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.