Regulatory Effects of Mowing on Biomass Allocation and Compensation Growth Mechanisms in Elymus Species

Abstract

Mowing is a crucial grassland management practice; however, its effects on biomass allocation and compensatory mechanisms across different growth stages remain insufficiently understood. This study investigated five Elymus forage species (Elymus nutans ‘Aba’, Elymus sibiricus ‘Qingmu No.1’, Elymus submuticus ’Tongde’, Elymus breviaristatus ‘Tongde’, and Elymus sibiricus ‘Tongde’). Four mowing intensities (control, light, moderate, and heavy) were applied at three phenological stages (jointing, booting, and flowering). Biomass allocation patterns among plant components (roots, stems, leaves, and spikes) were assessed, and allometric growth relationships were analyzed. Structural equation modeling (SEM) was used to evaluate the contributions of mowing timing and organ biomass to overall compensatory ability. The results showed that mowing significantly altered biomass allocation patterns, characterized by an increase in root-biomass proportion, a decrease in stem and spike proportions, and species- and stage-specific changes in leaf proportion. The allometric growth relationships between plant organs varied across growth stages and were significantly influenced by mowing intensity, affecting organ growth coordination. SEM analysis revealed that mowing timing and root biomass were the primary drivers of total biomass compensation, with root biomass playing a particularly critical role under moderate to heavy mowing. Mowing exerts complex regulatory effects on biomass allocation and compensatory growth in Elymus species, with impacts varying by intensity, growth stage, and species. To enhance overcompensatory growth, moderate mowing at the jointing stage is recommended, while heavy mowing during the flowering stage should be avoided. Furthermore, maintaining root health is crucial for improving compensatory growth capacity. These findings provide valuable insights for the sustainable management of Elymus grasslands.

1. Introduction

Grasslands represent one of the largest terrestrial ecosystems on Earth, playing an irreplaceable role in supporting biodiversity, acting as carbon sinks, and sustaining livestock production [1]. Mowing, one of the most commonly employed grassland management practices, plays a crucial role in regulating plant growth, enhancing forage quality, and maintaining ecosystem stability. In recent years, researchers have paid increasing attention to the role of mowing in regulating biomass allocation and plant compensatory growth mechanisms. Mowing not only has a profound impact on grassland productivity but also determines the ecosystem’s capacity to recover from external disturbances [2,3,4,5]. Understanding the response mechanisms of species with perennial life cycles and clonal reproduction strategies to mowing is crucial for enhancing grassland resilience and productivity under management pressure [6,7,8].

Mowing intensity and timing are key factors influencing plant compensatory ability. Moderate mowing can enhance plant recovery and growth by optimizing resource allocation. However, excessive mowing intensity may lead to the rapid depletion of carbohydrate reserves, disrupt resource-allocation patterns, weaken plant resilience, and significantly reduce productivity [9,10]. Additionally, the timing of mowing is another critical factor influencing plant compensatory ability. At different phenological stages, plants exhibit significant shifts in resource-allocation priorities between vegetative and reproductive organs [11,12,13,14]. Although previous studies have revealed certain patterns of how mowing affects plant growth and resource allocation, research on stage-specific mowing responses remains insufficient, particularly in clonal grasses such as Elymus species. These species exhibit unique adaptive strategies in response to environmental disturbances; however, their compensatory growth mechanisms and resource-allocation strategies remain underexplored. Therefore, further investigation in this area is crucial for improving the management and sustainable utilization of grassland ecosystems.

Biomass allocation refers to the distribution of resources among roots, stems, leaves, and reproductive structures, playing a crucial role in plant growth and recovery. Plants enhance their survival under adverse environmental conditions through the coordinated regulation of biomass allocation, morphological plasticity, and a series of physiological processes [15,16,17]. In response to disturbances such as mowing, plants often exhibit compensatory responses by adjusting biomass allocation strategies to adapt to environmental changes. For instance, moderate mowing can effectively stimulate resource allocation to belowground structures, promoting root growth and nutrient uptake. This process not only enhances root health but also supports shoot regeneration and tiller formation [18,19,20,21]. Compensatory growth is a crucial mechanism by which plants recover rapidly from tissue damage caused by mowing or grazing to compensate for biomass loss [22]. This mechanism plays a key role in maintaining ecosystem function under disturbance conditions [23].

