1 INTRODUCTION
Negative density dependence in a plant community is mainly reflected in the density dependence, distance dependence, and community compensation effect, which is not only a reflection of intraspecific or interspecific competition but also a reflection of light, soil moisture and nutrient status after competition. Negative density dependence represents a self-thinning type of attenuation and the embodiment of the realized niche. In the past 20 years, ecologists have studied the patterns and strengths of conspecific - and heterogeneous negative density dependence (Adler et al., 2018; Chisholm & Fung, 2018) in various plant taxa, including grassland and woodland plant species (Johnson et al., 2012; Detto et al., 2019; Forrister et al., 2019) and especially tropical rainforest species (Bagchi et al., 2011; Johnson et al., 2017; Kellner et al., 2018). Some ecologists have explored the relationship between negative density dependence and biodiversity. For example, abundant species exhibit weaker negative density dependence than rare species, and species-rich regions show stronger negative density dependence than species-poor regions (Johnson et al., 2012). However, the driving mechanisms of negative density dependence continue to attract the interest of ecologists.
Plant density is one of the most important community characteristics (Grime, 2001) and is the basis of plant community biodiversity (Tilman, 2000). Density dependence is a very common property of plant populations or communities. Generally, plant density exhibits a Malthusian or logistic growth process in the growth process of plant populations. When density dependence occurs, plant density is followed by negative feedback of growth. Whether the competition resulting from this negative feedback is consistent with the intraspecific (Verhulust, 1838) or interspecific competition model (Volterra, 1926), plant populations are all manifestations of the exploitive competition of plants for resources. In particular, plant density is strongly affected by competition for soil moisture and nutrients, which can influence plant stoichiometry, e.g., the N:P ratio.
The law of minimum of Liebig (1840) suggests that plant growth depends on the availability of the scarcest resource. The Shelford’s (1913) law of tolerance suggests that a species has tolerance limits for certain factors, beyond which it cannot survive. These two laws reveal the mechanism by which a plant’s stoichiometric N:P ratio can affect vegetation. The stoichiometric N:P ratio in plant tissues can predict the supply of nutrients such as N and P (Koerselman & Meuleman, 1996; Kranabetter, Harman-Denhoed, & Hawkins, 2019; Tian et al., 2019). Some studies have shown that the nutrient contents of leaves can reflect the soil nutrient supply (Luo et al., 2017; Liu et al., 2018), and plant stoichiometric N:P ratio has been used as an indicator of the environmental supply of nutrients to plants (Aerts & Chapin, 2000; Güsewell, 2004; Matzek & Vitousek, 2009). Shaver & Chapin (1995) and Bedford, Walbridge, & Aldou (1999) point out that the stoichiometric N:P ratio in plant tissues can serve as an indicator of whether vegetation is restricted or plant growth is affected by N or P, conditions that are significant for maintaining species richness in ecosystems and biodiversity. Güsewell, Koerselman, & Verhoeven (2003) point out that the relative utilization rate of N and P in plant communities could be reflected by the stoichiometric N:P ratio in plant tissues and that deficiencies in N and P could be predicted.
Although plant density is related to productivity (Grace, 1999; Grime, 2001), it is also affected by the plant N:P ratio. Some studies have shown that the stoichiometric N:P ratio in plants reflects the degree of species endangerment. For example, Venterink et al. (2003) measured the N:P ratio in vascular plant tissues at 150 wet-point sites in Poland, Belgium, and the Netherlands to determine whether each site was restricted by N or P by examining the growth of plant communities at various sites and compiling data on endangered species. They found that endangered species were growing predominantly in phosphorus-limited wet sites and that the abundance and productivity of endangered species decreased with increasing P. At present, there is a lack of a clear statistical relationship between plant N:P ratio and plant density.
Theoretical analysis indicates that the plant N:P ratio can affect plant density (Elser et al., 2000), especially in arid and semi-arid regions. Here, we hypothesize that in arid and semi-arid regions, the transition to the late stage of a population or a community is accompanied by an increase in the vegetation N:P ratio due to P consumption by plants. Furthermore, we hypothesize that the higher vegetation N:P ratio leads to a self-thinning type of attenuation of plant density, which occurs during the negative density dependence. In this study, we investigated the effect of changes in the stoichiometric N:P ratio of vegetation and associated characteristics on plant density in a succession series in a semi-arid region. Our goal was to determine whether the vegetation N:P ratio is the mechanism driving negative density dependence.