Discussion
Although root morphological traits generally decreased, root mass allocation increased, decreased or canalized, with increased density, depending on different soil conditions and growth stages. These suggested the responses of plant roots to density is not merely a single or simple reaction, but of complexity, including multiple responsive trends within and among traits. Density effects included effects of above- and below-ground interactions respectively, the strength of which can vary with soil conditions and growth stage in actuality. The response of plant total mass mainly reflected the passive effects of intraspecific interactions and its intensity, while responses of root:shoot ratio and root morphological traits explicitly revealed the strategies of plants in dealing with such biotic environmental effects (plant active reactions).
Due to the complexity of results, we used a table to illustrate this issue, by classifying plant response to density into three circumstances (Table 3): (a) response to below-ground interaction only (when above-ground interaction is negligible), (b) response to above-ground interaction only (when below-ground interaction is negligible), and (c) response to both kinds of interactions or response to density (effects of both interactions are not negligible). Within each circumstance, the intensity of above- or (and) below-ground interactions can be estimated as low (L), moderate (M) or high (H) level according to the response of plant total mass. To reinforce our conclusions, we also cited results from other studies in each circumstance.