Root morphological responses to population density vary with soil conditions and growth stages

How plants cope with increases in population density via root plasticity
is not well documented, although abiotic environments and plant ontogeny
may have important roles in determining root response to density. To
investigate how plant root plasticity in response to density varies with
soil conditions and growth stages, we conducted a field experiment with an
annual herbaceous species (Abutilon theophrasti). Plants were grown at
low, medium and high densities (13.4, 36.0 and 121.0 plants ▪ m-2,
respectively), under fertile and infertile soil conditions and a series of
root traits were measured after 30, 50 and 70 d. Root allocation
increased, decreased or canalized in response to density, depending on
soil conditions and stages of plant growth, indicating the complex effects
of population density, including both competitive and facilitative
effects. Root allocation was promoted by neighbor roots at early stages
and in abundant resource availability, due to low to moderate belowground
interactions among smaller plants, leading to facilitation. As plants
grew, competition intensified and infertile soil aggravated belowground
competition, leading to decreased root allocation in response to density.
Root growth may be more likely restricted horizontally rather than
vertically by the presence of neighbor, suggesting a spatial-orientation
effect in their responses to density. We emphasized the importance of
considering effects of abiotic conditions and plant growth stages in
elucidating the complexity of density effects on root traits.