Response to density
As plants grew large enough, both above- and below-ground interactions
occurred, plant root response to density may actually depend on the
relative strength of competition vs. facilitation. When competition
became intense enough to counteract or surpass facilitative effects,
increased density either did not affect root allocation (Forsteret al. 2011), or decreased root allocation (Maliakal et
al. 1999), depending on resource levels (Table 3c). Nutrient depletion
can intensify below-ground competition (Tilman 1988, Wilson and Tilman
1991). As below-ground resources decreased, competition among plants
transformed from primarily above-ground to primarily below-ground (Grime
1973, Grime 1979). Consequently, when a plant in dense population
suffers resource deficiency, its root allocation is more likely
decreased, due to intensified below-ground competition primarily, as the
case in infertile soil conditions at 50 days of plant growth in this
study. Otherwise, sufficient resources may alleviate below-ground
competition, leading to no response to density in roots, such as in
fertile soil at the same stage in this study. However, no response to
density in root allocation also occurred in infertile soil at 70 days of
growth, suggested interaction intensity first increases then decreases
over time (Hutchings and Budd 1981, Wang et al. 2017).