Grade-specific biomass allocation and allometric growth strategies in Pinus yunnanensis seedlings

Abstract

Background

Pinus yunnanensis, an evergreen coniferous species native to southwestern China, possesses considerable ecological and economic importance. In this study, two-year-old P. yunnanensis seedlings were categorized into three distinct grades according to seedling height to investigate variations in biomass accumulation, allocation patterns, and allometric growth relationships.

Results

Total biomass gradually decreases with decreasing seedling grades. Aboveground biomass consistently exceeded belowground biomass across all grades. The biomass levels of various organs followed the sequence: leaf biomass > root biomass > stem biomass > sprout biomass, with relatively stable allocation ratios maintained among them. Allometric and isometric relationships between organs, as well as between organs and whole-plant size, varied with seedling grade, reflecting differences in relative growth rates among plant components. Roots and aboveground parts share a common slope demonstrating a general consistency in resource allocation strategy during growth. In contrast, sprouts exhibit isokinetic growth relationships with other organs, showing poorer linear fitting. Correlation network heatmaps further illustrated divergent growth patterns and resource allocation strategies among grades. Most indicators within each grade showed significant positive correlations. Weaker correlations were observed between Grade I and Grade II seedlings, between seedling height and various indicators, and between sprouts and indicators. Conversely, Grade III seedlings exhibited significant negative correlations.

Conclusions

These findings reflect resource trade-off mechanisms governing biomass allocation among organs in seedlings of varying grades. Through stable biomass allocation ratios and plastic allometric growth relationships, P. yunnanensis seedlings adapt to varying growth conditions.

Introduction

Biomass represents a crucial growth strategy for plants, reflecting their capacity for energy accumulation [1]. The allocation of biomass among plant components is influenced by multiple factors, including environmental conditions, plant age, and individual plant size [2]. Limited survival resources constitute the key determinant shaping plant biomass allocation strategies [3]. The Optimal Allocation Hypothesis posits that plants adapt to external environments by regulating biomass distribution among different components; this adaptation enabled them to maximize the acquisition of limited resources (e.g. light, water, and nutrients), thereby sustaining maximum growth rates [4–6]. Individual size was one of the most critical characteristics of an organism. Therefore, applying allometric growth allocation theory to analyze organ biomass allocation patterns in seedlings of different sizes (based on individual size) was a key approach to understanding the growth patterns and life-history trade-off strategies of this plant group [7].

Allometric growth denotes a growth relationship in which two phenotypic traits of an organism exhibit disproportionately scaled relative growth rates [8]. According to the allometric allocation hypothesis, plant resource allocation is regulated by individual size; specifically, small individuals prioritize resource investment in leaves to maximize the acquisition of light energy [9, 10]. However, when biomass allocation ratios between organs exhibit significant variation, the relative growth relationships among organs may not be apparent. Focusing solely on biomass allocation patterns across organs risks obscuring substantive ecological issues. Therefore, employing allometric allocation theory to evaluate biomass distribution patterns among organs in seedlings of different sizes (classified by individual size) is crucial for understanding the growth and life-history trade-off strategies of such plants [11, 12].

