4.2 Changes in the above- and belowground biomass allocation
fractions among different functional types
The RMFs of the two annuals in this study, C. glaucum andS. viridis , were less than 4%, which was much lower than their
SMFs (Figure 3). Since this experiment lasted for only one growing
season, the peak biomass represents the NPP. The patterns of above- and
belowground biomass allocation for annuals were further supported by
findings at the community level. We synthesized paired ANPP and BNPP
data from global grassland communities and found that despite
interference from other nondominant species (e.g., perennials), BNPP was
much lower than ANPP in grassland communities where annuals were the
dominant species (Figure 6a-c); this finding is consistent with the
overall trend in annual plant biomass allocation in this study. This
pattern may be related to the opportunistic survival strategies of
annuals. Due to their rapid growth and short life history cycles, they
need to allocate more photosynthetic products to shoots to maximize
photosynthesis and reproductive yield (Grime, 2001). In addition,
annuals are less tolerant to environmental stress than perennials, and
the belowground parts of annuals often have no energy storage organs;
together, these factors result in the aboveground biomass of annuals
being larger than their belowground biomass (Gedroc et al., 1996; Zhou
et al., 2014). In contrast, perennials in arid and semiarid grasslands
are often water- and nutrient-limited during growth processes and
generally opt for a ‘conservative’ strategy, allocating a large
proportion of their productivity to the root system in order to better
adapt to drought conditions or nutrient-poor environments (Balachowski
& Volaire, 2018).
The RMF and R/S of the perennial C4 plant C.
squarrosa were significantly lower than those of the three other
C3 perennials in this study (Figure 3). This result is
consistent with the results from global field community studies showing
that the BNPP/ANPP ratio was lower in communities dominated by
C4 plants than in communities dominated by
C3 plants (Figure 6d-f). This may be attributed to the
fact that plants have evolved thinner roots throughout their
evolutionary history; C4 plants, which are recognized as
being younger than C3 plants, consistently have thinner
absorbing roots and higher water and nitrogen uptake efficiencies than
C3 plants (Edwards et al., 2004; Ma et al., 2018;
Osborne, 2008). In addition, the C4 photosynthetic
pathway has a higher affinity for CO2 than the
C3 photosynthetic pathway, which allows
C4 plants to use smaller stomatal openings for
CO2 uptake and minimize their water losses through
transpiration (Chapin et al., 2011). Therefore, C4plants do not require excessive C to be allocated to their roots.