Abstract
During the range expansion of invasive plants, competitors shared different co-evolutionary history with invasive plants, as well as population differentiation, would have different effects on the response of invaders to global change factors such as increased nitrogen deposition. Disregard the community responses and potential adaptations of invaders during the range expansion might bring misleading answers. To address these challenges, we conducted a greenhouse experiment to explore the synergistic effects between population differentiation during range expansion and competitors on the invasion ofGalinsoga quadriradiata in response to increased nitrogen deposition. Competitors (new or old that shared short or long co-evolutionary history with the invader, respectively) were set to compete with the invasive central and edge populations under different nitrogen addition treatments.Galinsoga quadriradiata from the central population (i.e., with longer residence time since invasion) showed significantly higher total mass, reproduction, interspecific competitiveness when compared to the individuals from the edge population, and the magnitude of response to nitrogen addition treatments was larger in the central population when planted in isolation (single-culture). Nitrogen addition promoted growth and reproductive performance of G. quadriradiata in single-culture, in the presence of competitors this effect was weakened. The old competitors acted more effectively than new competitors in inhibiting the invader performance, mainly for the central population. Our results indicate that population differentiation on growth and competitiveness occurred during the range expansion of G. quadriradiata , with the central population displaying higher invasiveness than the edge population. The co-evolutionary history between invasive species and its competitors has been suggested to be probably not in favor of invasive plants, especially for central populations. Our results highlight the synergistic and non-additive role of population differentiation and shared co-evolution history between invasive species and its competitors in the range expansion of invaders in the context of global change factors.
Keywords: central or edge population, co-evolutionary history, interspecific competition, invasive plant, nitrogen deposition, population differentiation, range expansion
INTRODUCTION
Human activities have dramatically changed the nitrogen cycle in natural ecosystems since the industrial revolution (Liu et al., 2020; Yu et al., 2019; Zhang et al., 2021). This is particularly true for China, as the largest developing country, it is expected to experience a rapid increase in nitrogen deposition: the average flux of total nitrogen deposition for China has been estimated as 20.4 ± 2.6 kg ha-1 yr-1 as a result of the industrialization process (Xu et al., 2019; Yu et al., 2019). The increase of nitrogen deposition leads to anthropogenic disturbances that increase available soil nitrogen, and different plants have different responses to this process. Studies have shown that the increase of available soil nitrogen is beneficial to fast-growing plants that can quickly transform nitrogen into new biomass, but not to slow-growing species (Feng et al., 2009; Liu et al., 2017a; Nordin et al., 2005). Many studies have shown that increased nitrogen deposition will promote the colonization, diffusion and other invasive processes of exotic plants, because one of the major characteristics of invasive plants is rapid growth and preemption of resources (Davis et al., 2000; Radford, 2013; Yu et al., 2020; Zheng et al., 2020). Therefore, it is considered that the impact of invasive plants may be further strengthened in the context of future climate change. However, it is still unclear how the increase of nitrogen deposition affects the invasion process of widespread alien plants due to the complexity of interactions during their range expansion.
During their range expansion, invasive plants encounter environmental heterogeneity that might lead to differentiation across populations (Dematteis et al., 2020; Helsen et al., 2020; Zhang et al., 2022). This differentiation may reflect upon species performance and to diverse responses to the increase of nitrogen deposition. Drastic differences across populations usually encountered between a central population where the species initially established in the introduced range and an edge population where the invasive plant has established recently. While the central population shares a longer co-evolutionary history with co-occurring competitors (old competitors), the edge population recently co-occurs with its competitors at the expansion front (new competitors) (Dematteis et al., 2020; Halbritter et al., 2015; Tabassum & Leishman, 2020). The differentiation on growth and competitiveness among invasive populations since time of establishment can significantly affect the competition dynamic with their competitors and determine invaders further expansion (Farooq et al., 2017; Kilkenny & Galloway, 2016). For example, compared with the edge population, the central population of an invasive plant usually needs to deal with the strong competition caused by the denser number of individuals due to the longer time since establishment, so the central population often tends to be more competitive (Phillips et al., 2010; Shine et al., 2011). This differentiation may be driven by evolutionary processes (such as spatial sorting, natural selection, gene flow, mutation, and genetic drift / gene surfing) as well as ecological processes (such as demography, dispersal, expansion speed and shape, and expansion variability), and could be modified by landscape features, trait genetics and biotic interactions (Miller et al., 2020).
The new and old competitors interact differently with the invasive plant along the expansion range (Callaway & Aschehoug, 2000; Sun et al., 2013; Sun & He, 2018), and the outcome of their interactions usually brings idiosyncratic and mixed results. In general, new competitors tend to inhibit the growth of invasive species more significantly than old competitors (Alexander et al., 2015; Sun & He, 2018). Unlike new competitors, the co-evolution between an older competitor and the invader seems lead to a gradual development from competition to coexistence (Huang et al., 2018; Lankau, 2013; Sheppard & Schurr, 2019). Other studies show that invasive species might have an initial advantage over their competitors, but over time, this advantage decreases and competitors may limit the performance of invaders (Oduor, 2013; Saul & Jeschke, 2015; Sheppard & Schurr, 2019). Although the study of pairwise interactions between invasive plants and different competitors is relatively common across the invasion biology literature, there is a lack of understanding on whether old and new competitors have different effects on the response of invasive plants.
