Response to above-ground interaction only
Our study did not include results on response to above-ground
interaction only. To interpret this issue more explicitly, however, we
quoted results from light-effect studies (Table 3b), which simulated
density effects via manipulating variations in light quantity and
quality. Although light signals cannot completely equate to above-ground
interaction, but these results provided essential information, which is
difficult to acquire using density or interaction treatments, especially
when plants are expected to grow in more nature or normal state.
From these results, we found above-ground interactions can had no
effects at low to moderate levels, or had stronger negative effects on
root allocation (Table 3b) than below-ground interactions (Table 3a).
Light competition may not affect total mass and root allocation at low
intensity (Murphy and Dudley 2007), or decrease them at high intensity
(Kennedy et al. 2007). Similarly as below-ground interaction, the
strength of above-ground interaction may first increase then decrease
over time (Hutchings and Budd 1981, Wang et al. 2017). And plants
interacting with neighbors may alter biomass allocation while
maintaining total mass at first, then alter total mass as interaction
intensifies. When total mass did not respond to light while root
allocation decreased (Forster et al. 2011), it suggested
above-ground interaction was getting increasingly intense and began to
affect root allocation, but not strong enough to affect total mass. No
response in root allocation and a decrease in total mass indicated
attenuated above-ground interaction (Casper et al. 1998). It is
interesting to note that root mass allocation in shade was lower than
that in full light, but unaffected by density (Table 3b, 3c; Forsteret al. 2011), suggesting the decrease of root allocation by
above-ground interaction can be alleviated by facilitative effects of
below-ground interactions in dense population .