4.1 Clone architectures of C. udensis diploids and tetraploids in the Hualongshan Mountains
Different species have been found to possess different clonal architecture patterns. Furthermore, the clonal architectures of species might represent malleable changes under different habitat conditions (Kartzinel et al., 2015; Wang et al., 2018b; Huang et al., 2018). Within heterogeneous environments, clonal plants can adjust their strategies to secure resources through the pliable changes in their clonal architectures, which ultimately result in changes of the properties of its ramets (Kartzinel et al., 2015; Wang et al., 2018a). Our field investigations found that there was only a single active bud on many rhizomes of different C. udensis ploidies.
When C. udensis individuals normally grow and develop, they require only one active bud on each rhizome knot to develop aboveground parts every year. However, in some cases, there were 2-4 active buds on the rhizome knot, which began to grow through the year, all of which formed basal rosette seedlings. Each newly sprouted rhizome generated an indefinite number of buds on its rhizome, so that a former ”one” style of belowground rhizome could generate new branching. Each new branching continues to sprout new buds in the later growth process (Fig. 2) and extend along a different direction from the original branching of the former rhizome. Thus, the active buds on each rhizome knot could form a symmetrical or 90° angle, which determined and affected the characteristics of the outward growth of new branches. Furthermore, the newly formed branching continued to grow and produce additional rhizome nodes to occupy more spatial positions, to gradually form a clonal plant comprised of multiple ramets (Liu et al., 2013; Ott et al., 2019).
The number and position of buds on the rhizome knot develop and form before the winter. When the temperature drops in the autumn and winter, the aboveground portions of C. udensis individuals die. The rhizome and formed rhizome buds continue to live underground through the winter, and during the ensuing spring, some of the active buds sprout aboveground to form caespitose clonal architectures (Qian et al., 2017). The number of active and dormant buds on many rhizome knots are approximately equivalent for the C. udensis diploids and tetraploids. Each spring, the active buds begin to germinate and form caespitose seedling sprouts, while the dormant buds remain in a dormant state continuously (Chen et al., 2016).
The distance between the rhizome internodes of the genet in C. udensis was relatively short (1-2 cm). The buds of the rhizome typically occurred at the rhizome knot. Generally, the ramets would grow adjacent to each other, whereas more collective clone patchiness in the microspace was formed. According to the annual statistical analysis for diploid and tetraploid clone reproduction, the ratio of individuals having clone reproduction was relatively low for both ploidies. The number of ramets generated by the diploid and tetraploid individuals was also limited, where the spatial distribution pattern of these ramets was quite repetitive (Fig. 2). Thus, the clonal architecture of C. udensis was a phalanx pattern (Fig. 3). The spatial pattern and architecture of the clonal population was thought to give rise to the capacity for utilizing environmental heterogenicity (e.g., various resources) for the clonal plants. Although this clonal structure ofC. udensis had a limited expansion ability, it facilitated the easy utilization of environmental resources at localized sites (Wang & Zhao, 2007; Wang et al., 2008).
Clonal plants can improve the efficacy of clonal reproduction to optimize the survival rates of ramets through physiological integration and foraging behavior (Van Drunen et al., 2015). Due to the continuous expansion of rhizomes, the growth pattern of C. udensis gradually formed from a phalanx to a relatively scattered ramet in flocks. This change would meet the growing demands for space and nutrition for clonal growth, while enabling the diploid and tetraploid populations to more reasonably occupy and utilize environmental resources to maintain the sustainable development and survival of the C. udensis diploid and tetraploid population (Wang et al., 2008; Wang et al., 2010; Wang et al., 2011). However, C. udensis is not suitable for larger clonal populations, as there are no long-wandering spacers in its diploids or tetraploids.
The clonal growth of C. udensis belongs to the phalanx mode. If plants with phalanx clonal growth, the source of heterologous pollen is lower than that of plants with guerrilla clonal growth. The limitation of pollen resources would change the mating system of plants from outcrossing to inbreeding. Long-distance pollination by pollinators and the number of clonal ramets visited by pollinators in a flight round can maintain a high outcrossing rate for clonal plants (Bai et al., 2009).C. udensis is entomophilous pollination, and its breeding system is partial self-compatibility and outcrossing (Bai et al., 2009). This is a trade-off relationship between clonal growth and sexual reproduction and is a adaptability to the diverse environment (Zhang et al., 2006).
The relative proportion of clonal reproduction and sexual reproduction in plants are impacted by environmental conditions such as water and light. The balance between this two-breeding system primarily depends on environmental changes, resource demands, and interactions between organisms (Pirog et al., 2017; Alonso-Marcos et al., 2019). For C. udensis , the clone reproduction level of the tetraploid was higher than that of the diploid (Table 4). Compared with the sexual reproduction of plants, the growth probability and survival rate were much higher than those seedlings produced by seeds, although the resource input was higher when each ramet was produced through clonal reproduction (Van Drunen et al., 2015; Geremew et al., 2018; Smith et al., 2020).
In undisturbed environments, individual plants are consistently in a strong competitive state with each other. Newly generated clonal ramets more easily survive and grow in contrast to plants that are germinated by seeds. Therefore, plants are more likely to adopt clone reproduction when they grow under a long-term and stable environmental selection pressures. This might be related to the difficulty of establishing seedlings from seed reproduction (Chen et al., 2016). When the tetraploids of C. udensis form through genome duplication (Li et al., 1996), they explore new surroundings that are completely different from the ancestral diploids. The tetraploids might adapt to this new environment through higher level of clonal growth than its ancestral diploids and grow at relatively lower altitudes than that of the diploids in the Hualongshan Mountains.
To occupy and maintain spatial territories, C. udensis had to maintain sexual reproduction due to the instability of water, heat, climate, and other environmental factors in the subalpine environment. Thus, both the diploid and tetraploid populations preserved the sexual reproduction system through seeds as the main forms, and the ancestral diploids sustained a higher rate of sexual reproduction than did the tetraploids. Furthermore, C. udensis is a perennial rosette herbaceous species that grows under the shade of forests. Its population distribution was limited by changes in the canopy density of forest communities, environmental temperature, soil moisture, and the thickness of the humus layer. In conjunction with the lower altitudes of the geographical distribution areas of C. udensis , the decomposition period of humus gradually shortened. The humus soil layer that the seeds of C. udensis depended on for dormancy and germination gradually became a limiting factor for sustaining the population. During the evolutionary process, the tetraploids might decrease the rate of sexual reproduction and increase clonal growth to stabilize the population.
As both the sexual reproduction and asexual reproduction of plants can contribute to risk sharing and resource acquisition capacities in plaque habitats, as well as the maintenance of genetic diversity, plants can adopt reproductive trade-off strategies under different environments (Zhang et al., 2015). Generally, habitats under forest shade are not helpful for seed germination and seedling growth. C. udensiswould propagate offspring through clonal growth that could overcome the disadvantage of seedlings that are growing under the forest canopy. When fierce intraspecific competition is restricted by density, plants can avoid intraspecific competition by creating more resources to invest in seed reproduction through its own hormone regulation within a limited space. Therefore, to adapt to the changing subalpine environment, the diploids and tetraploids of C. udensis evolved into a facultative clone species, which would maintain population stability and reproduction through both seed reproduction and clone growth.