Developmental canalization
Theoretically, it is predicted increments in environmental variations may lead to relatively higher levels of plasticity in morphological traits (Bradshaw, 1965; de Jong, 1995), whereas fitness-traits may be more likely to maintain stable under a range of environmental conditions (Lerner, 1954; Stearns & Kawecki, 1994; Waddington, 1957; Wagner et al., 1997). Characters more closely related to fitness are expected to be better buffered against environmental effects because deviations from the optimal phenotype will be strongly selected against (Clarke, 1995; Lerner, 1954; Stearns & Kawecki, 1994; Waddington, 1957). These predictions have been proved by the contrasting performances in the degree of plasticity in response to temperature between morphological traits and fitness traits (Liefting et al. , 2009). However, we did not find reproductive mass ratio had lower plasticity in response to density than other traits, and its among-individual variation (CV) was higher than other traits at different densities, consistent with other studies (Woodset al. , 1999). It suggested the relative stability of fitness traits depends on the level or range of environmental variations, and increased phenotypic variations can be produced under highly stressful conditions that plants are incapable to adapt (Woods et al. , 1999). When the stress is severe enough, the buffering against drastic changes may no longer be able to prevent such overt changes, and phenotypic variability in more robust traits might assist survival at the population level (Elgart et al. , 2015). The increase of density should have been severe enough to cause the decrease of reproductive allocation in this study.