Seed development through to seedling emergence is one of the most pivotal phases in the life history of plants (Harper, 1977). Seeds are different from other plant organs because they can exist independently of the parent plant, and their traits vary greatly both within- (Liu et al., 2016; Marchelli and Gallo, 1999; Winn and Werner, 1987) and between-species (Guo et al., 2010; Moles et al., 2007; Murray et al., 2004; Soper Gorden et al., 2016). This has led to different species having distinct reproductive strategies as a way of encompassing diverse abiotic conditions (latitude, altitude, etc.). However, in contrast to the vegetative organs of the plant (e.g., leaves), ecologists have only recently begun investigating the large geographical patterns of plant reproductive organs (e.g., seed).
One way to study the relationship between plant traits and environmental factors is to assess changes in traits at the macro-geographic scale using either latitudinal or altitudinal gradients. This approach provides a natural testing ground for exploring the response of widespread living organisms to biological and environmental change (Körner 2007; De Frenne et al. 2013). We would expect the form and function, of both plants and their seeds to vary in the diverse environments found along such gradients indicating different resource acquisition strategies of plant species (Díaz et al. 2016). Therefore, there has been much interest in determining how latitudinal gradients affect seed mass, an important plant functional trait that plays a major role in reproductive success (Daco et al. 2021; Ulrich et al. 2023). However, conflicting results have been found when seed mass has been compared along altitudinal gradients with some studies showing a reduction in seed mass with increasing latitude (Donoso, 1979; Gallagher and Leishman, 2012; Marchelli and Gallo, 1999; Moles et al., 2007; Moles and Westoby, 2003) and others showing the opposite trend (Graae et al. 2009; Carón et al. 2014). Moreover, almost all studies of seed traits along latitudinal gradients concentrate on seed size and mass, and much less is known about the effects on other seed traits, e.g., seed set, seed production, and seed nutrient concentration. To our knowledge, research on latitudinal gradients of seed nutrient concentrations is particularly scarce (De Frenne et al. 2010). Given the importance of seed nutrients for initial seedling growth, surprisingly little attention has been paid to seeds compared to plant leaves and fine roots (Hu et al. 2020; You et al. 2023). Meanwhile, these patterns observed in leaves and roots, which can be explained by the limitation of soil nutrients at higher latitudes (geochemical hypothesis) and by direct temperature effects on plant physiology (growth rate hypothesis) (Lovelock et al. 2007; Elser et al. 2000), may not comparable to seed. This lack of knowledge may limit our understanding of resource acquisition strategies of plants across latitudes. Furthermore, it is also possible that within-species variations in the traits of species with a widespread distribution may be related to environmental factors that also vary along the geographic gradient, especially across different latitudes (Wright et al. 2017; Ren et al. 2020). Numerous studies have examined the association of seed traits with variation across environmental gradients (Murray et al. 2004; Soper Gorden et al. 2016; Wu et al. 2018). However, most previous studies focused only on climate-dependent abiotic factors without considering the effects of soil, and their interaction or the relative importance of climate and soil factors.
Knowledge about seeds and their traits along latitudinal gradients, which almost always reflect a climatic range, is of particular importance in investigating the threat to native ecosystems of invasive alien species (Guo et al., 2023; Soper Gorden et al., 2016; Wu et al., 2018). Invasive species have been identified as one of the problems associated with global warming, this is driven by globalization as well as climate change and is increasing in speed, scale, and damaging impact with little evidence that this impact will diminish (Seebens et al. 2017). The enhanced sexual reproductive capacity of some invasive species relative to native species has been shown to be an important factor in their success (van Kleunen et al. 2015). Seed mass and size are important indicators for assessing the invasive ability of many invasive plants, but the relationship between seed size and invasive ability can vary depending on species (Leishman et al. 2000), with some alien species relying on small seeds for long-distance dispersal and invasion (Moodley et al. 2013), and others relying on large seeds for survival in specific habitats (Burke and Grime, 1996). Therefore, to achieve an evidence-based approach for predicting potential harm caused by invasive species, the first step is to understand how species reproduce and the factors that affect reproductive success (Andersen et al. 2004). Here, we compare the seed traits of an invasive species relative to a native one on a latitudinal gradient along the coast of China.
The native species is Phragmites australis, a cosmopolitan plant growing mainly in wetlands. This species has high morphological diversity and plays an instrumental role in the maintenance of biodiversity and ecosystem function. It is often used as a model species, i.e. the native counterpart, for studies on biological invasions and responses to climate change (Ren et al. 2020; Liu et al. 2024). This makes it an excellent choice to compare with the non-native macrophyte Spartina alterniflora along the eastern coast of China, where is encroaching extensively upon indigenous P. australis ecosystems (Strong and Ayres 2013). S. alterniflora is native to the Atlantic and Gulf Coasts of America and has become widely invasive subsequent to its introduction in 1979 (Meng et al. 2020). Both species are distributed over a large latitudinal range and have a wide range of climatic habitats and ecological niches (Liu et al., 2016; Packer et al., 2017). Recent empirical studies have found latitudinal variations in morphology (Liu et al., 2022; Meyerson et al., 2016), growth (Liu et al. 2016; Ren et al. 2020), and phenology (Bastlová et al. 2006) of both species. However, knowledge of possible latitudinal variation in seed traits, vital for plant recruitment and population regeneration, is lacking.
Here, we compared the pattern of latitudinal variation influencing seed traits in both species, the native P. australis, versus the invasive alien S. alterniflora, an invasive alien) and assessed the impacts of climatic and soil factors that might influence them across a 20o-latitudinal transect along the coastal marshes of China. In doing so, we address two main questions: (1) Are there any variation differences in seed traits between the native and alien species along the coastal latitudinal gradient in China? (2) If so, which environmental factors are most important in influencing these variations? Furthermore, we hypothesized that: (i) there will be significant latitudinal patterns in most seed trait variables for at least one species; (ii) seed traits will exhibit parallel latitudinal clines in both species across the latitudinal gradient; and, (iii) climate variables will be the dominant factor driving the latitudinal pattern of different species, compared to soil ones. This study also provides new insights into the ecological mechanisms of latitudinal variation of the reproductive organ morphology in native and invasive species.