P. yunnanensis, an evergreen tree belonging to the genus Pinus (family Pinaceae), is widely distributed across southwestern China, including Yunnan, Guizhou, Sichuan, Tibet, and Guangxi [13]. As a primary native tree species and key pioneer species for afforestation in southwest China, it also serves as a major economic species in Yunnan Province. Characterized by straight trunks, it is valued as high-quality timber for construction and industrial use [14, 15]. Additionally, P. yunnanensis is rich in resin; its roots are used for cultivating Poria cocos, its bark yields tannin, its seeds produce oil, its trunk exudes resin, its needles yield pine needle oil, and its wood can be distilled to produce various chemical products [16]. Pine resin, turpentine, branches, leaves, young fruits, and pine pollen all have medicinal properties, conferring significant nutritional and economic value [17]. However, in practical afforestation, P. yunnanensis plantations often exhibit low quality, low productivity, and low timber volume per unit area. Seedling quality is a critical factor affecting afforestation success, and the use of high-quality seedlings can significantly improve survival rates and forest productivity [18]. Sapling height (H) and diameter at breast height (DBH) are intuitive indicators for seedlings and represent important, highly practical metrics for evaluating seedling quality. Thus, in production practice, these two indicators are commonly used for seedling grading [19]. Existing studies on P. yunnanensis biomass allocation primarily focus on homogeneous seedlings or single environmental gradients, lacking systematic analysis of how seedling size heterogeneity (graded by height) regulates allometric growth relationships among organs. This creates a theoretical gap in understanding the adaptive strategies of seedlings with different growth potentials. To address this, this study proposes two core hypotheses: Seedling grade significantly affects biomass accumulation and allocation, with higher grades showing higher aboveground biomass ratios and total biomass accumulation; Allometric relationships among organs are plastic across grades, but root-aboveground relationships maintain conservative growth trajectories. The findings are expected to extend allometric theory by demonstrating whether and how conserved vs. plastic allocation strategies manifest across distinct size grades within a single species under uniform conditions, thereby linking practical grading criteria with fundamental growth models. This study investigated biomass allocation patterns in different-grade P. yunnanensis seedlings by grading two-year-old seedlings. It compared allometric growth differences between biomass components and individual seedlings across grades, aiming to reveal growth rate variations and adaptive mechanisms in structural components and morphological indicators of graded seedlings. This research enhances understanding of growth processes among seedlings of varying sizes and provides theoretical references for cultivating high-quality P. yunnanensis seedlings.

Materials and methods

Study area overview

This experiment was conducted at the Nursery of Southwest Forestry University, located at 102°45′41″E, 25°04′00″N, at an elevation of 1945 m. The study area has a subtropical semi-humid plateau monsoon climate, with an annual mean temperature of 14.7 °C, an absolute minimum temperature of -9 °C, and an absolute maximum temperature of 32.5 °C. Seasonal temperature fluctuations are slight, and rainfall is relatively abundant, with annual precipitation ranging from 700 to 1100 mm and a mean relative humidity of 68.2%. Precipitation is distinctly divided into wet and dry seasons: the wet season occurs from May to October, and the dry season from November to April. The regional soil is acidic (pH 6.0–6.2) with low phosphorus content, where total phosphorus ranges from 0.9 to 1.22 g/kg. The described environmental conditions represent the typical natural range of P. yunnanensis. The growth variation observed here therefore provides a valid basis for analyzing size-dependent allocation strategies, and the subsequent grading and allometric analyses reflect the species’ adaptive responses within its ecological context.

Sources of material

Test seeds of P. yunnanensis were collected from mixed seed lots at the Clonal Seed Orchard in Midu, Yunnan. The selected mother trees were mature and exhibited good fruiting performance. After proper marking and drying, well-developed seeds were screened for subsequent use. Before sowing, the selected seeds underwent uniform disinfection and soaking treatments. Once seedlings had fully germinated, they were transplanted into nursery pots (18 cm bottom diameter × 32 cm height). These pots were pre-filled with a substrate mixture of humus soil and red soil at a 3:1 volume ratio. Regular seedling care, including watering, fertilization, and pesticide application, was provided throughout the seedling stage.

Measurement of indicators

A total of 185 P. yunnanensis seedlings were uprooted using the whole-plant excavation method. Seedling height was measured with a ruler (accuracy: 0.01 cm), and stem diameter at ground level was determined using a vernier caliper (accuracy: 0.01 mm). Upon returning to the laboratory, the roots were severed from the stem base with scissors, followed by separation of roots, stems, leaves, and sprouts. “sprout” refers to all axillary shoots and lateral branches emerging from the main stem. Roots were rinsed with deionized water and air-drained. The fresh weight of each organ was measured (accuracy: 0.001 g); subsequent to weighing, each organ was placed in a separate paper bag and clearly labeled. All samples were oven-dried simultaneously in two identical ovens to ensure consistent temperature conditions and control potential inter-oven systematic errors, with an even distribution of samples across all seedling grades in each oven. The samples were first de-enzymed at 105 °C for 40 min, followed by drying at a reduced temperature of 80 °C to a constant weight. The dry weight of each organ was then precisely measured (precision: 0.001 g), and used to characterize the biomass of the corresponding organ.

