1 Introduction

Biological invasion is one of the most serious ecological problems worldwide, with detrimental effects on the economy, environment, and public health (Ferreras and Galetto 2010; Mack et al. 2000; Mazia et al. 2001; Schlaepfer et al. 2010; Vilà and Weiner 2004). It poses a significant threat to biodiversity (Beckmann et al. 2011; Lake and Leishman 2004; Perglová et al. 2009; Binggeli 1996; Simberloff et al. 2013; Verlinden et al. 2014; Vilà and Weiner 2004) and has become a key factor in recent extinctions (Thomas et al. 2004). Despite the formulation of several theories, such as the inherent superiority hypothesis (Lowry et al. 2013), the empty niche hypothesis (Hierro et al. 2005), the resource opportunity theory (Davis et al. 2000), the enemy release hypothesis (Keane 2002), the novel weapon hypothesis (Callaway and Ridenour 2004), and the evolution of increased competitive ability hypothesis (EICA; Blossey and Notzold 1995), the underlying mechanisms of biological invasion remain unknown. The ecological substance of plant invasion lies in the colonisation, population establishment, and proliferation of certain alien species in new environments.

Plant regeneration from seeds plays a pivotal role in plant invasions as it is often the initial stage in the invasion process. Although some invasive plant species rely entirely on asexual reproduction, many benefit from both sexual and asexual reproduction once established (Huebner 2022). Colonisation success is determined by many processes, with seed germination being the first step, regardless of whether the site offers an ideal growth habitat or presents a stressful environment with aggressive competitors (Luo and Cardina 2012). Seeds, as primary dispersal and propagation units, have many important functional traits that reflect plant adaptations to their surroundings, including size, shape, structure, germination requirements, and responses to abiotic stress (Saatkamp et al. 2018). Therefore, seed studies provide valuable insights into the processes involved in plant invasion.

Siam weed (Chromolaena odorata) and Crofton weed (Ageratina adenophora), both native to Mexico, are rapidly spreading across tropical and subtropical regions globally, including in China (He 2012; Global Invasive Species Database GISD 2024; Ma 2013; Xu and Qiang 2004). In southern Yunnan, these two invasive species exhibit a mosaic distribution pattern. Chromolaena odoratum usually grows in areas of low latitude and low altitude, such as Menglun in Xishuangbanna, whereas Ageratina adenophora is found at higher latitudes, such as Kunming. The two species co-occur in transition zones, including regions with middle latitudes, such as Pu'er, and areas at low latitudes but with high altitudes, such as Kongmingshan in Xishuangbanna. These distribution patterns make them ideal candidates for comparative studies on plant invasion.

We hypothesised that thermal interactions between these species and their habitats contribute to this mosaic distribution. To test this hypothesis, we investigated how this plant–habitat interaction affects seed traits and performance across various biological processes. These include seed viability and high-temperature tolerance, seed longevity in soil seed banks, and temperature requirements for germination. In this study, we examine the high-temperature tolerance and germination responses under high temperature and water stress of seeds from three of four invaded provenances to better understand how seed trait–habitat interactions influence the distribution of these two invasive species.

2 Materials and Methods

3 Results

3.2 Seed Tolerance to Extremely High Temperatures

All factors—species, seed provenance, hydration state, and heating temperature—as well as their interactions significantly affected seed viability (p < 0.001 for all factors, except for S × P × H × T, where p = 0.074; Supporting Information S1). An anomaly was observed in the low viability (32.33% ± 1.89%) of air-dried Chromolaena odoratum seeds from Pu'er after heating at 35°C; however, a later verification test determined a viability of 71% ± 3.38% (Figure 1b). Most dry seeds exhibited minimal viability loss after heating at temperatures up to 90°C, though none survived at 95°C. Heating for 30 min at 60°C markedly reduced the viability of imbibed seeds, with few Ageratina adenophora seeds surviving and none of the Chromolaena odoratum seeds surviving after heating at 70°C or higher. Amongst the provenances, Ageratina adenophora seeds from Kongmingshan had the best germination performance, whereas Chromolaena odoratum seeds from Menglun performed best across the three provenances (Figure 1).

