It is well known that biological invasion has become a serious global problem which causes great damage to native ecosystems (Callaway and Aschehoug, 2000; Rejmánek and Marcel, 2015; Richardson et al., 2000; Van Kleunen et al., 2010, Van Kleunen et al., 2015). Clonal plants play an important role in biological invasion. Among 2670 invasive plant species originating from Central Europe, 66.5% (about 2000 plant species) have clonal life-history traits (Klimes et al., 1997). In China, 196 out of 515 recorded invasive plant species are clonal plants (Wang et al., 2016a), and 49.2% of 315 plant species distributed in Northeast China are clonal plants (Song and Dong, 2002; Song et al., 2001). In general, the most important trait associated with clonal growth is clonal integration, which is an ability of clonal plants to transport the substances and/or signals between connected ramets to adjust and adapt to various environmental stress (Dong and Yu, 2007). Clonal integration can increase the growth and propagation of connected ramets, either individually or as a whole, and promote the spread of clonal ramets from original habitats where the seeds are planted (Alpert, 1999; Hartnett and Bazzaz, 1983; Roiloa and Retuerto, 2006; Slade and Hutchings, 1987; Saitoh et al., 2002). Since clonal integration might be associated with the adaptability of invasive plants to the environment, it is of great importance to elucidate the relationship between clonal integration and adaptability of invasive plants in the early stage of invasion.
To successfully invade new habitats, clonal plants have to cope with the heterogeneity in environmental resources such as light, nutrients, water, etc. (You et al., 2016; Wang et al., 2017; Chen et al., 2018). The adaptive strategy of clonal plants to environmental heterogeneity has always been one of the hot topics in the field of biological invasion. Clonal integration could enable clonal plants to share the products of photosynthesis, mineral nutrient, or water between ramets, thereby increasing the survival and growth of individual ramets when they experience different conditions (Alpert, 1996; Hutchings & Wijesinghe, 1997; Dong et al., 2015; Lyu et al., 2016; Shi et al., 2021). For example, when some ramets are damaged or restricted by regional environmental stress, unaffected ramets could help these ramets survive or escape from adverse habitats through translocation of resources among connected ramets (Hellström et al., 2006; Wang et al., 2009; Lyu et al., 2016). In heterogeneous environments, clonal integration can improve the performance of mother and daughter ramets in Wedelia trilobata, a typical invasive clonal plant, to facilitate their invasion into waterlogged areas (Saptiningsih et al.,2018). In homogeneous environments, however, clonal integration has different effects on the growth of clonal plants. For example, clonal integration enhances the growth of the aboveground parts of connected ramets of a clonal weed Alternanthera philoxeroides and increase their resistance to pathogenic fungi (Qi et al., 2017), but it does not affect the growth of the other invasive plant Phalaris arundinacea treated with homogeneous simulated herbivory (Yu et al., 2018). Therefore, it is necessary to understand the adaptive strategy of invasive clonal plants in either homogeneous or heterogeneous environments.
In China, the most dangerous invasive plants are clonal plants, such as Mikania micrantha, Alternanthera philoxeroides, Spartina alterniflora, Eupatorium adenophora, and so on. Recent studies mainly focus on clonal plants with rhizomes and stolons, but little is known about clonal plants with root suckers. In clonal plants with root suckers, adventitious buds on the roots of parent plants may grow and spread to form new colonies (Alpert, 1996; Li et al., 2011; Peng et al., 2014). As for clonal plants with root suckers, the number of ramets that can produce aboveground shoots from adventitious buds on the roots is a key factor for clonal growth, distribution pattern and population size of new ramets (Zheng et al., 2019). The current knowledge about clonal integration in plants with rhizomes and stolons is not sufficient to completely elucidate the mechanisms underlying the invasion of clonal plants with root suckers.
R. amphibia and R. sylvestris, belonging to the family Brassicaceae Burnett (Cruciferae), are perennial herbaceous plants, which propagate primarily by root suckering. These two plants prefer a humid environment and are tolerant to waterlogging. In Liaoning province, they are first found in lawns, probably introduced via the horticulture trade. These plants have spread rapidly in recent years, formed patchy distributions and become the dominant plant species in lawns, which creates challenges for the maintenance and management of lawns (Zhang et al., 2009; Wang et al., 2016b; Miao et al., 2019). At the same time, improper irrigation might lead to the spatial heterogeneity of soil moisture in lawns. Therefore, understanding the mechanism by which clonal integration influences the spread of clonal plants with root suckers in water-heterogeneous environment is necessary.