Abstract: Objective The study was intended to explore the response difference of nutrient limitation of typical urban ornamental tree species along the urban and rural environmental gradients, in order to provide a scientific basis for nutrient management of urban forest ornamental tree species under urbanization. Method The mature and defoliated leaves of typical ornamental tree species Acer truncatum and Syringa oblata were collected along the urban-suburban-rural gradients of Beijing in summer and autumn, respectively. Six leaf functional traits (leaf fresh weight, leaf dry weight, leaf area, specific leaf area, leaf mass per unit area and leaf water content) and the contents of six elements [carbon (C), nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg)] were determined. The stoichiometric ratio and nutrient resorption efficiency were calculated. The differences of leaf functional traits, eco-stoichiometric characteristics and nutrient resorption efficiency between Acer truncatum and Syringa oblata were compared. The correlations of environmental factors such as mean annual temperature, mean annual precipitation and mean annual solar radiation with leaf functional traits, eco-stoichiometric characteristics and nutrient resorption efficiency of Acer truncatum and Syringa oblata were analyzed. Result Leaf functional traits, element content and stoichiometric ratio had significant response to urbanization, and there were interspecific differences. In mature stage, along the gradients from the urban area to the rural area, the leaf fresh weight, leaf dry weight, leaf area, specific leaf area and leaf water content of Acer truncatum and Syringa oblata increased as a whole, while the leaf mass per unit area decreased. During the abscission period, the leaf fresh weight and leaf dry weight of Acer truncatum decreased gradually along the gradients between the urban area and the rural area. The leaf dry weight and leaf mass per unit area of Syringa oblata increased at first and then decreased along the gradients from the urban area to the rural area, while the leaf area decreased gradually. The specific leaf area of defoliated leaves of the two tree species decreased at first and then increased along the urban and rural gradient. At mature stage, the N and K contents of Acer truncatum and Syringa oblata in the suburban gradient were lower than those in the urban and rural gradients. The P and Mg contents of mature leaves of Acer truncatum did not change significantly in the urban and rural area, but the P and Mg contents of mature leaves of Syringa oblata in the suburban area were significantly higher than those in the urban and rural gradient. During the defoliation period, the P and Mg contents of Acer truncatum had no obvious change along the gradients between the urban area and the rural area, while the P content of Syringa oblata in the suburban gradient was significantly higher than that in the urban and rural gradients, and the Mg content in the rural area was significantly lower than that in the urban and suburban areas. In defoliating stage, the K content of Acer truncatum and Syringa oblata decreased at first and then increased in the urban and rural areas. There was no significant difference in nutrient resorption efficiency of N, P, K, Ca and Mg between the two tree species along the urban and rural gradients. Conclusion Urbanization leads to smaller leaves of Acer truncatum and Syringa oblata. The growth of Acer truncatum and Syringa oblata were restricted by N, while Acer truncatum in the urban gradient and Syringa oblata in the rural gradient had stronger drought tolerance. The resorption efficiency of K, Ca and Mg of the two tree species in different gradients was lower than the global average, and the element resorption ability of Acer truncatum was stronger than that of Syringa oblata. Compared with leaf nutrient content and nutrient resorption efficiency, leaf functional traits were more closely related to environmental factors.

Key words: urban-rural gradient, leaf, ecological stoichiometry, nutrient resorption, ornamental tree species