Compensatory growth responses of plants to the removal of different organs may vary significantly. Previous studies have shown that perennial grasses exhibit significant compensatory growth after moderate mowing, primarily achieved through the redistribution of belowground carbohydrate reserves [24]. Another study found that moderate mowing promotes new leaf growth and alters the carbon–nitrogen allocation pattern in the root–shoot system, reflecting the ability of forage grasses to adjust nutrient distribution strategies in response to mowing disturbances [25]. However, the specific mechanisms underlying compensatory biomass growth in different plant organs in response to mowing remain poorly understood and require further investigation. Exploring the effects of mowing at different growth stages on biomass allocation and the compensatory growth of plant organs and uncovering the driving mechanisms of such responses are crucial for understanding grassland ecosystem responses to climate change and for optimizing management strategies. However, studies on the biomass allocation of the Elymus grasslands in the Qinghai–Tibet Plateau and their dynamic responses to mowing management remain scarce. To address this research gap, we selected key Elymus forage species on the Qinghai–Tibet Plateau, including Elymus nutans ‘Aba’ (ENUAB), Elymus sibiricus ‘Qingmu No.1’ (ESIQN), Elymus submuticus ‘Tongde’ (ESUTD), Elymus breviaristatus ‘Tongde’ (EBRTD), and Elymus sibiricus ‘Tongde’ (ESITD). From the perspective of mowing management, four mowing treatments (no mowing, light mowing, moderate mowing, and heavy mowing) were applied at three growth stages (jointing, booting, and flowering). This study aimed to systematically examine the effects of mowing at different growth stages on biomass allocation and compensatory growth in plant components (roots, stems, leaves, and spikes) and propose the following hypotheses: (1) How do changes in the biomass of different plant organs (roots, stems, leaves, and spikes) exhibit specific responses to mowing treatments at different growth stages? (2) Will mowing treatments at different growth stages have more pronounced effects on tile biomass compensation, and will these effects be consistent across mowing intensities? The findings will provide critical theoretical and practical insights into the dynamic response mechanisms of Elymus grassland ecosystems on the Qinghai–Tibetan Plateau, contributing to the development of scientifically sound mowing management strategies.

2. Materials and Methods

2.1. Study Area

The experiment was conducted at Bakatai Farm in Qinghai Province (longitude 100°55′ E, latitude 36°17′ N), with an average altitude of 3300 m. The region is characterized by a distinct climate, featuring long, cold winters and short, cool summers, which are typical of a plateau continental climate. Precipitation is unevenly distributed throughout the year, with most occurring between July and October. The region has an annual average rainfall of approximately 300 mm, while evaporation ranges between 2000 and 2400 mm per year, with an average annual temperature of 4.1 °C. Additionally, the soil types in this region are primarily alpine meadow soil and loessial soil [26].

From 2022 to 2023, this study selected five Elymus forage species cultivated for four years as research subjects: ENUAB, ESIQN, ESUTD, EBRTD, and ESITD. To ensure experimental randomness, a randomized block design was adopted, with each species subjected to mowing treatment at the jointing, booting, and flowering stages. In the selected plots, five species of Elymus forage, each with a size of approximately 10 cm, were randomly chosen for the experimental study conducted during 2022–2023, which included four mowing treatments and three mowing stages. Based on previous studies [27], four mowing treatments were applied at each stage: no mowing (CK), light mowing (LM, stubble height 15 cm), moderate mowing (MM, stubble height 10 cm), and heavy mowing (HM, stubble height 5 cm). In 2022, mowing treatments for the five Elymus species were conducted at the booting stage (20 June, 15 June, 16 June, 17 June, and 17 June) and at the flowering stage (21 July, 12 July, 13 July, 15 July, and 16 July). In 2023, treatments were applied at the jointing stage (5 June, 9 June, 10 June, 11 June, and 10 June), booting stage (17 June, 21 June, 23 June, 24 June, and 22 June), and flowering stage (19 July, 16 July, 15 July, 16 July, and 14 July). Each treatment was replicated three times, resulting in a total of 12 plots, with each plot covering an area of 4 × 4 m. Isolation strips of approximately 1 m were established. Mowing treatments were performed at the jointing, booting, and flowering stages. Within each plot, two 1 × 1 m quadrats were randomly selected, marked, and recorded. Seven weeks after the mowing treatments, at each growth stage, five clumps of each of the five Elymus species were sampled from each plot, and the average measurements from five plants per quadrat were used as one experimental replicate. Each species had six replicates for all the treatments.

