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).