Different competitors might have contrasting effects on the response of invasive plants to elevated nitrogen. Many studies have shown that invasive plants use nitrogen more efficiently than native species, with responding more quickly and greatly to the increase of nitrogen deposition (Parepa et al., 2019; Yu et al., 2020). This differential response could lead to a predicted enhanced expansion rate of invasive plants (Liu et al., 2017b; Liu et al., 2019). However, there is no guarantee that invasive plants will succeed in the future, as their success will also depend on the outcome of the competition with their competitors. Some studies have shown that, compared to native species, invasive plants usually maintain a larger magnitude of response to increased nitrogen deposition and increased relative competitive advantage (Eller & Oliveira, 2017; Qin et al., 2018; Valliere et al., 2017). Other studies suggest that the presence of competitors significantly reduces the response magnitude of invasive species to increased nitrogen deposition, and that some native species even gain a higher competitive advantage over invaders as nitrogen availability increases (Eskelinen & Harrison, 2014; Luo et al., 2014; Wang et al., 2017). Thus, the competition outcome between invasive plants and their competitors under elevated nitrogen deposition is still context dependent and needs further clarification.
Galinsoga quadriradiata is an annual invasive forb of Asteraceae, which is native to tropical America (Du et al., 2014; He et al., 2020; Liu et al., 2016b). This invasive plant has strong adaptability and has been widely distributed in most areas of subtropical, temperate and warm temperate zones around the world, mainly growing in farmland, abandoned land, roadsides, grasslands and other habitats with frequent human disturbance (Kabuce & Priede, 2010; Liu et al., 2016b; Liu et al., 2021b; Zhang et al., 2022). It was first introduced into East China (Jiangxi Province) around the beginning of the last century (earlier than 1943) (Liu et al., 2018; Liu et al., 2016b; Yang et al., 2018). It then gradually spread and established in the Southwest, Central, and North China (Yang et al., 2018). Galinsoga quadriradiata arrived at the southern slope of Qinling Mountains, where our study takes place, in Shaanxi Province about 10 years ago (Liu et al., 2016b; Liu et al., 2021b). This invasive plant impacts natural ecosystems and reduce crop production by about 50% when under high invasion (Kabuce & Priede, 2010; Liu et al., 2021a). Whereas its impacts are well understood, the mechanisms of invasion and the predicted expansion under increasing nitrogen deposition, an ongoing scenario in China, are still far from been fully uncovered. In this study, we conducted a common garden experiment to detect the differences in response between central and edge populations ofG. quadriradiata to their competitors with different co-evolutionary histories in the context of increased soil nitrogen deposition. We hypothesized that: (1) the central population of G. quadriradiata would display higher competitive ability and growth when compared to the edge population; (2) nitrogen addition will increase the growth and reproductive performance of both populations, but the central population will have a stronger response to increases in nitrogen deposition than the edge population; (3) the competition outcome in the context of increased nitrogen will depend on the co-evolutionary history between G. quadriradiataand its competitors with the new competitors showing a stronger inhibition on the invader’s response under high nitrogen, compared to the old competitors.
MATERIALS AND METHODS
Greenhouse experimental design
We chose Glycine max , Zea mays , and Glebionis coronaria , as old competitors which have longer co-evolutionary history with G. quadriradiata ; while Medicago sativa ,Achnatherum splendens , Sorghum bicolor , Sonchus wightianu , and Artemisia capillaris , as the new competitors. These competitors belong to Poaceae, Fabaceae and Asteraceae respectively. They are common species in the distribution range ofG. quadriradiata , and they are also the main competitors. Since the invasive plant commonly colonize in crop lands, we chose three common crop species as competitors. The new competitor species are commonly distributed in north and west China, while the old competitors are found in central and east China. We chose the species based on field surveys prior to the beginning of this experiment (Liu et al., 2021a; Liu et al., 2016b; Yang et al., 2018). The seeds of the central population ofG.quadriradiata and the old competitors were collected from four sampling sites in Lushan, Jiangxi Province in the Fall of 2017, while the seeds of the edge population were collected from three sampling sites (Fig. 1, Table S1) at different elevations in Qinling Mountains, Shaanxi Province in the Fall of 2015. All collected seeds were stored in a 4 °C refrigerator until set to germinate. The seeds of the new competitors were purchased in the market prior to the beginning of the experiment in 2018.
In early May 2018, the seeds of all species were sowed in nursery pots filled with sterile substrate and they were let to germinate in an artificial climate chamber. Four weeks later, the seedlings (about 5 cm high) were transplanted into plastic pots (diameter 16 cm, height 14 cm), and moved into the greenhouse. The pots were filled with sand and soil in a 1:1 proportion by volume. To compare the inter-specific competition intensity, the seedlings were planted in isolation (single-culture) or together with a native competitor (mixed-culture).