2.2. Sampling and Measurement of Indicators

Based on the mowing treatments conducted at different growth stages of five Elymus species during 2022–2023, we did not collect the removed biomass after mowing. Biomass sampling was conducted seven weeks after mowing treatments applied at the jointing, booting, and flowering stages. In the designated quadrats for mowing treatments, whole-plant excavation was used for sample collection. The specific sampling dates were as follows: For the booting-stage treatments in 2022, the sampling dates for ENUAB, ESIQN, ESUTD, EBRTD, and ESITD were 8 August, 3 August, 4 August, 5 August, and 5 August. For the flowering-stage treatments, the sampling dates were 8 September, 30 August, 31 August, 2 September, and 3 September, respectively, for the same five species. In 2023, ENUAB, ESIQN, ESUTD, EBRTD, and ESITD occurred at the jointing (24 July, 28 July, 29 July, 30 July, and 29 July), booting (5 August, 9 August, 10 August, 11 August, and 9 August), and flowering (6 September, 4 September, 3 September, 4 September, and 2 September) stages. The collected plant samples included five Elymus species, which were separated into aboveground and belowground parts, bagged, and carefully handled to maintain the root-system integrity. Subsequently, the aboveground parts were further divided into spikes, stems, and leaves to facilitate the biomass measurement of each component. Finally, each component was placed in an envelope and dried at 65 °C to a constant weight for dry biomass determination.

2.3. Statistical Analysis

The total biomass of the five Elymus species in 2022–2023 was the biomass accumulated over seven weeks following mowing at three different growth stages. The total biomass of an individual Elymus forage was calculated as the sum of the biomass of its components (roots, stems, leaves, and spikes).

Biomass proportion of each component (roots, stems, leaves, and spikes) = biomass of each component/total individual biomass × 100%

We calculated the total biomass and biomass compensation index of each component (roots, stems, leaves, and spikes) for the five Elymus species after seven weeks of regrowth following mowing at three different growth stages in 2022–2023. The compensation index for the total biomass and individual components (roots, stems, leaves, and spikes) of the five Elymus species was calculated as follows:

$${\mathrm{CI}}_{\mathrm{i}}={\displaystyle \frac{{\mathrm{N}}_{\mathrm{it}}}{{\mathrm{N}}_{\mathrm{ic}}}}$$

CI_i represents the compensation index for the total biomass and individual components (including roots, stems, leaves, and spikes) of the five Elymus species. The subscript i refers to the total biomass and the biomass of each component (roots, stems, leaves, and spikes) of the Elymus individuals harvested seven weeks after mowing. N_it and N_ic represent the identifiers for the total biomass and each component (roots, stems, leaves, and spikes) of the five Elymus species individuals under mowing and non-mowing treatments, respectively, seven weeks after treatment at different growth stages. CI > 1 indicates overcompensation (OCG). At CI = 1, the compensation is balanced. CI < 1 indicates undercompensation.

Before data analysis, all the data were transformed exponentially to meet the requirements of a normal distribution. A linear mixed-effects model was first used to analyze the effects of mowing treatments and stages on the biomass of five Elymus components (roots, stems, leaves, and spikes), total biomass, biomass allocation, total biomass compensation index, and component biomass compensation index. Mowing treatments, mowing stages, and the five Elymus species and their interactions were set as fixed factors, whereas years and blocks were treated as random factors. Subsequently, mixed-effects models were applied to examine the effects of mowing treatments, Elymus species, and their interactions at the jointing, booting, and flowering stages on these indices. The “nlme” package was used to fit the linear mixed-effects models, and Tukey’s HSD test was applied at the 5% level for multiple comparisons. One-way ANOVA was used to compare the total biomass compensation and the biomass compensation of individual components (roots, stems, leaves, and spikes) between mowing treatments at each stage. A one-sample t-test was used to analyze the differences between the CI and the 1. The standardized major axis (SMA) method was used to investigate the biomass allocation characteristics of the plant components (roots, stems, leaves, and spikes) under mowing treatments at different growth stages. Additionally, this study analyzed how mowing treatments affected the biomass allocation strategies of these components. To achieve this, the allometric growth equation Y = b·Xa was used to test the biomass allocation strategies among the different components under various mowing treatments. In this equation, Y represents the biomass of a specific plant organ, X represents the biomass of another organ, and a and b are allometric growth coefficients. Furthermore, the equation was log-transformed into a linear form, ln(Y) = ln(c) + a·ln(X) [28], to facilitate the analysis and comparison. A Mantel test was used to study the relationships between the total biomass compensation and component biomass under mowing treatments across different growth stages. Finally, structural equation modeling (SEM) was applied to further explore the driving mechanisms of the mowing stages and component biomass on total biomass compensation under various treatments. To optimize the SEM, Fisher’s C statistic (p < 0.05) and Akaike’s information criterion (AICc) were used to evaluate the fit of the global model. All data analyses were conducted using R 4.0.3 (R Development Core Team, 2020). SMA, Mantel, and SEM analyses were performed using the R packages “smart” [29], “linkET”, and “piecewiseSEM” [30].

