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.