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