LAUSR

Understory vegetation plays a crucial role in nutrient turnover and cycling in plantations, but it also competes for nutrients with crop trees when only a single species is present due to its specific nutrient requirements. However, it remains unclear whether this competition can be alleviated when the species richness of understory vegetation increases. In this study, we tested different gradients of understory vegetation species richness, including understory vegetation removal (UR), the retention of a single main understory vegetation species (RS), and the retention of natural diverse understory vegetation (RD) as part of a poplar (Populus deltoides ‘Nanlin-3804′) plantation, to study their effects on poplar growth, and to evaluate nitrogen (N) usage and how this was affected by the interactions between the poplar and understory vegetation. The results showed a generally lower periodic growth, and a significant decline in the foliar chlorophyll content and glutamine synthetase activity of poplar under treatment with RS and RD compared to those under UR treatment conducted in July 2019, which clearly indicated N competition between the understory vegetation and poplar trees. However, no significant difference was detected in the foliar chlorophyll content and glutamine synthetase activity of poplar under RD and RS treatment; only the nitrate reductase activity in poplar leaves under RD treatment declined significantly, by 22.25%, in June 2019. On the contrary, the diameter at breast height (DBH) of the poplar under RD treatment showed an increase of 34.69% from July to August 2019, compared with that under RS treatment. Furthermore, the increase in the species richness of understory vegetation resulted in an increase in the δ15N values in the poplar leaves, which was strongly regulated by the NH4+-N pool in the 10–20 cm soil layer, indicating the effective coordination of N utilization between poplar and understory vegetation when diversified understory plant species were present. These findings demonstrate the essential role of understory vegetation species diversity in alleviating N competition with crop trees, and provide guidance for understory vegetation management in poplar plantations.