3.3 Effects of Continuous High-Temperature Treatment on Seed Viability

All investigated factors and their interactions significantly affected seed viability (p < 0.001 for all; Supporting Information S1). Overall, Chromolaena odoratum seeds had notably higher tolerance to continuous high-temperature stress at 40°C compared to Ageratina adenophora seeds (Figure 2). After 10 days of exposure, Ageratina adenophora seeds lost more than half of their viability, with seeds from Pu'er showing the greatest decline (< 10% survival). In contrast, Chromolaena odoratum seeds experienced little to no viability loss, with seeds from Menglun showing no loss and those from Kongmingshan and Pu'er showing only minimal reductions.

3.4 Effect of Incubation Temperature on Seed Germination

Analysis of variation found that all investigated factors, along with their interactions, significantly affected seed germination (species at α = 0.05 level, S × P at a marginal level, and the remaining factors at α = 0.01 level; Supporting Information S1). The two species had distinct temperature requirements for germination. At 10°C, about half of Ageratina adenophora seeds germinated, with seeds from Kunming showing the highest germination percentages, while only a few Chromolaena odoratum seeds germinated at this temperature. In contrast, more than half of Chromolaena odoratum seeds germinated at 35°C, with seeds from Menglun having the highest germination, while only a few Ageratina adenophora seeds germinated. No seeds of either species germinated at 40°C (Figure 3).

3.5 Effects of Periodic High Temperature on Seed Germination

All investigated factors and their interactions significantly affected seed germination (p < 0.001 for all; Supporting Material 1). Daily heating ≥ 7 h at 40°C significantly reduced Ageratina adenophora seed germination, with seeds from Pu'er consistently showing the lowest germination percentages. Chromolaena odoratum seeds demonstrated marked intraspecific variation. Seeds from Pu'er displayed significantly reduced germination after ≥ 5 h of daily heating, while seeds from the other two provenances did not exhibit a reduction in germination until the daily heating duration reached 12 h. Seeds from Pu'er had the lowest germination and the highest sensitivity to high temperatures (Figure 4).

3.6 Effects of Water Availability on Seed Germination

Ageratina adenophora seeds were more sensitive to water stress than Chromolaena odoratum seeds, with the former quickly showing a marked reduction in germination as water potential decreased. Only a few Ageratina adenophora seeds germinated when water potential was reduced to −0.8 MPa and below, while this turning point was −1.2 MPa for Chromolaena odoratum seeds. Seed provenance significantly affected the germination of Chromolaena odoratum seeds but had little effect on Ageratina adenophora seeds. At the water potentials between 0 and −1.2 MPa, Chromolaena odoratum seeds from Menglun exhibited the highest germination percentage, while those from Pu'er showed the lowest, with seeds from Kongmingshan falling in between (Figure 5; Supporting Information S1). Most seeds that failed to germinate when incubated at water potentials between −0.8 and −1.5 MPa resumed germination once the water stress was removed (data not shown).

3.7 Effects of NaCl Stress on Seed Germination

Similar to the water availability experiment, Ageratina adenophora seeds were more sensitive to NaCl stress than Chromolaena odoratum seeds. Incubated under NaCl stress with a water potential of −0.8 MPa, 40–70% Chromolaena odoratum seeds germinated, while only a few Ageratina adenophora seeds (≤ 5%) did. Seed provenance affected seed germination in both species, although the effect was more pronounced in Chromolaena odoratum. For Chromolaena odoratum, the order of sensitivity to NaCl stress was PE > KMS > ML, whereas it was KM > KMS > PE for Ageratina adenophora (Figure 6; Supporting Information S1). As with the water availability experiment, most ungerminated seeds incubated at water potentials between −0.8 and −1.5 MPa germinated soon after the stress was removed (data not shown).

4 Discussion

Although Ageratina adenophora spread naturally into China, Chromolaena odorata was intentionally introduced as a spice plant. Both species entered the country via similar invasive routes around the same time, from Myanmar into South Yunnan around the 1930s—marking the first recorded instances of their appearance in China (He 2012; Hu et al. 2013; Liu et al. 1985; Xu and Qiang 2004). Ageratina adenophora was first documented in Menghai County, Xishuangbanna, in 1940, from where it spread northward to Pu'er and Kunming and eastward to Kongmingshan (Sang et al. 2010). Recent research has found that Ageratina adenophora in China has a distinctly expanded ecological niche and exhibits greater invasiveness compared to its counterpart in Mexico (Xian et al. 2023).

