can exert significant selective pressures on plant physiology (Shi et al., 2020; Wang et al., 2022b). Key leaf traits reflect the physiological and ecological adaptation strategies of plants under different environmental conditions (Li et al., 2021). Therefore, studying the variation patterns of key leaf traits along elevational gradients is crucial for understanding the survival and adaptation mechanisms of plants in different elevation environments (Zhang et al., 2021; Rixen et al., 2022). Moreover, research on elevational gradients can uncover how environmental factors such as climate change and soil nutrients, as well as stand factors like stand age and forest density, jointly influence key leaf traits, thereby providing a theoretical basis for predicting ecosystem functions in the context of global change. Thus, exploring the variation of key leaf traits across elevational gradients not only deepens our understanding of ecological processes but also provides strong support for species adaptive evolution and ecosystem management (Wang et al., 2022b).

Preliminary research has found that with increasing elevation, SLA significantly decreases and LDMC significantly increases, indicating a shift towards conservative survival strategies in plants (Bjorkman et al., 2018; Thomas et al., 2020). Leaf N content increases (Xu et al., 2021a), while leaf P content decreases, as low temperatures limit nitrogen mineralization rates, causing plants to accumulate more nitrogen in leaves to maintain adequate photosynthetic efficiency. Phosphorus elements are often harder for plants to absorb and utilize at high altitudes, resulting in lower leaf phosphorus content. However, some studies have observed decreases in leaf nitrogen and phosphorus contents with increasing elevation (Zhao et al., 2016; Zhang et al., 2023), suggesting that in certain ecosystems, low temperatures and soil nutrient limitations reduce the efficiency of nutrient absorption and utilization. Therefore, the patterns of key leaf traits along elevational gradients at the macro scale have not yet reached a consensus.

Along the elevational gradient, climatic factors, especially temperature and precipitation, undergo significant changes (An et al., 2020; Wang et al., 2022b). Numerous studies have found that key leaf traits are regulated by climatic factors. Under suitable temperature and precipitation conditions, trees allocate more resources to compete for light, increasing SLA and decreasing LDMC (Kuppler et al., 2020; Wang et al., 2022a; Zheng et al., 2024). Conversely, cold and arid environments increase survival stress in trees, forcing them to allocate more resources to survival. This is achieved by reducing SLA and increasing LDMC to prolong life span and slow growth, resulting in a conservative resource utilization strategy (Kramp et al., 2022; Yu et al., 2022). In cold environments, plant growth rates usually slow down, affecting their absorption and utilization of nutrients like nitrogen and phosphorus. Under drought conditions, plants may enhance nutrient acquisition by increasing root growth or altering root distribution (Wang et al., 2021a). Additionally, plants adjust the content and ratio of leaf nitrogen and phosphorus to adapt to water-limited environments (Tian et al., 2019). Furthermore, sunlight duration is another important climatic factor affecting key leaf traits. With decreasing elevation, the effective duration of sunlight reduces, leading trees to increase SLA and leaf nitrogen content and decrease LDMC. This strategy ensures efficient use of light energy and survival in intense resource competition (Reich et al., 1997).

Soil, as the direct living environment for plants, also plays a non-negligible role in key leaf traits (Wang et al., 2021b; Cui et al., 2022). Existing research shows that an increase in soil nitrogen typically leads to an increase in plant leaf nitrogen content. Higher soil nitrogen levels may cause a decrease in leaf LDMC, as plants might produce thinner, less fibrous leaves to optimize photosynthesis (Liu et al., 2021a; Pichon et al., 2022; Waring et al., 2023). Phosphorus limitation can lead to plants producing smaller leaves and higher LDMC to reduce growth demands and improve resource efficiency. Increased soil phosphorus enhances leaf phosphorus content, aiding in the synthesis of DNA and RNA and energy conversion processes, which could affect plant growth rates and leaf traits (Yang et al., 2023; Yan et al., 2024). Soil microbial activity is regulated by soil pH, and soil microbes participate in the decomposition of organic matter. Therefore, soil pH has a significant impact on plant nutrient utilization strategies and leaf nutrient traits (Kang et al., 2021; Han et al., 2023).

