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.