3. Results

3.1. Effects of Mowing on the Individual Biomass and Component Allocation of Elymus Species

Mowing treatments had a significant impact on the total biomass of Elymus forage species and the biomass of its individual organs (roots, stems, leaves, and spikes). Moreover, the effects of mowing stages and forage varieties exhibited significant differences for these indicators (p < 0.05, Table S1, Figure 1). Regarding total biomass, mowing treatments significantly affected total biomass at different growth stages (p < 0.05). Moderate mowing at the jointing stage significantly increased total biomass (p < 0.05), with ESIQN performing the best. Light mowing at the booting stage promoted an increase in total biomass (p < 0.05), although differences among varieties were not significant (p > 0.05). At the flowering stage, total biomass decreased significantly as mowing intensity increased (p < 0.05). Under the no-mowing treatment, ESUTD had the highest total biomass, while under heavy mowing, ESITD had the lowest total biomass. For root biomass, mowing treatments significantly increased root biomass at the jointing and booting stages (p < 0.05), with variations among different forage varieties under different mowing treatments. Under moderate mowing at the jointing and booting stages, ESIQN exhibited the highest root biomass. At the flowering stage, ESIQN also had the highest root biomass under light mowing. However, under moderate mowing at the flowering stage, there were no significant differences in root biomass among the different varieties. In terms of stem biomass, at all three growth stages, stem biomass decreased as mowing intensity increased (p < 0.05). Under moderate mowing at the jointing stage, ESIQN had the highest stem biomass. At the booting stage, ESUTD exhibited the highest stem biomass under light mowing. During the flowering stage, ESUTD had the highest stem biomass under the no-mowing treatment. For leaf biomass, mowing treatments significantly increased leaf biomass at the jointing and booting stages while reducing it at the flowering stage (p < 0.05). Moderate mowing at the jointing stage significantly increased leaf biomass (p < 0.05), with ESIQN performing the best. Light mowing at the booting stage promoted an increase in leaf biomass, though differences among varieties were not significant. At the flowering stage, leaf biomass decreased with increasing mowing intensity, with ESIQN exhibiting the highest leaf biomass under the no-mowing treatment. Regarding spike biomass, mowing treatments significantly reduced spike biomass at all three growth stages (p < 0.05). At the jointing stage, under the no-mowing treatment, ESIQN had the highest spike biomass. At the booting stage, there were no significant differences in spike biomass among the different varieties under various mowing treatments (p > 0.05). During the flowering stage, under the no-mowing treatment, ESUTD and EBRTD exhibited significantly higher spike biomass than other treatments (p < 0.05). Furthermore, under light, moderate, and heavy mowing treatments, there were no significant differences in spike biomass among the Elymus species varieties (p > 0.05).

3.2. Effects of Mowing on the Component Biomass Allocation in Elymus Species

Mowing treatments, mowing stages, and their interactions significantly affected the biomass allocation ratios of different organs in Elymus species (p < 0.05). Overall, at all three mowing stages, the root-biomass allocation ratio was the highest, followed by that of the leaves, stems, and spikes. The mowing treatments significantly increased the root-biomass allocation ratio and reduced the allocation ratios of stems and spikes during the jointing, booting, and flowering stages (p < 0.05, Table S2, Figure 2). At the jointing stage, mowing treatments had no significant effect on leaf-biomass allocation in ESUTD (p > 0.05) but was significantly reduced in the other species (ENUAB, ESIQN and EBRTD), except for ESITD (p < 0.05). At the booting stage, the mowing treatments had no significant effect on the leaf-biomass allocation ratios of ENUAB, ESUTD, and EBRTD (p > 0.05). However, light mowing significantly increased the leaf-biomass allocation ratio in ESIQN, whereas moderate mowing significantly increased it in ESITD (p < 0.05). At the flowering stage, the mowing treatments had no significant effect on the leaf-biomass allocation ratios in ENUAB, and ESITD (p > 0.05). However, moderate mowing significantly increased the leaf-biomass allocation in ESUTD and EBRTD significantly reduced it in ESIQN (p < 0.05). Overall, the results indicated that mowing treatments altered the biomass allocation patterns of Elymus species by increasing the root-biomass allocation ratio and reducing the stem and spike allocation ratios. At different mowing stages, the leaf-biomass allocation ratios of the Elymus species varied depending on the mowing treatments.

