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 .