Seedling grading

Seedling height (H) and stem diameter at ground level (D) are typically used as quality indicators for seedling grading. However, due to the strong correlations among various quality indicators, the use of multiple metrics can lead to information redundancy. Seedling grading criteria should not only be scientifically sound but also align with production practices and practical applications, emphasizing applicability and operability. In this study, the coefficient of variation (CV) for seedling height was greater than that for ground-level stem diameter, indicating that variation in height is more pronounced than variation in diameter. Moreover, seedling height is more visually intuitive during morphological observation and offers stronger operability. Therefore, this study used seedling height as the sole quality indicator for grading. Following the “mean ± 1/2 standard deviation” as the grading criterion, seedlings were categorized into three grades: Grade I, Grade II, and Grade III. With reference to national and regional seedling quality standards, the classification is defined as: Grade I seedlings: ≥ M + 1/2SD; Grade II seedlings: M − 0.5SD ≤ value < M + 0.5SD; Grade III seedlings: < M − 1/2SD [20]. Here, M represents the mean seedling height, and SD represents the standard deviation of seedling height. The sample sizes for each grade were: Grade I: n = 42, Grade II: n = 98, Grade III: n = 45.

Data processing

Statistical analyses were performed using Excel 2021, SPSS 21.0, and R software (smatr package). Prior to analysis, data were checked for normality (Shapiro–Wilk test) and homogeneity of variance (Levene’s test). SPSS 21.0 was used to calculate means and standard deviations and to conduct one-way analysis of variance (ANOVA) on biomass traits across seedling grades, with post-hoc Duncan’s multiple range test (α = 0.05). Phenotypic plasticity index (PI) is used to measure the degree of phenotypic change of organisms under different environmental conditions. PI = (Max - Min) / Max. The Max and Min refer to the maximum and the minimum mean value of a variable [21]. Allometric growth analysis allometric relationships were analyzed using Standardized Major Axis (SMA) regression on log10‑transformed biomass data, following the model log10(Y) = α + β · log10(X). Slopes, 95% confidence intervals, coefficients of determination (R²), and tests of isometry (slope = 1) are reported [22]. Graphical representations were generated using OriginPro 2024 software. All data are presented as mean ± standard deviation.

Results

Effects of different grades on growth characteristics of P. yunnanensis seedlings

Grade I seedlings exhibited the greatest height (M ± SD: 11.03 ± 2.15 cm), followed by Grade II (8.95 ± 0.85 cm) and Grade III (6.94 ± 1.23 cm) (Fig. 1a). Stem diameter at ground level showed a similar trend across grades, with Grade II seedlings having the highest mean diameter (13.03 mm), followed by Grade I (12.33 mm) and Grade III (11.08 mm) (Fig. 1b). The distribution of height-to-diameter ratios indicated that most seedlings across all grades prioritized horizontal expansion (ratio < 1). However, the proportion of seedlings exhibiting vertical growth emphasis (ratio > 1) increased with seedling grade, following the order: Grade I > Grade II > Grade III (Fig. 1c).

Fig. 1.

Analysis of seedling height (a), diameter at ground level (b), and height-to-diameter ratio (c) for different grades of P. yunnanensis seedlings

Biomass accumulation of P. yunnanensis seedlings of different growth grades

Biomass accumulation was strongly influenced by seedling grade. Total biomass decreased significantly with declining grade, with Grade I seedlings accumulating the most biomass and Grade III seedlings the least (Fig. 2a). Across all grades, aboveground biomass consistently exceeded belowground biomass (Fig. 2b). At the organ level, biomass allocation followed a consistent hierarchy: leaves > roots > stems > sprouts (Fig. 2c). While root, stem, and leaf biomass declined with decreasing seedling grade, sprout biomass exhibited a distinct pattern, being highest in Grade III seedlings, followed by Grade I and Grade II.

