In natural communities, resources (i.e., light, water and nutrients) usually show a heterogeneous distribution both spatially and temporally (Hutchings and De Kroon, 1994; Lajtha, 1995). The heterogeneous distribution of resources increases the difficulty for organisms, especially plants, to obtain effective resources (Hutchings and De Kroon, 1994), which has led to the evolution of diverse and effective ecological strategies throughout their long-term evolutionary process (Wang et al., 2011). Clonal integration and morphological plasticity of clonal plants are well known as strategies to adapt heterogeneous environments in a certain time period (De Kroon and Hutchings, 1995; Roiloa and Retuerto, 2006; Liu et al., 2018). Previous studies have shown that connections among ramets of clonal plants by rhizomes or stolons allow the transport of photosynthates, water, and nutrients from established to developing ramets (Alpert, 1999; Gao et al., 2008). Clonal plants exhibit a particularly foraging behavior of rhizome or stolon as a response to resource heterogeneity, such as clones would reduce the length of rhizome (stolon) spacers or increase branching in favorable environments, and place more ramets selectively in nutrient-rich habitats to promote access to heterogeneous resources (Hutchings and De Kroon, 1994; De Kroon and Hutchings, 1995). However, there is still a lack of experimental evidence on the mechanism of clonal plant spatial expansion under heterogeneous environments or interactive stresses. A number of studies have focused on changes in the above-ground parts of clonal plants in different habitats, while few have considered with changes in belowground root system. The clonal expansion of plants is a complex and important process that not only affects their performance, but also determines their mode of access to environmental resources (De Kroon and Hutchings, 1995; Alpert, 1999). The lateral rhizome and rhizome buds of clonal plants grow to form the above-ground ramets, and then the belowground rhizome buds of these ramets continue to repeat this process to achieve clonal expansion. In addition, this expansion process of clonal plants is also affected by both abiotic and biological factors (such as competition, herbivory, mycorrhiza, and environmental pressure) (Gao et al., 2012). Therefore, it can lead to the development of different plant adaptive strategies (Gao et al., 2008; Schiffers et al., 2011). For example, clonal plants reduce the size of the canopy and distribute more biomass to their belowground organs under severe grazing conditions (Wang et al., 2019). However, because of the complexity of root growth in natural grassland plants, belowground clonal growth remains poorly understood.

Environmental heterogeneity is diverse and multifaceted. Saline-alkali heterogeneity is one of the most important soil chemical properties affecting plant growth. Soil salinization has become an increasingly serious global environmental problem because of the large-scale degradation of grasslands (Munns and Tester, 2008; Yang et al., 2011). Salinity is always heterogeneously distributed in local habitats, especially on degraded grasslands. Because of different forms, proportions, and quantities of salt in the soil, soil alkalization often occurs along with salinization, which leads to the complexity of soil salinity heterogeneity (Wallender and Tanji, 2011). Moreover, with climate, topography, grazing, and other factors, soil salinization and patch distribution are intensified (Shi and Sheng, 2005). Researchers have found that saline-alkali stress can slow down plant growth rate and reduce the individual’s size, leaf area, root activity, and induce leaf chlorosis (i.e., yellowing of the leaves), resulting in a decrease in biomass accumulation and various other physiological processes (Wei et al., 2008; Zhao et al., 2008). However, clonal plants have a higher endurance than non-clonal plants in adverse habitats (such as salt, low nutrition, drought, etc.) (Zhao et al., 2008; Gao et al., 2012). A typical grassland clone plant, Leymus chinensis, has a high tolerance to saline-alkali environments, and can distribute its rhizomes or clonal ramets non-randomly in heterogeneous soil types (Ba et al., 2006; Li et al., 2014). Previous studies have shown that the root system of L. chinensis can have a “guerilla” distribution by increasing the rhizome branching angle and spacer length, which is helpful in avoiding the stress damage of saline-alkali soil patches (Shen and Ma, 2013). The heterogeneous distribution of salt and alkali cations in the soil promotes the formation of diversified adaptive strategies in saline-alkali tolerant plants, which is an important driving force for expansion of its population (Zhao et al., 2008; Türkana and Demiral, 2009).