Chromolaena odorata has two independent introduction recordings in Asia: one from Jamaica in 1845 and another from the West Indies around 1920 (Yu et al. 2014). It was first introduced to South Yunnan in 1934 (He 2012). Yu et al. (2014) found that Chromolaen aodorata populations in Asia have extremely low genetic diversity compared to those in its native range, but the genotype in Asia demonstrates strong competitive ability.

These two invasive species co-occur in South Yunnan, where they demonstrate a mosaic distribution pattern. Ageratina adenophora predominantly thrives in northern regions, whereas Chromolaena odorata is more common in the south. Transition areas, particularly those at intermediate latitudes or low latitudes with high altitudes, appear to be less suitable for both species.

Using Ageratina adenophora and Chromolaena odoratum seeds collected from three of the four provenances, we investigated their tolerance to high temperatures and their germination under stress conditions. The study revealed both inter- and intraspecific differences in seed responses. Both species produced seeds exhibiting tolerance to extremely high temperatures, confirming earlier findings on the strong heat tolerance of invasive species growing in open ground in Xishuangbanna, such as Tithonia diversifolia (Wen 2015), Amaranthus spinosus (Ye and Wen 2017), Ageratum conyzoides, and Conyza Canadensis (Yuan and Wen 2018). In this study, Ageratina adenophora and Chromolaena odorata seeds in air-dried status showed no significant viability loss after 30 min of heating up to 90°C (Figure 1). However, inter-specific differences were revealed in the following experiments.

The seeds of both species demonstrated a wide temperature range for germination, from 10°C to 35°C. Compared with Ageratina adenophora seeds, Chromolaena odorata seeds generally had a higher temperature requirement for germination. While most Chromolaena odorata seeds germinated at 35°C, only a few germinated at 10°C. In contrast, Ageratina adenophora seeds had good germination performance at 10°C but low germination percentages at 35°C (Figure 3). Neither species germinated at a constant temperature of 40°C. However, Chromolaena odorata seeds were more tolerant to longer daily exposures to 40°C than Ageratina adenophora seeds under conditions with periodic heating. Interestingly, Chromolaena odorata seeds from Pu'er were sensitive to daily 5-h exposure at 40°C, while Ageratina adenophora seeds exhibited tolerance to this treatment (Figure 4).

In continuous high-temperature experiments, Chromolaena odorata seeds exhibited superior tolerance. After 10 days of exposure to 40°C, most Ageratina adenophora seeds lost viability, while Chromolaena odorata seeds survived (Figure 2). This suggests that Chromolaena odorata seeds have greater longevity under high-temperature stress (Long et al. 2008). Furthermore, Chromolaena odorata seeds were much less sensitive to water and NaCl stress than Ageratina adenophora seeds (Figures 5 and 6). These differences underline the distinct ecological characteristics of the two species: Chromolaena odorata is a tropical plant, while Ageratina adenophora is more temperate.

The differing, yet overlapping, climate niches of these two invaders in South Yunnan are shaped by the region's mountainous topography, which provides both suitable and marginal habitats for their growth. The thermal interaction between plants and their habitats affects the seed survival and germination of invaders in the field, contributing to the success or failure of these species' invasions into new areas.

At the same time, this study revealed marked intraspecific variations in both invaders. Despite a large difference in seed weight—Chromolaena odorata seeds were approximately five times heavier than Ageratina adenophora seeds (0.224 g vs. 0.0447 g per 100 seeds)—both species had high initial viability, though with obvious intraspecific variation depending on seed provenance (64–82% germination). Seeds harvested from sites where the species grew better generally had higher quality. Specifically, Chromolaena odorata seeds from Menglun and Ageratina adenophora seeds from Kunming and Kongmingshan had higher initial viability than their counterparts from less suitable sites (Table 1). Similarly, Chromolaena odorata seeds from Menglun often exhibited the highest tolerance across all high-temperature experiments in this study, while those from Pu'er were the most sensitive, and those from Kongmingshan exhibited intermediate tolerance (Figures 1-4). This pattern was particularly evident in the daily periodic heating experiment, in which Chromolaena odorata seeds from Pu'er were the most sensitive to heat amongst the three provenances (Figure 4b). For Ageratina adenophora, seeds from Kunming had better germination performance across a wider temperature range than those from Kongmingshan (Figure 3a) but displayed lower tolerance to high temperatures. Seeds from Pu'er consistently performed the worst (Figures 1, 2, and 4a).