Fig. 2.

Individual biomass (a), root sprout ratio (b), and organ-specific biomass (c) of P. yunnanensis at different growth grade (Note: All values = mean ± standard error; different lowercase letters indicate significant inter-grade differences (P < 0.05))

Allocation of biomass in P. yunnanensis seedlings of different growth grades

For Grade I seedlings, aboveground biomass accounts for 71% and root biomass for 29%; for Grade II seedlings, aboveground biomass constitutes 70% and root biomass 30%; and for Grade III seedlings, aboveground biomass represents 69.7% while root biomass makes up 30.3% (Fig. 3 abc). Among organ-specific biomass distributions, leaf biomass exhibited the highest proportion, ranging from approximately 32% to 36%. Root biomass followed, accounting for 29% to 30%, while stem biomass constituted about 25% to 28%. Sprout biomass showed the lowest proportion with significant variability, fluctuating between 7% and 13% (Fig. 3 def). These results indicate that biomass in P. yunnanensis seedlings of different grades is preferentially allocated to the aboveground portion, with a consistent pattern: leaf biomass > root biomass> stem biomass > sprout biomass.

Fig. 3.

Above-ground and below-ground biomass (a, b, c) and organ-specific biomass allocation ratios (d, e, f) of seedlings at different growth grade

Analysis of phenotypic plasticity

Phenotypic plasticity analysis of measured biomass indices revealed that, all plasticity indices for biomass parameters were below 0.5. Among these, the plasticity of sprout biomass allocation ratio (0.427), stem biomass (0.412), and leaf biomass (0.424) exhibited relatively higher plasticity with greater variation, while the above-ground biomass allocation ratio showed the lowest plasticity (0.035). From a morphological perspective, the plasticity index for seedling height (0.371) exceeded that for stem diameter at ground level (0.149), indicating lower phenotypic plasticity in stem diameter (Fig. 4). These findings suggest that during growth, P. yunnanensis seedlings primarily balance resource utilization by adjusting biomass allocation to meet survival requirements.

Fig. 4.

Analysis of Phenotypic Plasticity in Morphological and Growth Parameters of P. yunnanensis

Allometric growth relationships among components in seedling grading

Different growth rates exist among various organs of P. yunnanensis seedlings across different grades. Regarding the allometric relationship between root-stem biomass, the allometric growth indices between roots and stems showed no significant difference across grades (P > 0.05), with a common slope of 0.787. Grade I seedlings exhibited an allometric relationship with a slope < 1. Grade II and III seedlings showed isometric growth (slope not significantly different from 1) (Fig. 5a). For root-leaf, significant differences existed among grades, with no common slope. Both Grade I and II seedlings exhibited isometric growth in roots and leaves (slope not significantly different from 1), while Grade III seedlings showed allometric growth with a slope greater than 1, meaning root biomass accumulation outpaced leaf biomass accumulation (Fig. 5b). When comparing root-aboveground, the allometric growth index was not significant between grades, with a common slope of 0.368. Seedlings of grades I and II exhibited allometric growth with slopes less than 1. Seedlings of grade III showed isometric growth (slope not significantly different from 1) (Fig. 5d). From the stem-leaf perspective, significant differences exist among different grades. Seedlings of grades I and II exhibited isometric growth (slope not significantly different from 1), while seedlings of grade III showed allometric growth with slopes greater than 1 (Fig. 5e). In contrast, isometric growth relationships (slope not significantly different from 1) were observed among all grades for root-sprout, stem-sprout, and leaf-sprout (Fig. 5c, f, g).

Fig. 5.

Allometric growth relationships for different parts of P. yunnanensis: root-stem (a), root-leaf (b), root-sprout (c), root-aboveground (d), stem-leaf (e), stem-sprout (f), and leaf-sprout (g). (Note: “b” = slope; P_− 1.0: significance of slope difference from theoretical value 1.0; “A” = allometric relationship, “I” = isometric relationship (same below).)

