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.