Forest age and stand density, among other stand factors, also have a critical impact on key leaf traits (Louis et al., 2012; Zhang et al., 2022). In the early stages of forest development, trees tend to grow rapidly, producing larger, thinner leaves (high SLA, low LDMC) to maximize photosynthesis. In the later stages of forest development, plants allocate more resources to structural reinforcement of leaves, thus increasing LDMC (Craven et al., 2018; Engbersen et al., 2022; Kramp et al., 2022). In high-density stands, competition among trees (especially for light) is more intense. This may lead to the production of smaller leaves (low SLA) and higher LDMC, adapting to lower light levels and increased competitive pressure. During the early stages of forest development, trees have a higher demand for soil nutrients, accelerating the absorption of N and P from the soil, resulting in higher leaf nitrogen and phosphorus contents (Li et al., 2017). In the later stages of forest development, the available nitrogen and phosphorus in the soil may decrease due to slower decomposition of organic matter and intensified competition between microbes and plants for nitrogen. Therefore, the leaf nitrogen and phosphorus contents in mature forests may be lower (Liu et al., 2021b, Liu et al., 2021c; Yang et al., 2021).

Based on field surveys and literature collection from 2008 to 2020, data from 697 forest sites across China were used to explore the patterns of key leaf traits along elevational gradients at a macro scale and their dominant factors. To address the above issues, we propose the following hypotheses: 1) With increasing elevation, SLA significantly decreases, and LDMC increases, indicating a more conservative plant resource utilization strategy. 2) Climatic factors are the dominant environmental factors influencing the variation of key leaf traits along elevational gradients, with stand factors (forest age) also playing a significant role. 3) The direct impact of climatic factors on the elevational variation patterns of key leaf traits is greater than their indirect effects (by affecting soil nutrient and stand factors).

Trees at different elevations adopt varying survival strategies to adapt to complex habitats, often reflected in changes in key leaf traits. Our results indicate that trees in lower elevations possess higher SLA and lower LDMC, while those in higher elevations have lower SLA and higher LDMC. Rixen et al. (2022) studied intraspecific variation in aboveground functional traits across 66 alpine plant species in four countries, exploring how these traits vary with elevation. They found that SLA significantly decreases and LDMC significantly increases with increasing elevation. These findings are consistent with the results of our study. This is primarily because lower elevation areas generally have better water and thermal conditions ( Supplementary Figure 2 ) (Gao and Liu, 2018), leading trees to adopt faster investment-return resource utilization strategies to cope with biotic competition in the community. These trees typically increase their light capture area, reduce leaf construction investment, resulting in higher SLA and lower LDMC (Coble et al., 2017). In contrast, higher elevation areas are generally characterized by colder temperatures, less rainfall, and fewer available soil nutrients ( Supplementary Figures 2 , 3 ). Trees in these areas face survival pressure mainly from these environmental factors and adopt conservative survival strategies by reducing leaf area and increasing leaf construction investment, typically resulting in lower SLA and higher LDMC (Liu et al., 2023a). With increasing elevation, LN and LP also tend to increase. Xu et al. (2021a) sampled 428 plant individuals across 18 sampling sites along a 3000-meter elevation gradient on Gongga Mountain in China and found that leaf nitrogen content increased with elevation, consistent with the results of our study. This is mainly because the rise in elevation is usually accompanied by a decrease in temperature. Under low temperature conditions, trees’ metabolic rates decrease, leading to an increased demand for nitrogen and phosphorus to maintain normal physiological functions (Michaletz, 2018). Additionally, soils in high-altitude areas are often poor, with fewer available nutrients such as nitrogen and phosphorus. Trees adapt to these nutrient-poor environments by enhancing the efficiency of nitrogen and phosphorus absorption in roots, thereby accumulating more nitrogen and phosphorus in their leaves (Wang et al., 2021a). Our study emphasizes the significant influence of elevation on functional traits, and the observed variation in key leaf traits along the elevational gradient aligns with the leaf economics spectrum theory (Wright et al., 2004). Our findings are crucial for understanding plant adaptation, competition, ecosystem functioning, and responses to climate change across different elevations, providing valuable insights for future ecological research and forest management.

