Abstracts
Root hemiparasiticPedicularis kansuensis is an invasive native species in China and has been expanding in the Bayanbulak Grassland of Xinjiang Uygur Autonomous Region over the past decade, threatening the local livestock industry. To understand why this damaging species expands in some areas but not in others, we compared soil water content, soil nutrient status, and plant community structures between heavily infected and non-infected sites. We hypothesized that soil nutrient levels and plant species composition would be more beneficial to the spatial expansion of P. kansuensis in heavily infected areas than in non-infected ones. Quadrat (1 m × 1 m) surveys were carried out in the Bayanbulak Grassland. Species number, percent vegetation cover, aboveground plant biomass, plant height, and relative frequency were recorded. Three topsoil samples were taken at 0–10 cm depth from each quadrat and their nutrient statuses were determined. The results showed that (1)P. kansuensis invasion reduces the species richness of the community compared to the uninvaded area (3.07%), but increases the canopy cover overall (16.99%); (2) soil water content (SW) and soil nutrient content are the main factors that determine invasion by P. kansuensis , and SW plays the more important role; (3) SW controls P. kansuensis leaf stoichiometry, decreases the Ratio of leaf nitrogen (LN) to leaf phosphorus (LP), and affects LP (P < 0.005). Finally, after combining the results, we found that the soil factor accounted for 46.50% and plants accounted for 22.50% of P. kansuensis invasions in the Bayanbulak Grassland.
Keywords: root hemiparasitic plant; alpine grassland;Pedicularis kansuensis ; native species
Introduction
Biological invasion is a serious global environmental issue and is a major problem associated with global change (Drenovsky and Batten, 2007). In 1958, Elton (1958) defined biological invasion as being when a species expanded into a new area where its offspring could reproduce, spread, and sustain themselves. Theoretically, invasive species include invasive non-native plants and invasive native plants, but both of them have the common characteristic that they can spread and cause harm. However, current research on invasive ecology is mainly focused on invasive alien species (Ai Zemin et al., 2017) and little attention had been paid to invasive native species, especially, invasive native species that have parasitic properties.
Parasitic plants are special plants that complete their life cycle by obtaining nutrients from other living plants using parasitic organs called haustoria. There are about 270–275 genera of known parasitic plants. They contain nearly 4500 species and account for about 1% of the total number of angiosperm species (260 000) (Press and Phoenix, 2005). About half of the parasitic plant species are root hemiparasitic plants (Joel et al., 2013). They retain a certain photosynthetic capacity, but still need to obtain water, mineral nutrients, and some carbohydrate supplies from their host plants via haustoria (Tӗšitel et al., 2011). Aboveground, they compete with their host plants for light, but belowground, they depend on their host plant and obtain nutrients and water from the soil via the roots of their host (Cameron et al., 2008; Demey et al., 2013). Therefore, the relationship between hemiparasitic plants and the soil is more important and complicated than for other types of plants.
In general, as a significant component of the soil environment, soil water content (SW) plays a decisive role in the composition and distribution of plant communities (Li et al., 2003) and it has been shown that the addition of water will increase the occurrence of invasive plants (Blumenthal et al., 2008). In contrast, drought decreases stomatal conductance, and thus, reduces the aboveground biomass of plants (Knapp et al., 2002; Loydi et al., 2018). However, under natural conditions, grasslands are relatively dry or in dry periods, which means that the occurrence of hemiparasitic plants has little impact on the vegetation community in which they are located. However, they do reduce the resources available for community growth and development (Ameloot et al., 2005). Previous research has also found that under drought conditions, the hemiparasitic Rhinanthus minorhad little impact on the host and community species, which meant that it could not develop an advantage by changing species diversity, but in higher soil moisture content areas, such as in mesic grasslands,R. minor can effectively improve the diversity of community species (Těšitel et al., 2018).
Apart from SW, soil nutrients are essential nutrients for plant life, and soil organic carbon (SOC), nitrogen (N) and phosphorus (P) are the main nutrient indexes for soil. Soil organic carbon is also a vital factor in the formation of soil structure, which directly affects soil fertility, water holding capacity, soil erosion resistance and soil bulk density (Sterner and Elser, 2002; Yu et al., 2010). There is a general consensus that resources play an important role in determining the success or not of invasive species (Harpole, 2006). For example, non-native plants invasions have been correlated with changes in soil nutrients (Eller and Oliveira, 2017). Furthermore, studies also have confirmed that soil nutrients have important effects on the growth and development of the genus Pedicularis (Li and Guan, 2006; Decleer et al., 2013). For example, in areas where soil fertility and species richness are low, Pedicularis canadensis densities can reach a maximum (Hedberg et al., 2005). In contrast, the growth and development of the host plants are significantly affected by hemiparasitic plants in poor fertility soil and under good light conditions (Press and Phoenix, 2005; Borowicz and Armstrong, 2012). This could be related to macroelements, such as N, P, K, Ca, and Mg, in root hemiparasitic plants and microelements, such as Zn and B, that are mainly obtained from their host plants (Cameron and Seel, 2007).
Pedicularis kansuensis is an annual or biennial root hemiparasite plant that belongs to the genus Pedicularis , an endemic species to China, and it is mainly distributed in southwest and western China (Zhong, 1963). In recent years, it has occurred in Qinghai and Gansu Province, especially in the Bayanbulak alpine grassland of Xinjiang Uygur Autonomous Region. Its appearance has seriously affected the yield and quality of forage grass and threatens the local livestock industry (Li et al., 2006; Sui et al., 2015; Bao et al., 2015). Similar to other invasive plants, it has the characteristics of a greater ripening rate and strong adaptability, and often spreads rapidly in grassland ecosystems in the form of a cluster distribution within a short period of time (Liu et al., 2008; Sui et al., 2016). According to the incomplete statistics, its hazard area is up to 2.33×104 ha and it is spreading at a rate of 3.3×103 ha year–1 (Song, 2006; Liu et al., 2008). Therefore, clarifying the expansion factors associated with P. kansuensis in the Bayanbulak alpine grassland ecosystem in Xinjiang is important.
According to a field survey report, P. kansuensis is generally distributed in cold and high-altitude areas, it has a greater requirement for moist conditions (Wang et al., 2007), and is a native species in the Bayanbulak alpine grassland. Our previous study indicated that the nearer the water source, the higher the P. kansuensisdensity and the greater its distribution area (Liu et al., 2011). Our previous research also found that the P. kansuensis became extinct when 90 kg N ha−1 year−1 of nitrogen was added (Liu et al., 2017). Therefore, it is important to identify the roles played by SW and nutrient contents in the rapid spread of P. kansuensis . To investigate these factors, we set up eight sampling sites each in infected and non-infected areas of the Bayanbulak alpine grassland based on different constructive species, investigated plant community structure, species diversity, SW, and soil nutrient differences between the heavily infected and non-infected sites, and addressed the following questions: (1) how does the community structure change after P. kansuensis invasion; (2) what kind of ecological environment is beneficial to P. kansuensis ; and (3) is it water or nutrients that determine invasion by P. kansuensis ?
Materials and methods