3.3. The Allometric Growth Relationships of Biomass Among Different Components of Elymus Species Under Mowing Treatments

The allometric relationships of biomass among the different components of the Elymus species showed significant differences under mowing treatments at different growth stages (Figure 3). At the jointing stage, in the absence of mowing treatment, the relationships between the leaf stem, leaf root, and stem root exhibited isometric growth. Under the light mowing treatment, all components except the leaf stem also exhibited isometric growth. Under moderate mowing treatment, leaf stem and leaf root showed isometric growth, whereas leaf spike, stem root, stem spike, and root spike exhibited allometric growth relationships. Under the heavy mowing treatment, most biomass components exhibited allometric growth relationships. At the booting stage, in all mowing treatments, significant allometric growth relationships were observed among leaf spikes, stem roots, and root spikes. Under the no mowing treatment, leaf stem and leaf spike exhibited isometric growth, while stem spikes exhibited isometric growth under light and moderate mowing treatments. However, under heavy mowing treatment, all components showed significant allometric growth relationships. At the flowering stage, the allometric growth indices of the leaf stem, leaf spike, root spike, and stem spike in Elymus species were all less than 1, indicating allometric growth relationships among these components. In contrast, the allometric growth indices for leaf roots and stem roots under all mowing treatments were greater than 1. Additionally, stem spikes exhibited isometric growth under both moderate and heavy mowing treatments.

3.4. Effects of Mowing on the Compensatory Growth of Biomass Components in Elymus Species

The total biomass and biomass compensation indices of all components of the Elymus species showed significant differences among the three growth stages (p < 0.05, Table S3, Figure 4). For total biomass compensation, moderate mowing treatment at the jointing stage significantly increased the total biomass compensation index (p < 0.05). In all mowing treatments, the total biomass compensation index was either equal or overcompensated. The total biomass compensation index at the booting and flowering stages decreased with increasing mowing intensity (p < 0.05). At the booting stage, moderate and heavy mowing treatments exhibited equal and overcompensation; however, under heavy mowing, ESIQN showed undercompensated growth. During the flowering stage, under all mowing treatments, ESUTD and EBRTD exhibited an undercompensated total biomass index. Mowing at the jointing stage significantly increased the root-biomass compensation index (p < 0.05), with all mowing treatments exhibiting overcompensated growth. Mowing at the booting stage had no significant effect on the root-biomass compensation index (p > 0.05), but generally showed equal or overcompensated growth. At the flowering stage, heavy mowing significantly reduced the root-biomass compensation index (p < 0.05); however, under different mowing intensities, it still exhibited equal or overcompensated growth was observed. The mowing treatments significantly reduced the stem-biomass compensation index at the jointing, booting, and flowering stages (p < 0.05). Under light and moderate mowing, ESIQN exhibited equal compensation growth. Under heavy mowing conditions, all varieties showed undercompensated growth, except for ENUAB at the jointing stage. Mowing at the jointing stage had no significant effect on the leaf-biomass compensation index (p > 0.05), with ESIQN and ESUTD exhibited equal compensation growth. Mowing during the booting and flowering stages significantly reduced the leaf-biomass compensation index. During the booting stage, heavy mowing caused ESITD exhibited undercompensated growth, whereas the other varieties exhibited equal compensation under light and moderate mowing. At the flowering stage, all varieties exhibited equal or overcompensated growth under light mowing. Under moderate mowing, some varieties exhibited equal compensation, while under heavy mowing, all varieties showed undercompensated growth. The mowing treatments significantly reduced the spike-biomass compensation index across the three growth stages (p < 0.05). At the jointing stage, under light and moderate mowing, spike biomass exhibited equal or undercompensated growth. At the booting and flowering stages, spike biomass exhibited undercompensated growth, with the exception of ENUAB, which exhibited equal compensation under the different light mowing treatment, and was mowed during the booting stage.

3.5. The Pathways Through Which Mowing Affects Compensatory Growth in Elymus Species

Across different growth stages, the contributions of various organs to the total biomass compensation in Elymus species were significantly different under mowing treatments (Figure 5). Under the light mowing treatment, total biomass compensation at the jointing stage was not significantly influenced by any organ’s biomass; at the booting stage, it was mainly driven by root and leaf biomass; and at the flowering stage, spike biomass had no significant impact on compensation (Figure 5a). Under the moderate mowing treatment, the total biomass compensation at the jointing stage was jointly influenced by root, stem, leaf, and spike biomass; at the booting stage, it was dominated by root and leaf biomass; and at the flowering stage, it was not affected by stem biomass (Figure 5b). Under the heavy mowing treatment, total biomass compensation at the jointing stage was not significantly affected by any organ; at the booting stage, it was mainly driven by root biomass; and at the flowering stage, it was jointly influenced by root, stem, and leaf biomass. Overall, spike biomass played a significant role in total biomass compensation across all growth stages under the moderate and heavy cutting treatments (Figure 5c).