In natural grasslands, plant communities are dominated by perennial herbaceous species. Grazing and mowing play an important role in maintaining grassland ecological health and production stability (Wang et al., 2004; Diaz et al., 2007; Bi et al., 2018). It is generally believed that grazing and mowing affect the plants directly by defoliation and indirectly by modifying their growth allocation and resource availability (Gao et al., 2008; Liu et al., 2015). Plant compensatory growth is an important adaptive response to herbivory, which may alleviate the potential deleterious effects of tissue damage. Many studies have confirmed that moderate clipping has positive effects on the growth of L. chinensis (i.e., increasing above-ground biomass, rhizome tillers and stimulating outgrowth of buds) (Wang, 2004; Zhao et al., 2008; Liu et al., 2012), but its compensation capacity requires both adequate nutrients and meristems to support vegetative regrowth or reproduction. In suitable habitats, clonal plants were able to produce sufficient active rhizome buds when basal meristems were stimulated by clipping. The degree of plant compensation capacity depends on asexual ramet production and expansion (Gao et al., 2012). Thus, a trade-off related to carbon allocation with the potential of morphological plasticity would be an important strategy to cope with herbivory.

Previous studies have shown that clonal growth in a heterogeneous environment is controlled by plasticity in branch production, internode distance, and angle of rhizome growth (Evans and Cain, 1995). A variety of morphological plasticity and differential foraging behaviors affect plant clonal growth and population dynamics (De Kroon and Hutchings, 1995; Kembel et al., 2008; Mcnickle et al., 2009). However, these studies overlooked the spatial clonal expansion strategies and failed to quantify and predict the associational effects, when plants faced contrasting stress and heterogeneous environments. Therefore, we explored population expansion strategies of clonal plants, providing direct evidence for the adaptation patterns of L. chinensis under heterogeneous environmental stress. Specifically, we addressed the following hypotheses: i) Soil saline-alkali heterogeneous patches are beneficial to the spatial expansion of clonal plant; ii) The effect of environmental stress (saline-alkali) on the expansion of plant is more significant than that of external disturbances (grazing or clipping).

Clonal plants exhibit a variety of adaptations in response to environmental heterogeneity and stress, including changes in morphological plasticity, physiological integration, carbon allocation between functioning organs (De Kroon and Mommer, 2006; Kembel et al., 2008; Cahill et al., 2010). In the present study, we tested whether soil environmental heterogeneity may facilitate spatial expansion of clonal plant in Songnen grassland. Our results indicate that leaf areas, daughter plant and rhizome bud numbers of clonal plant were benefited from low saline-alkali heterogeneity patches. The underlying mechanism for the environmental heterogeneity effect might be plays a key role in plant population densities increase and spatial expansion. Although previous studies ascribed the positive relationship between biomass allocation and clonal expansion to mowing or grazing disturbance (Gao et al., 2008; Zhao et al., 2008; Wang et al., 2019), this relationship could also be driven by soil saline-alkali heterogeneity patches. The inter-relationship between herbivores and plants in grassland ecosystems is complex, with factors such as herbivore species, interspecific competition, climatic, and soil physicochemical properties greatly affecting the interspecific balance and ecosystem stability.

Biomass allocation is an important strategy for plants to respond to changes in resource availability and is a key factor in understanding the dynamic changes in plant population in various habitats (Yan et al., 2016; Zhang et al., 2017). In natural grassland, clipping and grazing directly affect the above-ground plant parts. Different frequencies and intensities of mowing or grazing can have different effects on plant dry matter distribution and biomass accumulation. In the present study, we found that clipping significantly decreased the above-ground biomass of L. chinensis in homogeneous patches, which was consistent with the results of previous studies (Bi et al., 2018). Clipping significantly reduced plant height, leaf area and weakened their photosynthetic capacity (Figure 3D), resulting in under-compensatory growth (Gao et al., 2012; Lu et al., 2020). In low saline-alkali heterogeneous patches, clipping had no significant effect on the biomass accumulation (Figures 2A–C), and similar findings were also reported in the study of grazing on Bromus ircutensis and Psammochloa villosa (Liu et al., 2009). This is related to the fact that low saline-alkali heterogeneity significantly increased the number of rhizome internode buds as well as the number of daughter plants (Figures 3B, C), therefore expanding its population size. These results implied that low saline-alkali heterogeneity buffered the negative effects of clipping on plant growth and promoted compensatory growth, which suggested that the compensatory response can increase the grazing tolerance of plant in low saline-alkali patches grassland. However, compensatory growth was not observed in the heavy saline-alkali heterogeneous patches (Figures 2A–C), because high salinity reduced the soil water potential, increase the difficulty of water absorption by plant roots, and caused physiological drought in plants (Munns and Tester, 2008; Geng et al., 2019). At the same time, the entering of salt into plant cells has a toxic effect, leading to a decrease in a series of physiological activities such as photosynthesis and resulting in undercompensated growth (Munns and Tester, 2008; Lu et al., 2020).