Biologically, these statistical patterns reflect grade-specific allocation strategies. The allometric slope < 1 for root-stem in Grade I seedlings indicates a relative prioritization of stem biomass, likely supporting vertical structure in larger seedlings. In Grade III seedlings, the root-leaf slope > 1 suggests a proportionally greater investment in belowground resources. The conserved isometry in sprout-related pairs implies that sprout growth remains proportionally coupled to main organs across all sizes, possibly serving a modular functional role.

Allometric growth relationships among organs and individual seedlings of P. yunnanensis of different growth grades

Significant or highly significant allometric or isometric growth relationships exist between various components and individual trees across different growth grade of P. yunnanensis seedlings. Overall, significant differences (P < 0.05) were observed between roots, stems, aboveground biomass, and individual tree biomass, while no significant differences were found between leaves, sprout biomass, and individual tree biomass (Fig. 6). Roots and individual tree biomass exhibited allometric growth relationships with a common slope of 0.567 that was significantly less than 1 across all grades (Fig. 6a). Stems and individual tree biomass showed isometric growth relationships with a common slope of 0.981 that was not significantly different from 1 (Fig. 6b). Leaf and sprout biomass exhibited allometric growth relationships with individual plant biomass across all three grades (slopes significantly different from 1), with no common slope (Fig. 6c, d). Above-ground biomass and individual plant biomass showed an allometric relationship (slope significantly different from 1) for Grade I seedlings, whereas Grade II and III seedlings exhibited isometric relationship (slopes not significantly different from 1). This indicates that Grade I seedlings possessed a higher growth rate relative to Grade II and III seedlings (Fig. 6e).

Fig. 6.

Allometric growth relationships between roots (a), stems (b), leaves (c), sprouts (d), aboveground parts (e), and individual plants in P. yunnanensis seedlings of different grades

Correlation between different grades and growth parameters

Most indicators among different grades of P. yunnanensis seedlings exhibit positive correlations, with the majority being strong correlations. In Grade II seedlings, positive correlations between seedling height-sprout biomass and leaf biomass-sprout biomass are relatively weak. In contrast, Grade I and Grade III seedlings show negative correlations between these indicators, with Grade III seedlings exhibiting a significantly negative correlation (Fig. 7).

Fig. 7.

Correlation between biomass accumulation and its components in P. yunnanensis under different growth grade, The color gradient represents Pearson’s correlation coefficient, ranging from − 1 (blue) to + 1 (red) (Note: * p < 0.05)

Discussion

To better adapt to environmental changes, plants have evolved multiple regulatory strategies during their growth, with biomass investment and allocation being the most common approach [23]. Analysis of biomass investment and allocation across different growth grades of P. yunnanensis seedlings reveals that all seedling biomass allocation ratios show above-ground biomass significantly exceeding root biomass. This pattern aligns with findings by Guangpeng Tang [22] and reflects a general prioritization of aerial growth in young trees under non-limiting soil conditions. Differences in biomass allocation ratios among organs within different growth grade were not significant, following the pattern: leaf biomass > root biomass > stem biomass > sprout biomass [6, 24]. This indicates that P. yunnanensis seedlings invest more biomass in leaves during the early growth stage, a pattern that may enhance light capture efficiency and support growth through photosynthesis [25]. Root biomass proportion remained relatively stable across grades, suggesting a consistent allocation toward belowground resources that likely supports water and nutrient uptake even under varying size and biomass accumulation [26, 27]. The biomass proportion of sprouts varied notably among grades, following the order III > I > II. This variation may reflect differences in biomass allocation strategies between Grade I and III seedlings, which represent contrasting size extremes. The higher sprout biomass in Grade III seedlings could be associated with their smaller stature, possibly relating to a morphological response to light availability, while the intermediate Grade II seedlings displayed a more stable allocation profile [28, 29].