We also found that with increasing temperature, SLA significantly increases, and LDMC significantly decreases ( Figures 3A, E ). Wei et al. (2023) found that warming significantly increased SLA based on a seven-year field warming experiment, which is also consistent with our findings. This is because as temperatures rise, trees often increase their leaf area to enhance photosynthesis, thus more effectively utilizing available light. Higher SLA indicates thinner leaves, which is beneficial for light capture and gas exchange in photosynthesis (Huang et al., 2020). Increased temperatures also lead to a reduction in the accumulation of organic carbon compounds (such as cellulose and lignin) in leaves, thereby lowering LDMC. This reduction in investment in light energy allows the plant to be more efficient in growth and metabolic processes (Shi et al., 2022). As temperature increases, LN shows an increasing trend ( Figure 3I ), while with increased precipitation, both LN and LP tend to decrease ( Figures 3J, N ), which is consistent with the study findings of Chen et al. (2013). This may be because rising temperatures enhance soil microbial activity, increasing nitrogen release in the soil. Plants effectively absorb this nitrogen through their vascular systems, indirectly increasing leaf nitrogen content (Xu et al., 2021b). With continued increases in precipitation, surface runoff and subsurface flow carry away a significant amount of soil organic matter, leading to nutrient (especially nitrogen and phosphorus) depletion in the soil. Therefore, intense rainfall results in lower nitrogen and phosphorus content in plant leaves (Wang et al., 2014).

Soil, as the immediate living environment for plants, significantly influences key leaf traits. Our study found that as soil nitrogen (N) content increases, leaf nitrogen (LN) initially decreases and then increases ( Figure 4G ), which is consistent with the study findings of Chen et al. (2013). This is because when soil nitrogen content is low, plants are limited by nitrogen nutrition. They adapt by expanding their root system to seek more nitrogen sources and reduce the allocation of nitrogen in their leaves to cope with nitrogen deficiency (Wang et al., 2022c). As soil nitrogen content increases, plants can more easily absorb nitrogen. This leads to an increase in nitrogen content in leaves, enhancing photosynthesis efficiency and growth rate (Waring et al., 2023). With an increase in soil phosphorus, leaf nitrogen and phosphorus contents decrease ( Figures 4H, K ), which is contrary to the findings of Chen et al., 2013; Chen et al., 2024. According to the Nutrient Balance Theory, when a particular nutrient in the soil (e.g., phosphorus) becomes abundant, plants may alter their absorption and distribution strategies for other nutrients (e.g., nitrogen) to maintain nutrient balance (Li et al., 2023). In such cases, even if nitrogen is abundant, plants might reduce nitrogen absorption, leading to a decrease in leaf nitrogen content. The Dilution Effect also suggests that as leaf phosphorus content increases, plant growth may accelerate, causing a relative dilution of absolute nitrogen and phosphorus contents in leaves compared to the total leaf mass (Zhang et al., 2022; Liang et al., 2023). Therefore, even though the total nitrogen and phosphorus content in plants increases, the concentrations of nitrogen and phosphorus calculated per dry leaf weight might decrease. With increasing soil pH, the nitrogen and phosphorus contents in plant leaves significantly increase ( Figures 4I, L ), which is contrary to the findings of Lin et al. (2022). This discrepancy may be because acidic soil conditions can inhibit the release of soil nitrogen and phosphorus by affecting microbial activity (Liu et al., 2023b). As the pH rises, soil nitrogen and phosphorus gradually release, indirectly increasing the nitrogen and phosphorus content in plant leaves.

