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.