4.1 Large discrepancies between the R/S ratios from greenhouse
experiments and field investigations
In this study, we found that the individual-level mean R/S values for
the six common species from the temperate grasslands in northern China
ranged from 0.04 to 0.92 (Figure 3), which were lower than those from
global and regional field community investigations (Table 2).
Specifically, our result is much lower than that
(1.66~15.21) of a field community survey performed in
the same region (Wang et al., 2008); the R/S of perennials in this study
is close to the result (0.84) of individual-level field observations
(Wang et al., 2010) but still lower than that
(0.75~2.98) of genus-level surveys (Luo et al., 2013).
Considering the field sampling process, we suggest that the R/S obtained
in field community studies may be strongly overestimated for three main
reasons. First, field sampling methods are usually not equipped to
accurately identify dead and live roots and are more prone to including
dead roots in root samples, resulting in an overestimation of root
biomass and therefore of R/S (Wang et al., 2010). Indeed, the large
discrepancy between the R/S results at the individual and community
levels in the same area can also support this view. For example, Wang et
al. (2010) reported that the median R/S at the individual level in
northern China’s grasslands was only 0.84, which was much lower than
that at the community level (Table 2). In fact, root sampling at the
individual level is more accurate than sampling at the community level
because root systems of the same species are relatively uniform in
morphology and colour, which makes it easier to distinguish living roots
from dead roots. Second, grazing by herbivores can reduce shoot biomass
to some extent, resulting in an underestimation of shoot biomass and the
subsequent overestimation of R/S (Mikola et al., 2009). Third, for
mature communities in the field, the dominant species are mainly
perennials, and there are much fewer annuals. Our study found that the
RMFs of annual grasses were extremely low. Therefore, if the R/S derived
from perennial plant samples is used to estimate the root biomass of
annuals, it will be highly biased.
The overestimation of R/S can occur during field sampling, but
greenhouse pot experiments can effectively circumvent some of the
shortcomings of field sampling. First, the roots of greenhouse plants
can be fully sampled and estimated. The cultivation substrate in this
study was homogeneous and uncomplicated and could be easily separated
from the root system. Second, the greenhouse growth environment is
consistent and stable among replicates; shoot biomass is not lost due to
animal grazing, and the influence of microenvironmental variability on
biomass allocation is avoided. We also acknowledge that greenhouse
cultivation experiments have some drawbacks, such as the possibility of
underestimating root biomass due to the height limitation of the
planting pots, which limits the root growth space to some extent. In
addition, for perennials, cultivation for only one growing season may
result in some underestimation of root biomass if the root turnover time
is greater than one year (Gill & Jackson, 2000). However, despite
certain drawbacks, greenhouse cultivation experiments may still be a
more accurate method than field sampling for measuring the R/S of
different species in the future.