Conclusions
In conclusion, plant root growth may increase, decrease or canalize, in response to the increase of density, depending on the strength of above- and below-ground interactions, which can vary with soil conditions and plant growth stage. (1) As interaction intensity increases, plants first alter biomass allocation to keep plant size stable, then suffer a reduction of plant size. Low to moderate below-ground interaction is more likely to facilitate root growth, but the effects fade away when interaction is intense; above-ground interaction mainly induce negative root response at high intensity. (2) Soil conditions altered root response mainly through effects on below-ground interactions, growth stage affected root response mainly through effects on plant size and both kinds of interactions. (3) By comparing root and leaf responses to density, we found a negative relationship between them. It shed light on the necessity of considering both below-ground and various above-ground modules in competition studies. (4) Increased density was more likely to restrict root growth horizontally than vertically. It is more efficient for plants to forage greater depth for resources, than competing for the deficient resources in upper soil.
The study underscored the important roles of abiotic environmental factors and plant ontogeny in determining plant response to biotic environmental factor, which may at least partly explain the inconsistent results in relevant studies. The complexity of plant response to density can be better understood under different backgrounds of abiotic factors and plant ontogenetic stages.