4.3 Density effects on above- and belowground biomass allocation
and allometric relationships
Our greenhouse manipulation experiment revealed three trends in the RMF
and R/S with increasing density among the studied species: statistically
insignificant changes (C. glaucum , L. chinensis andS. grandis , Figure 4b, h, l), an increasing trend (C.
squarrosa , Figure 4f) and fluctuating patterns (S. viridis andM. sativa , Figure 4d, j). This result suggests that density has
different effects on biomass allocation across species. For C.
glaucum , L. chinensis and S. grandis , the density-induced
above- and belowground competition was symmetrical (Weiner & Thomas,
1986). For C. squarrosa , the RMF and R/S increased with
increasing density, indicating that competition was more intense
belowground than aboveground at higher densities; relatedly, a higher
proportion of biomass must be allocated to roots to achieve more
efficient nutrient and water uptake (Berendse & Möller, 2009). The
relative intensity of above- and belowground competition in S.
viridis and M. sativa varied with density. For example, M.
sativa had the highest R/S at a moderate density (D3, 225 plant
m-2). This density may be the threshold value for
above- and belowground biomass allocation in M. sativa ; when
planting densities are close to this value, this species is prone to
allocate more biomass to its roots, which is not conducive to increasing
forage yields.
Our results further showed that planting density significantly regulated
the allometric scaling exponents of shoot biomass against root biomass
for C. glaucum , M. sativa and S. grandis (Table 1;
Figure 5). These patterns do not support the ontogenetically fixed
scaling exponents predicted by allometric partitioning theory, which
suggests that scaling relationships are insensitive to biotic or abiotic
factors (McCarthy & Enquist, 2007; Müller et al., 2000; Weiner, 2004).
Rather, the allometric relationships in this study were altered by the
planting density. This inconsistency is likely due to that these three
species do not possess the typical fractal structure (e.g., S.
grandis has an expanded tiller node rather than a fractal structure)
that is the basis for the vascular plant allometry model (Wang et al.,
2010; West et al., 1999). In contrast, for S. viridis , C.squarrosa and L. chinensis , the planting density
had statistically insignificant effects on the allometric scaling
exponents of shoot biomass against root biomass; for M. sativa,the scaling exponents gradually decreased with increasing density (Table
1; Figure 5). This difference suggests that the effects of planting
density on plant allometric relationships between above- and belowground
biomass are species-specific. The stability or instability of the
scaling exponent response to planting density might be attributed to the
balanced or unbalanced limitations on light and nutrients under
different competitive conditions (Poorter et al., 2015). Overall, the
allometric partitioning theory derived from global data analyses of
vascular plants is not applicable to the studied herbaceous plants, and
the effects of planting density on allometric relationships should be
considered when using predefined scaling relationships to quantify the
root biomass of herbaceous plants (Yan et al., 2019).