Allometric relationships are ubiquitous in biology, serving to examine biomass allocation in relation to individual size [30]. This allows allometric allocation theory to effectively explain how biomass distribution is influenced by both species and individual size, holding profound implications for the utilization, growth, development, and reproduction of biological resources [22, 31, 32]. The allometric relationships between roots and stems, as well as roots and aboveground parts, across different seedling grades exhibit a common slope, indicating that growth trajectories remain unchanged [33]. This consistent scaling may reflect coordinated development between root and shoot systems, which is fundamental to maintaining functional integration during early growth [34]. The isometric growth relationship observed between roots and sprouts, stems and sprouts, and leaves and sprouts indicates that sprout growth exhibits weak correlation with other organs. This likely relates to the plant’s redundant growth strategy for expanding growth space, serving an “auxiliary” function in overall plant development [35].

Regarding the association between structural components and individual biomass, the isometric growth of stems relative to individual biomass indicates that stems serve as the primary vehicle for biomass accumulation, with their growth directly reflecting the overall growth status of seedlings [33, 36]. Conversely, the allometric growth of roots relative to individual biomass reveals that root growth lags behind overall individual growth, consistent with the strategy of prioritizing above-ground development [33, 37]. Concerning the relationship between aboveground biomass and individual plants, the allometric growth of Grade I seedlings indicates that higher-grade seedlings exhibit greater growth efficiency in their aboveground parts, accumulating biomass at a faster rate. This also represents a key manifestation of their grade advantage [38].

Correlation network heatmap analysis further revealed distinct differences in growth performance and resource allocation patterns among different seedling grades [39]. Most indicators across grades exhibited significant positive correlations, while seedling height and sprout length showed weaker or negative correlations with other indicators. Grade III seedlings exhibited negative values in sprout biomass distribution, differing from Grades I and II seedlings. This discrepancy may stem from environmental influences or developmental Grade variations affecting growth status or resource allocation [40]. These variations indicate that seedling grade significantly influences growth indicators. Seedlings of different grades may undergo adaptive adjustments in growth strategies and biomass allocation [41]. In this study, using seedling height as the sole criterion for grading may not fully reflect seedling quality. We recommend that future research adopt a multi-trait approach to enable a more comprehensive grading system, which would contribute to a better understanding of seedling growth patterns and ecological adaptation. Further investigation into the specific environmental or physiological mechanisms underlying these differences is also warranted.

Conclusions

In this study, biomass accumulation dynamics, resource allocation characteristics, and allometric relationships were systematically analyzed for two-year-old P. yunnanensis seedlings of varying grades. The findings indicate that seedling grade significantly influences total biomass accumulation and allocation. Biomass decreases with lower seedling grades, with above-ground biomass consistently exceeding below-ground biomass and maintaining a relatively stable allocation ratio. The biomass hierarchy across organs follows the order: leaves > roots > stems > sprouts. Allometric and isometric relationships among organs, as well as between organs and whole-plant size, varied with seedling grade, reflecting differences in relative growth rates and suggesting size-mediated trade-offs in resource allocation. These findings enhance our understanding of how P. yunnanensis seedlings adjust biomass allocation and growth trajectories in relation to size class under nursery conditions. The observed patterns may inform seedling grading and early cultivation practices, though further field validation is needed to assess their applicability in diverse afforestation settings.

Authors’ contributions

Na Li: Writing – original draft, investigation, review & editing, Conceptualization. Xiaoyue Zhang: Investigation, Formal analysis, Data curation. Nianhui Cai: Investigation, review & editing. Yulan Xu: Writing – review & editing, supervision, Funding acquisition, Conceptualization.

Funding

This research was supported by Yunnan Fundamental Research Projects (grant NO. 202401BD070001-023), the Project of Science and Technology Talent and Platform Program of YunnanProvincial Department of Science and Technology (202605AF350003), the Yunnan Provincial First-Class Discipline Construction Fund for Forestry at Southwest Forestry University (LXXK-2025D01), Yunnan Graduate Tutor Team Building Project (2024-61).

Data availability

The data are available from the corresponding author.

Declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.