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.