In addition to climatic and soil nutrient factors, stand factors, particularly forest age, significantly influence key leaf traits. As forest age increases, SLA significantly increases, LDMC decreases, and both LN and LP significantly decrease ( Figure 5 ). Zhang et al. (2024) also found in their study on resource utilization strategies of trees in planted and natural forests that SLA significantly increases and LDMC significantly decreases with increasing forest age, which is consistent with our findings. As forest age changes, photosynthetic capacity and nutrient demand generally shift as trees grow. Trees at different growth stages exhibit substantial differences in physiological processes and nutrient requirements, leading to changes in resource utilization strategies and nutrient stoichiometry characteristics with increasing forest age (Zhang et al., 2022). As trees grow and mature, they may allocate more resources to the growth and maintenance of stems and roots rather than leaves (Deng et al., 2024). This means that with increasing forest age, leaf nitrogen and phosphorus accumulation may decrease to support the growth of other parts of the tree. With forest development, tree diversity increases, intensifying intra- and interspecific competition. Trees respond by increasing their light-catching surface area, typically resulting in higher SLA and reduced LDMC (Bongers et al., 2020). This competition also leads to a reduction in soil nitrogen and phosphorus content, indirectly decreasing LN and LP levels.

Leaf nutrient traits (LN and NP) exhibit greater environmental plasticity compared to traits associated with resource utilization strategies (SLA and LDMC) ( Figures 1 – 8 ). In the study on the variation of plant leaf functional traits along environmental gradients and their driving factors, Akram et al. (2023) found that LN and LP are more influenced by environmental factors, particularly soil nutrients, than SLA and LDMC, which is consistent with our findings. This is primarily because they are directly related to the availability of nutrients in the environment and can quickly adjust to accommodate environmental changes (Guo et al., 2021). In contrast, SLA and LDMC, which reflect plants’ long-term adaptation strategies, change more slowly and steadily (Pierce et al., 2017). We found that stand factors, such as forest age, have the most significant direct impact on traits related to resource utilization strategies (SLA and LDMC), while climatic factors have the greatest direct influence on leaf nutrient traits (LN and LP). This is mainly because stand factors (especially forest age) directly affect traits related to resource utilization strategies by influencing plant growth strategies and resource allocation. On the other hand, climatic factors directly impact leaf nutrient traits by altering nutrient cycling and plant physiological processes, reflecting plants’ adaptability and ecological strategies under different environmental conditions (Liu et al., 2019).

Elevation not only directly affects key leaf traits but also indirectly influences them by regulating climatic, soil nutrient, and stand characteristics, with its direct impact being greater than its indirect impact ( Figure 8 ). This is primarily because high-elevation environments typically present extreme physiological stress conditions, such as low temperatures, intense ultraviolet radiation, and low oxygen environments. These conditions have a direct and significant impact on plant physiological processes, forcing plants to adapt directly to survive (Abbas et al., 2022). Moreover, environmental changes caused by elevation changes (such as temperature, humidity, and light) are rapid and direct. These environmental factors directly affect plants, influencing their growth and development processes, leading to rapid adjustments in leaf traits ( Supplementary Figure 4 ). Additionally, elevation’s impact on indirect factors like climate, soil nutrients, and stand characteristics is usually more complex and time-lagged (Gao and Liu, 2018). Therefore, although both direct and indirect effects of elevation influence plant leaf traits, in many cases, the direct effects, due to their immediacy, intensity, and necessity for physiological adaptation, may have a more significant impact on plants in the short term.

This extensive research across various forests in China illuminates how key leaf functional traits adapt along elevational gradients, crucial for understanding plant strategies under global change. Findings reveal that with increasing elevation, SLA decreases, while LDMC, LN, and LP significantly increase. These changes are predominantly influenced by climatic and soil nutrient factors, with forest age also playing a significant role. The study highlights the substantial impact of elevation on plant functional traits, offering insights into plant adaptation strategies and guiding future ecological research and forest management.