4.2 Nutrient effects on invasion by P. kansuensis
Some researchers have reported that it was nutrition that played a decisive role in the success of alien species (Driscoll and Strong, 2018). For example, invasive plants often show a superior competitive ability at acquiring and using nutrients than natives, even in nutrient deficient areas (Funk and Vitousek, 2007). For example, it has been reported that the established species were not easily attacked by hemiparasitic plants in nutrient-rich habitats, even though the established species were the superior hosts. However, the reverse was true in nutrient-poor areas (Marvier and Smith, 1997). In our study,P. kansuensis was more prone to emerge in areas with high SWs and organic matter contents. Therefore, our results confirmed the above studies. Other studies have found that water stress can reduce aboveground biomass and primary productivity (Knapp et al. 2002; Yahdjian and Sala, 2006). Generally, root hemiparasitic plants obtain nutrients, such as water and inorganic materials, from their host. Therefore, regions with higher SWs are more favorable to P. kansuensis growth. Other researchers think that nutrient allocation strategies are used. They have suggested that invasive plants might develop different biomass allocation strategies than native species in order to ensure a successful invasion (Press and Phoenix, 2005; Demey et al., 2013). For example, studies have found that under the interaction of temperature increase and nitrogen addition treatment (W × N), an invasive population of Plantago virginica (PV-In) and the native population (PV-Na) had different biomass allocation strategies when in competition with the local competitor P. asiatica (Luo et al., 2020).
We found that there was a significant positive correlation between SK and LK, and SP:SK and LP:LK. Soil water content was significantly negatively correlated with LP:LK and extremely significant positively and significantly positively correlated with LP and LN:LP, respectively. This could be related to the fact that the N:P ratios in plant organs can be regulated by internal nutrient transfer (Zhang et al., 2016). In other words, the plant itself has stoichiometric plasticity and can adjust its elemental balance under environmental pressure (Sterner and Elser, 2002). Furthermore, soil nutrients are widely used to manipulate resource availability to reduce invasion by exotic species in fragile ecosystems because they play a key role in the success of invasive species (Perry et al., 2004). Meanwhile, it has reported that increasing nutrient supply modulated the dominant plant defensive tactics from tolerance to induced resistance (Burghardt, 2016). For example,Bromus tectorum invasion was controlled by regulating the nutrient ratio in California (Walker et al., 2017), but in the wetlands, carbon addition combined with repeated biomass harvesting was used to reduce invasion by Phalaris arundinacea (Perry et al., 2004). The root hemiparasitic characteristics shown by P. kansuensis means that it mainly obtains water and other inorganic materials from its host, which is why sites with high SOC were used in this study. Our results also agreed with previous research that showed that invasion by root hemiparasitic plants was mainly affected by soil nutrient levels (Borowicz and Armstrong, 2012).