Conclusions
Our results showed an increase in phenotypic integration, plasticity and among-individual variation with density, and the positive correlations between integration and plasticity became stronger with higher densities but weaker over time in fertile soil, while the correlations among the three processes became stronger over time in infertile soil. These suggested certain shared or cooperation mechanisms for integration, canalization and plasticity in traits. Soil conditions and growth stage may affect the responses to density in correlations among the three processes via effects on plant size and competition intensity. Phenotypic integration may play a more important role in plant adaptation to environmental stresses, and the decrease in canalization level may not be harmful. The released variations among individuals due to attenuated canalization and enhanced integration may facilitate plant reaction and adaptation to environmental stresses, and inversely plasticity may lead to higher level of phenotypic integration and reduced canalization, especially when plants experience intense competition.