In the single-culture treatment, one seedling of the central population or edge population of G. quadriradiata was transplanted into a pot. In the mixed-culture treatment, one seedling of G. quadriradiata and one seedling of a new or old competitor were planted in the same pot. Seedlings from each population of G. quadriradiata were planted with all the other competitors, respectively. After transplanting, all potted plants were subjected to nitrogen treatments in a full factorial design. Each pot was watered with 5 ml deionized water (ambient nitrogen) or 3 g L-1 NH4NO3 solution (low nitrogen addition) or 6 g L-1NH4NO3 solution (high nitrogen addition) monthly. The low nitrogen treatment was set to simulate the current3 nitrogen deposition rate in China (Liu et al., 2013; Zhao et al., 2017) while the high nitrogen addition refers to double the current nitrogen deposition rate. For each combination of treatment, there were 10 replications for the single-culture and 8 replications for the mixed-culture. All pots were randomly arranged in an 80 m2 greenhouse to avoid the influence of herbivores, and watered every two days to keep the soil wet. The experiment had in total 888 pots and 1656 individuals planted.
From July to August 2018, the three uppermost fully expanded young leaves of each of three randomly selected individuals from each treatment were chosen for photosynthetic measurements. Photosynthesis was measured under sunny conditions at a sequence of light levels (1200, 1000, 800, 600, 400, 200, 100, 60, 20, 0 μmol m-2s-1 PPFD) using a Li-6800 Portable Photosynthesis System with a Red/Blue LED Light Source (Li-Cor, Lincoln, NE, USA). Light curves and maximum net photosynthetic rate (P max, μmol CO2 m-2 leaf area s-1) were calculated by a mechanistic model for the photosynthesis–light response based on the photosynthetic electron transport of photosystem II in C3 and C4species (Ye et al., 2013).
In early September 2018, all parts of the plants were harvested. The fresh leaf area was measured immediately after harvest by leaf analysis system WinFOLIA Pro (WinFOLIATM, Régent Instruments Inc., Québec, QC, Canada). The number of capitula per plant was counted before the harvesting as a metric of species performance. The dry weight of the root, leaf, capitula, and stem of each individual was separated and weighted after the samples had been dried to a constant weight in an oven at 60 °C. Elemental analysis for leaf phosphorus concentration (LPC) and leaf nitrogen concentration (LNC) was performed on a continuous flow analyzer (SEAL Auto Analyzer III, Germany). Specific leaf area (SLA) was calculated as the ratio of the leaf area to the leaf dry weight. For each treatment, 0.5 g leaf powder was burned in a muffle furnace at 600 °C for 6 hours to measure an average ash content. The ash content was calculated as ash mass divided by the sample mass. The leaf caloric value was measured by a calorimeter (IKA-C6000, IKA, Germany) with 0.5 g leaf powder from each sample. The SLA, ash content, leaf caloric value, LNC obtained were used to calculate the leaf construction cost per unit of mass (CCmass) following Liu et al. (Liu et al., 2016a; Shen et al., 2011). The CCmass was calculated using the ash content, leaf caloric value, SLA, and LNC. Root mass ratio (RMR) was calculated as the ratio of root dry mass and total dry mass.
The relative competition index (RCI), which represents the intensity of interspecific competition betweenG. quadriradiata and competitors, was calculated according to the following equation (Vilà & Weiner, 2004):
RCI = (M 1M 2) / M 1 (1)
Where, M 1 is the total dry mass of G. quadriradiata under single-culture conditions.M 2 is the total dry mass of G. quadriradiata in the mixed-culture conditions. The RCI values were generally positive, and the higher RCI value the higher intensity of interspecific competition.
Data analyses
A generalized linear mixed model (GLMM) was performed to evaluate the effects of the nitrogen treatments (NTC), invasive plant population types (PopType), culture types (Culture), and competitor types (CompType, nested in Culture) on the total mass, the number of capitula per plant, RMR, LPC, LNC, or P max. The effects of NTC, PopType, and CompType on the RCI or CCmass were also evaluated by GLMM.
All analyses were conducted using SAS 9.3 (SAS Institute Inc., Cary, NC, USA). The map with the sampling locations was made in ArcGIS version 10.2.2 (Environmental Systems Research Institute, Inc.), and other figures were made by SigmaPlot version 11.0 (Systat Software, Inc., San Jose, CA, USA).
RESULTS
Growth performance of G. quadriradiata
The result of the GLMM revealed that NTC (nitrogen addition), PopType (central vs. edge population), Culture (single vs mixed), and CompType (old vs. new competitors; nested in Culture) all had significant effects on plant total mass, and the interactions between CompType and PopType were also significant (Table 1). In general, the total mass ofG. quadriradiata increased with the nitrogen addition. The central population of G. quadriradiata obtained significantly higher total mass than the edge population. The total mass ofG. quadriradiata in the mixed-culture was significantly lower than that in the single-culture (Fig. 2). And the old competitors inhibited more the total mass of the central population when compared to the new competitors (Figs 2, S1).