Leaves are the primary plant photosynthetic organs and are also considered the main source of dry matter accumulation (Guo et al., 2019). The increase in leaf area and number promoted the photosynthesis of L. chinensis, which increased the plant’s ability to obtain more nutrients and feed back to the belowground organs. We found that low salinity heterogeneity patches could significantly increase leaf area and numbers (Figures 3A, E). This was the reason why L. chinensis able to achieve compensatory growth in a low-saline-alkali heterogeneous environment and induce reasonable biomass allocation patterns between above- and belowground functional organs.

In perennial plant communities, the metabolic and maintenance costs of buds are so low that maintaining a large number of dormant buds is a growth strategy for plants. One such benefit is that plant can quickly respond to environmental changes by producing a large population of tillers or ramets (David et al., 2006). Rhizome buds are critical organs for vegetative reproduction of clonal plants (Munier-Jolain et al., 1996; Costes, 2003). Rhizome internode buds can grow upward to form new ramets, and rhizome apical buds can grow horizontally to form new rhizomes, which contribute to the growth and expansion of plant population (Wang et al., 2022). Several studies have indicated that defoliation can promote the production of buds as a compensation for damage (Wang et al., 2004; Lu et al., 2020), but our results showed that clipping had no significant effects on the rhizome apical or internode buds in saline-alkali heterogeneous patches (Table 2; Figures 3C, F). Moderate clipping was up close to the intercalary meristems and young leaves, and it exerted stimulating effect on axillary bud output and led to the increase in the number of tillers (Liu et al., 2012). Heavy clipping was carried out near the basal meristems, and it produced a signal to stimulate rhizome expansion by increasing rhizome length and quantity. However, the rhizome internode bud number increased significantly in low saline-alkali soil patches (Figure 3C). The heterogeneous distribution patterns of saline-alkali promoted the “development” strategy of L. chinensis, and stimulated the expansion of rhizomes and germination of internode buds to compensate for the serious loss caused by defoliation, then obtain more available resources. In addition, the heterogeneity of saline-alkali soil inhibited the growth of rhizome apical buds (Figure 3F), suggesting that clonal plants change the ratio of rhizome apical buds to internode buds in order to achieve the optimal growth pattern in response to environmental stress.

Expansion through rhizomes (stolons) for greater physical space and resource in heterogeneous environment is important adaptive strategy for regrowth and population expansion of clonal plants after external disturbances (Wang et al., 2019). In the homogeneous environment without stress or disturbance, the expansion rate of L. chinensis was the fastest due to sufficient nutrition supply and growth space (Figure 4A). However, L. chinensis rapidly compensated for the loss of leaves and photosynthesis through tillering rather than rhizome expansion under moderate mowing or grazing. Therefore, rhizome expansion rate was reduced (Figure 4B). Heavy clipping was significantly accelerating the expansion rate (Figure 4C). Although, clipping greatly damaged the above-ground parts of the plant and reduced photosynthesis and material accumulation, but the loss of above-ground plant parts stimulates the expansion of belowground rhizome and production of rhizome buds or daughter plants, thus maintaining plant growth (Lu et al., 2020). In contrast, the expansion rate increased with increasing clipping or grazing intensity in the low-saline-alkali heterogeneous patches, but slowed down with increasing clipping intensity in high saline-alkali soil patches. Our result suggested that the saline-alkali heterogeneous of soil may mediate the effects of mowing or grazing in grasslands. Interactive effects to influence plant population expansion were not consistent at different levels of saline-alkali or patches in habitat. This emphasizes soil saline-alkali heterogeneity plays an important role in driving the spatial expansion of clonal plant, and provided a reference for utilization the ecological restoration of degraded and heterogeneous grassland patches.

Our study suggests that environmental pressure was the most important factor limiting the population size of clonal plant. The heterogeneous distribution of saline-alkali in the soil limited the growth, reproduction, and spatial expansion of L. chinensis by regulating the proportion of apical and internode rhizome buds. While grazing or clipping reduced its biomass accumulation, saline-alkali heterogeneity buffered the negative effects and promoted its compensatory growth. Therefore, besides human disturbance factors (grazing or clipping), we believed that saline-alkali heterogeneity distribution pattern in the soil was an important driving factor promoting the spatial expansion of plant. However, other factors affecting the formation and development of L. chinensis populations were not been investigated individually. Therefore, it is suggested that verify the environmental and biological factors affecting the expansion of plant populations in different environments and grassland types, as well as to explore the dynamic changes in L. chinensis populations.