Abstract
Greater shrub and herb abundance and diversity under the forest would potentially benefit biodiversity and improve forest ecosystem stability. Therefore, determining whether a high canopy density results in reduced growth of understory vegetation and associated thresholds may guide the reasonable planting density of planted forest. In this study, we selected an artificial coniferous forest planting area in Hubei Province, China as the research area, and adopted a constraint line to explore the relationship between the canopy density of different species, including Chinese fir (Cunninghamia lanceolata), cypress (Cupressus funebris), and Masson pine (Pinus massoniana), and the height and coverage of shrubs and herbs in the forests. The upper constraint lines, lower constraint lines, and mean lines were extracted by segmented quantile regression derived from the maximums, means, and minimums in the scatter clounds, which represented the relationships under the best, worst, and general environmental conditions, respectively. The results showed that for different species, the upper constraint lines were almost hump-shaped with thresholds, indicating that regardless of how good the environmental conditions were, the indicators of understory shrubs and herbs first increased and then decreased with an increase in canopy density. The canopy density thresholds for Chinese fir, cypress, and Masson pine were approximately 50%, 30%~40%, and 50%~60%, respectively. Overall, the thresholds of canopy density for herb indicators were greater than those of shrubs. Planning reasonable canopy density may enhance the growth of understory vegetation.