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.

Data collection

Plants were harvested at 30, 50 and 70 d of growth, representing developmental stages of early vegetative growth, late vegetative or early reproductive growth, and middle to late reproductive growth respectively. At each stage, six individual plants were randomly chosen from each plot, making a total of 6 replicates × 3 plots × 3 densities × 2 soils × 3 stages = 324 samplings. For each individual plant, the following traits were measured if applicable: diameter at the basal of the main root, length and number of lateral roots (above or equal to 1 mm in diameter along the main root). Morphological root traits were not measured at 30 d of growth due to small sizes of plants. Each individual was then separated into roots, stems, petioles, leaves, reproductive organs and branches (if any), enveloped respectively, and oven-dried at 75 ^oC for 2 d and weighed.

Statistical analysis

All statistical analyses were conducted using SAS statistical software (SAS Institute 9.0 Inc. 2002). Biomass traits including shoot mass, root mass, total biomass and root:shoot ratio, and morphological traits including main root length, main root diameter, lateral root number and lateral root length were analyzed. To minimize variance heterogeneity, all data were log-transformed before statistical analysis. Three-way ANOVA and ANCOVA were used to evaluate overall effects of growth stage, soil conditions and population density and their interactions on all traits, with total biomass nested in growth stage as a covariate in three-way ANCOVA. Within each soil condition at each stage, effects of density were analyzed by one-way ANOVAs for total mass, and one-way ANCOVAs for all the other traits with total mass as a covariate. For a given trait, it was considered to exhibit apparent plasticity whenever plant size (total biomass) accounted for significant variation in its response to density, and any variation in its expression that was independent of total biomass was considered an indication of true plasticity (after removal of size effects) (McConnaughay and Coleman 1999, Weiner 2004). Multiple comparisons used Least Significant Difference (LSD) method of the General Linear Model (GLM) program in one-way ANCOVA, which also produced adjusted mean values and standard errors.

The data includes two sheets, sheet1 is for traits of root mass, shoot mass and root to shoot ratio of all three stages, sheet2 is for root morphological traits of the latter two stages.

Line 1-5 for each sheet are original lables for data, can be neglected, and data of all blocks has been put together, as effects of blocks were not considered. 

Line 6-8 are factors considered in analyses, and Line 9-13 are values measured, each of which came from one individual plant.