Introduction
Population density, or competition, is one of major biotic environmental factors in nature. The increase of density can result in variations in multiple resources and plant-plant interactions (Casper et al.1997), affecting plant growth in many ways. However, the complexity of density effects is not well understood. Researches on effects of above- and below-ground competition separately (Cahill 2003, Murphy et al. 2007) or those of light quality or shade due to increased density (Bongers et al. 2018, Forster et al. 2011) contributed important information, but cannot help understanding how plants respond to density in an integrative perspective.
One aspect of the complicacy is the effects of abiotic environmental factors in plant response to density (Forster et al. 2011). Below-ground competition can be aggravated when below-ground resources limit plant growth (Casper and Jackson 1997, Schenk 2006). Increased soil resources can result in a shift in competition from occurring primarily below-ground to primarily above-ground (Tilman 1988, Wilsonet al. 1991), and an increased interaction between above- and below-ground competition (Cahill 1999). Consequently, soil conditions can significantly alter plant response to density (Poorter et al.2012). However, little evidence exists. A study showed above- and below-ground competition elicited independent responses, and the level of soil nutrient did not affect root response to the presence of neighbors (Murphy and Dudley 2007). Perhaps the low-nutrient regime did not cause true nutrient deficiencies, especially when competition was not intense. Substantial abiotic effects may result from effects of low vs. high levels of many resources, or infertile versus fertile soil conditions.
Another important aspect of the complicacy is temporal heterogeneity of density effects. For a dense population, as plant sizes grow, competition intensity first increases then weakens (Hutchings et al. 1981). Moreover, from the perspective of allometric growth, a plant experiences significant changes in allocation pattern at various developmental phases (Harper et al. 1970, Weiner 2004). However, allometric analysis or removal of size effect within a single stage cannot eliminate ontogenetic effects, as plasticity of allometric relationships in response to density is stage dependent as well (Liet al. 2013). Therefore trait plasticity in response to density is expected to differ among stages, which may explain the inconsistency in relevant results.
Compared to above-ground responses, below-ground responses to density or competition received much less attention. Studies have mostly focused on root: shoot ratio or root mass allocation, producing inconsistent results: 1) neither above- nor below-ground competition alters root allocation (Cahill 2003, Casper et al. 1998); 2) interactions among plants enhance root: shoot ratio (Gersani et al. 2001, O’Brien et al. 2005); 3) root mass ratio is lower at high density relative to low density (Forster et al. 2011, Poorter et al. 2016, Poorter et al. 2012). The inconsistency may result from distinct density effects in different research systems, which vary in manipulation of density treatments, amounts of species, abiotic environments, and stages of plant growth etc., wherein plant growth stage and abiotic environments should be the most significant of all.
Finally, but not the least importantly, meta-analyses showed that lab grown plants experience different abiotic and biotic environments from those grow in fields, and have a much shorter time for growing, which may strongly affect a plant’s overall morphology and physiology (Poorteret al. 2016). It is thereby necessary to apply growth regimes closer to field conditions for understanding density-induced plasticity that real occurs in nature (Gratani 2014, Poorter et al. 2016). Here we conducted a field experiment, growing plants of an annual species Abutilon theophrasti at different densities, under fertile versus infertile soil conditions, to measure a series of root traits at three stages of plant growth, in order to investigate whether and how plant root response to density can be altered by soil conditions and plant growth stage.