These intraspecific variations suggest an adaptative response of the invaders to local environmental conditions and highlight the maternal influence on seed development, viability, and high-temperature tolerance. A previous study reported that germination behaviour can vary based on the environmental conditions experienced by the mother plant during seed maturation (Corkidi et al. 1991). Invasive species growing in more favourable habitats tend to produce seeds with higher viability and greater tolerance, indicating the thermal interactions between plants and their environments during seed development.

The invasion of exotic plants occurs due to their ability to outperform native species in the areas where they are introduced, allowing them to proliferate and cause harmful effects on the economy, environment, and/or human health (Cui and He 2009). However, the underlying mechanisms enabling this outperformance remain unclear. Understanding invasiveness is key to unravelling the mechanisms of biological invasion. Comparative studies are important in this regard, as they offer new insights into the traits that make certain species successful invaders (Maillet and Lopez-Garcia 2000). Comparing closely related taxa is particularly effective for identifying characteristics of successful invaders, as it minimises biases associated with phylogenetic distance and habitat affinities between species compared (Maillet and Lopez-Garcia 2000; Perglová et al. 2009).

As invasive plants, Ageratina adenophora and Chromolaena odoratum are very closely related and share many traits: they are both perennial herbaceous shrubs from the same family (indicating close phylogenetic relatedness), originally from Mexico and Central America (sharing the same native range), and produce numerous small pappus-bearing achenes (high fecundity and anemochory). Both species also release allelochemicals (a ‘new weapon’ mechanism), are cosmopolitan toxic weeds naturalised in tropical and subtropical areas (invading similar areas), and are aggressive competitors that form dense stands, inhibiting the establishment of other plant species (ecologically damaging in similar ways). Furthermore, both species followed the same invasion route into China, and they have overlapping distributions in South Yunnan, where they co-occur in transitional areas. This overlap makes them an ideal case for a comparative study of plant invasion.

In this study, the two species investigated, Chromolaena odorata and Ageratina adenophora, were both invasive and aggressive, with the four seed provenances (Kunming, Pu'er, Menglun, and Kongmingshan) all representing areas susceptible to plant invasion. However, the question remains: Why do these species succeed at some sites but fail or perform less successfully at others? To address this, laboratory experiments using seeds collected from these provenances were performed, eliminating possible interference. The results suggest that ecological adaptation is the most critical factor for successful invasion.

Although plant invasion involves all stages of the invader's life cycle, seed production and germination are the most important phases during the invasion process (Huebner 2022; Luo and Cardina 2012; Perglová et al. 2009; Wen 2015). Several germination traits are associated with invasiveness, including the ability to germinate under a wide range of conditions, non-dormancy, the capacity to germinate without pretreatment, rapid germination, and high germination percentages (Goodwin et al. 1999; Luo and Cardina 2012; Radford and Cousens 2000; Perglová et al. 2009; Pyšek and Richardson 2007; Schlaepfer et al. 2010). Previous studies have suggested the importance of seed high-temperature tolerance for tropical invasive plants in open areas (Wen 2015; Ye and Wen 2017; Yuan and Wen 2018). This study further confirms that invaders growing on open ground in Xishuangbanna produce seeds with a strong tolerance to high temperatures.

In conclusion, this study suggests that the ecological adaptation of invaders to invaded areas is the most fundamental factor for invasion success. The thermal interaction between plants and their habitats affects seed development, leading to inter- and intraspecific differences in seed quality and other traits. These variations influence seed survival and germination in the field and ultimately determine the invasion success of Chromolaena odorata and Ageratina adenophora, playing a critical role in plant invasion dynamics.

Author Contributions

Bin Wen: conceptualisation, data curation, formal analysis, funding acquisition, methodology, project administration, supervision, validation, visualisation, writing – original draft, writing – review and editing. Zhufang Zhao: data curation, investigation. Siqi Wang: data curation, investigation. Xuejing Yin: data curation, investigation. Liping He: conceptualisation, writing – review and editing. Ligang Chen: conceptualisation, writing – review and editing.

Ethics Statement

The authors have nothing to report.

Conflicts of Interest

The authors declare no conflicts of interest.