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.