Root morphological responses to population density vary with soil conditions and growth stages: The complexity of...

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 days.

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.