1. Introduction
The allocation of biomass to above- and belowground organs is one of the
most important functional traits in plants (Dolezal et al., 2020;
Enquist & Niklas, 2002). This trait not only reflects plant survival
strategies but also has a significant influence on soil carbon (C)
inputs (Poeplau, 2016; Rasse et al., 2005; Umaña et al., 2020).
Grasslands are the most widespread terrestrial ecosystem and have
important regulatory functions in the global C cycle (Dixon et al.,
2014; White et al., 2000). The accurate assessment of the size of C
pools in belowground biomass is an important issue for future research
on grassland C cycling (Ahlstrom et al., 2015; Mokany et al., 2006). Due
to the difficulty of root sampling, the belowground biomass of
grasslands is usually indirectly estimated with the aid of the
easier-to-measure aboveground biomass and empirical root to shoot ratio
(R/S), which has become a key parameter for estimating the size of
grassland C pools (Jackson et al., 1996; Mokany et al., 2006).
Therefore, exploring the above- and belowground biomass allocation
patterns of herbaceous plants and their modulating factors is pivotal
for deepening our understanding of plant growth, survival, and carbon
cycling in terrestrial ecosystems (Poeplau, 2016).
For grassland ecosystems, the R/S reported from previous field surveys
is generally greater than 1 and varies widely among different studies.
For example, Jackson et al. (1996) reported that the global mean value
of R/S was 3.7; Mokany et al. (2006) combined data from community
investigations and calculated a global mean R/S of 4.5 for temperate
grassland communities. Yang et al. (2010) estimated a mean R/S value of
6.3 for grassland communities in China using field investigations from
265 sample sites, while Ma et al. (2006) obtained a mean R/S value of
61.3 for the Inner Mongolian grassland based on the synthesis of
field-measured data; the R/S value at some of these sample sites reached
over 400. In addition, results at the genus level differ greatly from
those at the community level. For example, a study based on grasslands
in northern China showed that the R/S of the four dominant genera
(Stipa spp ., Cleistogenes spp ., Agropyron spp ., andLeymus spp .) ranged from 0.75 to 2.98 (Luo et al., 2013), while
the median R/S at the individual level for this region was only 0.84
(Wang et al., 2010). Although a large amount of observational data has
been accumulated globally over the past few decades, it is still
difficult to reach a unified understanding at the global scale through
data syntheses, because large unreliable datasets can deteriorate
accurate statistical analyses and interpretations of the results (Mokany
et al., 2006). In addition, community-level studies are restricted to
reporting the weighted mean value of R/S for a given community without
considering the large variations in R/S at the intra- and interspecific
levels. To date, the reasons for the discrepancies in these results
among different studies are unclear, which presumably results in large
uncertainties in estimating the belowground C pools in grasslands.
Therefore, accurate manipulation experiments are necessary to broaden
our understanding of above- and belowground biomass allocation across
different species.
Previous studies have overwhelmingly concluded that plant biomass
allocation is jointly determined by both environmental and biotic
factors (Agathokleous et al., 2019; Gedroc et al., 1996; Poorter et al.,
2012). In terms of environmental factors, numerous studies have shown
that water conditions (Eziz et al., 2017), temperature (Reich et al.,
2014; Wang et al., 2016), nitrogen and phosphorus availability (Müller
et al., 2000; Peng & Yang, 2016; Yan et al., 2019) and their ratios
(Kumar et al., 2020), CO2 concentrations (Pei et al.,
2020), and soil texture (Xie et al., 2012) can have significant effects
on R/S. Among those biotic factors, planting density is not only an
important selection pressure in nature but also an important tool for
artificial pasture management (Boschma et al., 2019). It has been well
recognized that planting density can alter the above- and belowground
competition for light and nutrient among plants (Kira et al., 1953;
Weiner, 1986; Weiner & Freckleton, 2010; Yoda et al., 1963) and thus
influences biomass allocation. Through a global data synthesis analysis,
Poorter et al. (2012) found that on average, plants tended to increase
their R/S with increasing planting density. However, many site- or
species-specific studies have also found very complicated density
effects on R/S, with invariant (Casper et al., 1998), increasing
(Berendse & Möller, 2009) and decreasing (Hecht et al., 2016; Weiner,
1990) trends with increasing planting density. However, the density
effects on herbaceous plant biomass allocation, particularly across
multiple species of different plant functional types, are still less
well understood.
To address the knowledge gap mentioned above, we conducted a greenhouse
experiment with a total of six common species planted in five density
gradients. These species represent different plant functional types in
the temperate grasslands in northern China. We cultivated these plants
to the maturity stage and harvested the shoot and root biomass from
individuals of each species. We aim to address the following two
questions: (1) How does above- and belowground biomass allocation vary
among different plant functional types? and (2) How does planting
density regulate above- and belowground biomass allocation in these
species?