Structural equation modeling (SEM) was used to analyze the direct and indirect effects of different mowing intensities on the total biomass compensation index of Elymus species (Figure 6). The results are as follows: The SEM model explained 38%, 59%, and 57% of the variation in total biomass compensation under light (LM), moderate (MM), and heavy (HM) mowing treatments, respectively. Stem and spike biomass showed a significant positive correlation across all mowing treatments. Under the light mowing treatment, the mowing stage had a direct negative effect on total biomass compensation (path coefficient = −0.23), whereas it indirectly produced a positive effect via root, stem, and leaf biomass. Stem biomass (path coefficient = 0.63) had the most significant positive effect on total biomass compensation, followed by root biomass (0.39) and leaf biomass (0.22). Under the moderate mowing treatment, the direct negative effect of the mowing stage weakened (−0.21), and total biomass compensation was indirectly influenced by root biomass (0.45) and leaf biomass (0.18), with significant overall contributions. Under the heavy mowing treatment, the direct negative effect of the mowing stage further increased (−0.25), whereas the positive effects of root biomass (0.50) and leaf biomass (0.27) became more significant. The total effects of total biomass compensation variation further indicated that root, stem, and leaf biomass had a significant positive effect on the total biomass compensation variation under all mowing treatments, whereas spike biomass had a significant positive effect under the moderate mowing treatment. Overall, the negative impact of the mowing stage on total biomass compensation intensified with increased mowing intensity, whereas the positive effects of root and leaf biomass became more pronounced under moderate and heavy mowing treatments.

4. Discussion

4.1. Effects of Mowing on the Biomass Accumulation in Various Plant Organs

The accumulation and allocation of plant biomass represent a phenotypic plasticity response to environmental changes, reflecting strategies for resource acquisition and utilization while also influencing the terrestrial ecosystem carbon cycle [31]. This study demonstrates that mowing treatments and their timing significantly affect the total biomass of Elymus species, exhibiting a pronounced interaction effect between growth stage and mowing intensity. At the jointing stage, moderate mowing significantly promoted total biomass accumulation, indicating a typical compensatory growth response. During the booting stage, light mowing had the most beneficial effect. However, at the flowering stage, plants were highly sensitive to mowing, and heavy mowing resulted in a significant reduction in total biomass. This pattern supports the “intermediate disturbance hypothesis” and aligns with previous studies [32,33], suggesting that moderate mowing can stimulate compensatory growth, whereas excessive mowing surpasses the plant’s tolerance threshold, leading to reduced biomass accumulation. The effects of mowing on the biomass of different plant components (roots, stems, leaves, and spikes) exhibited an organ-specific, growth stage- and intensity-dependent pattern. This study found that moderate mowing significantly increased root biomass at the jointing and flowering stages, with light mowing at the flowering stage showing the most pronounced effect. This response reflects a strategic adjustment by plants to enhance water and nutrient uptake capacity [32,34,35]. However, heavy mowing resulted in a decrease in root biomass, suggesting that resources were preferentially allocated to the recovery of aboveground tissues [36,37,38]. Additionally, stem biomass generally declined after mowing, while leaf biomass exhibited compensatory growth under moderate mowing, particularly under moderate mowing at the jointing stage and light mowing at the booting stage. This finding reflects a plant strategy of prioritizing the recovery of photosynthetic organs [39,40,41] to reestablish energy acquisition capacity. Spikes, as reproductive organs, were the most sensitive to mowing. Mowing suppressed spike-biomass accumulation across all three growth stages, with the most significant inhibition occurring under heavy mowing. This suggests that under resource-limited conditions, plants prioritize vegetative growth over reproductive investment. Different Elymus species exhibited significant variations in their responses to mowing, highlighting the importance of genetic background. For example, ESIQN showed the strongest stem-biomass recovery under moderate mowing at the jointing stage, whereas ESITD performed the worst under heavy mowing. Similarly, ESUTD and EBRTD showed significantly higher spike biomass under no-mowing conditions than other species, suggesting that specific genotypes may have unique resource-allocation strategies. For the sustainable management of Elymus grasslands, mowing intensity should be optimized according to plant-growth stages. Moderate mowing is suitable at the jointing stage, light mowing is preferable during the booting stage, and heavy mowing should be avoided at the flowering stage. Additionally, species selection is a critical factor influencing grassland productivity and sustainability.

