Responses of different root traits
Different root morphological traits differed in response to density:
those mainly amplify into horizontal space such as lateral root length
and number, and main root diameter, were more likely to decline with
increased density, while those expands primarily into greater depth such
as main root length unaffected. These implied an effect of spatial
orientation of density on root propagation: the presence of neighbor
tended to restrict its expansion horizontally rather than vertically
(Gundel et al. 2014). For example, high density increased roots
of apple (Malus sp.) into deeper rather than upper soil layers
(Atkinson et al. 1976), similar to other results (Mason et
al. 1982, Pearson et al. 1985). Belowground modules of bulb that
enlarge horizontally were decreased by high density, but not for roots
that grow into greater depths (Li et al. 2011).
The presence of neighbors in the horizontal space may have reduced
nutrients available, making it is inefficient to forage a greater range
for satisfactory resources (Semchenko et al. 2007). Therefore, as
long as the deeper space is available, it is more intelligent to expand
roots into greater depth to acquire resources, than scrambling for the
deficient upper-soil resources (Gundel et al. 2014). Such
spatial-orientation effects of density can also be proved by alleviated
reduction by density in lateral root traits in fertile vs. infertile
soil. When inefficient root placement cannot be avoided, as plants grown
in pots with small depth, reproductive growth will inevitably decline,
exactly as in the phenomenon of ‘tragedy of commons’ (Gersani et
al. 2001). Due to the costs of producing plasticity, an extra root
proliferation will be costly itself. If it brings no benefits at all, or
sheer expenditure without subsequent income, plants will be unable to
maintain reproductive growth.