4.2. Effects of Mowing on the Biomass Allocation Among Plant Organs

Under resource limitations, plants must trade off biomass allocation among different organs [42,43]. This allocation strategy reflects an adaptive decision to optimize resource utilization, directly influencing plant growth and reproductive success under changing environmental conditions. Mowing disrupts apical dominance and reduces photosynthetic tissues, thereby inducing compensatory growth mechanisms. Light and moderate mowing promote leaf regeneration, as indicated by a significant increase in leaf-biomass proportion, reflecting a plant strategy that prioritizes the restoration of photosynthetic function [41,44,45,46]. In contrast, the stem-biomass proportion declines, weakening its support and transport functions. As a reproductive organ, spike recovery lags behind vegetative organs, particularly under heavy mowing, suggesting that mowing primarily affects reproductive investment [36]. Belowground organs play a crucial role in resource storage and compensatory growth. Moderate mowing increases root-biomass proportion, enhancing resource absorption and storage capacity, thereby providing a foundation for aboveground regeneration [47,48,49]. However, excessive mowing leads to the over-mobilization of root resources, impairing belowground organ functions. Mowing treatments, along with their interactions with growth stage and genotype, significantly affect the stem-to-leaf ratio and root-to-shoot ratio in Elymus species. Moderate mowing facilitates the coordinated growth of aboveground and belowground organs, reflected in optimized root-to-shoot ratio adjustments. In contrast, heavy mowing disrupts this balance, constraining overall plant-growth potential and resource-use efficiency [50,51]. Mowing intensity and mowing stage jointly regulate biomass allocation patterns. Moderate mowing promotes photosynthetic recovery while mobilizing root resources, facilitating coordinated organ growth. Different genotypes exhibit significant variation in mowing responses. However, across all growth stages, root-biomass proportion remains the highest regardless of mowing stage, indicating that plants tend to prioritize belowground investment in response to disturbance. The observed genotypic differences suggest that Elymus species have evolved diverse adaptive strategies. The allometric relationships among organs in Elymus species vary significantly across growth stages and mowing intensities. At the jointing stage, organs exhibit isometric growth under no-mowing conditions, maintaining balanced resource allocation. With increasing mowing intensity, an allometric relationship emerges between roots and spikes, favoring the recovery of functional organs. At the booting stage, stems and spikes maintain isometric growth under light and moderate mowing, whereas heavy mowing enhances allometric differences among vegetative organs, prioritizing resource allocation to photosynthetic organs. At the flowering stage, mowing disrupts resource-allocation balance, favoring reproductive organs. These findings suggest that light mowing facilitates functional recovery, moderate mowing balances functional and reproductive demands, whereas heavy mowing results in severe resource-allocation bias. The inconsistency between biomass accumulation rates and allocation changes among different plant components may stem from size constraints, representing a form of phenotypic plasticity [52,53].

4.3. Mowing Timing and Organ Biomass Jointly Drive the Total Biomass Compensation in Elymus Species

Mowing stage and intensity significantly affect the compensatory capacity of total biomass in Elymus forage grasses. This effect is mediated by trade-offs in resource allocation among different organs to achieve adaptive regulation in response to environmental conditions. The herbivory optimization hypothesis (HOH), the growth-rate model, the continuous compensation model, and the limited-resource model all suggest that environmental factors may either promote or inhibit compensatory growth in plants following mowing [54,55]. This study confirms that mowing at different growth stages significantly influences the overall and organ-specific compensation indices in Elymus species. The response varies depending on mowing intensity, organ type, and plant cultivar. The phenological stage at which mowing occurs largely determines the degree of biomass recovery. During the jointing stage (early growth phase), plants exhibit high compensatory potential due to the substantial accumulation of carbohydrates in the root system and storage organs. These stored resources can be rapidly mobilized to support aboveground tissue regeneration, facilitating compensatory growth. In contrast, mowing at the flowering stage (late growth phase) results in a significant decline in compensatory potential. This is because, at this stage, resource allocation shifts from vegetative growth to reproductive development, depleting stored reserves. Additionally, plants prioritize reproductive organ maintenance over the recovery of aboveground tissues. Different organs exhibit distinct compensatory responses to mowing, reflecting their specific roles in growth and resource allocation. Specifically, moderate mowing during the early growth stage (jointing stage) often promotes equal or overcompensatory growth, whereas severe mowing during the reproductive stage (flowering stage) may lead to prolonged growth suppression. During the booting and flowering stages, total biomass compensation mainly depends on contributions from roots and leaves, whereas spike compensation is relatively weak. This highlights the critical role of developmental stage in resource allocation. The contribution of different organs to total biomass compensation varies depending on mowing stage and intensity. Under light mowing, total biomass compensation during the jointing stage is minimally affected by specific organs, resulting in relatively balanced resource allocation. In contrast, compensation during the booting and flowering stages is primarily driven by root and leaf biomass. Under moderate mowing, compensation at the jointing stage is influenced by the combined effects of all organ biomasses, while root and leaf contributions remain dominant during the booting and flowering stages. Under severe mowing, root biomass plays an increasingly prominent role in total biomass compensation, particularly during the booting stage, where it becomes the primary driver. This shift in pattern suggests that as mowing intensity increases, plant resource-allocation strategies gradually transition from multi-organ coordination to preferential investment in specific organs. Notably, during the jointing stage, the root-biomass compensation index increases significantly, and all mowing treatments result in overcompensatory growth. This underscores the critical role of the root system in aboveground tissue recovery at this stage.

Biomass compensation in Elymus forage species is a critical ecological process for coping with mechanical damage, and the timing of mowing directly affects their compensatory growth. The variation in compensatory capacity across different growth stages is closely related to the physiological status and resource-accumulation patterns of Elymus forage species. Early-stage mowing (jointing stage) significantly promotes leaf and stem recovery due to the rapid growth phase of aboveground tissues, exhibiting strong compensatory capacity [56]. In contrast, late-stage mowing (flowering stage) leads to a marked decline in recovery capacity due to resource allocation shifting toward reproductive organs [57]. Mowing stages also regulate total biomass compensation by affecting carbon-assimilation efficiency and resource redistribution. Under early-stage mowing, residual stems and leaves enhance carbon-assimilation efficiency, providing sufficient energy for regeneration [44,58,59]. In contrast, late-stage mowing limits compensatory capacity due to reduced leaf area and diminished carbon-assimilation ability [57]. This indicates that mowing stages primarily influence total biomass compensation capacity by regulating the carbon strategy of Elymus forage species. Studies have shown significant functional differences among organs in their contributions to total biomass compensation. The root system plays a crucial role in resource storage and redistribution, particularly under moderate and severe mowing conditions, where the mobilization of carbon reserves supports aboveground recovery [60]. Its recovery and redistribution efficiency directly determine the plant’s compensatory capacity. The biomass compensation mechanism in Elymus forage species reflects their long-term adaptation strategy to environmental stress. Therefore, future research on optimal mowing management for Elymus forage species should integrate the interaction between growth stage and mowing intensity. For instance, moderate mowing at the jointing stage can promote overcompensatory growth, whereas severe mowing at the flowering stage should be avoided to reduce the risk of undercompensation. Additionally, the importance of root biomass should be emphasized. Future research on Elymus forage species should focus more on underground health to enhance overall biomass performance.

5. Conclusions

This study investigated the effects of different mowing treatments, mowing stages, and Elymus species varieties on the biomass of various plant components (roots, stems, leaves, and spikes) and revealed the regulatory effects of mowing on biomass allocation patterns and compensatory growth mechanisms. Mowing treatments significantly altered the biomass allocation patterns, with an increase in the proportion of root-biomass allocation, a decrease in the proportions of stem- and spike-biomass allocation, and a species-specific pattern in leaf-biomass allocation that varied with the mowing stage. Additionally, under different mowing intensities, the allometric growth relationships of component biomasses showed significant differences across growth stages, with increasing mowing intensity significantly affecting growth coordination among organs and resource-allocation strategies. Structural equation modeling (SEM) revealed that mowing stage and root biomass were the main factors driving total biomass compensation in Elymus species, with root biomass playing a particularly important role under moderate and heavy mowing treatments. The relative contributions of the component biomass to compensatory growth in Elymus species varied under different mowing intensities, with stem and leaf biomass showing higher contributions under light and heavy mowing treatments. In summary, this study revealed the complex regulatory effects of mowing disturbance on biomass allocation and compensatory growth mechanisms in Elymus species. It not only deepens the understanding of plant growth and resource-allocation mechanisms under mowing disturbances but also provides scientific evidence and theoretical support for optimizing grassland management strategies and understanding plant adaptation mechanisms to external disturbances.