The Influence of Climatic and Edaphic Conditions on the Development of Thuja occidentalis 'Smaragd' Under the Urban Conditions of a Large City

The negative effects of the urban environment on the morphological and physiological states of Thuja occidentalis 'Smaragd' have been investigated. For comparison, two plots were selected for the cultivation of eastern arborvitae cultivars: one in a public garden (the control plot) and the other one in street planting with its positive vertical temperature gradient associated with overheating and dehydration of the soil. This situation had a negative impact on the biometric and physiological parameters of the plants, and, in the end, the level of their vitality. If, under optimal growth conditions, the vertical temperature gradient is negative (the air temperature in the crown is higher and the soil temperature in the rhizosphere zone is lower), the physiological processes in Thuja ocidentalis 'Smaragd' (transpiration, water-holding capacity) proceed in a normal mode. A change in the temperature regime (soil temperature is higher than the air temperature in the crown), under the conditions of a positive temperature gradient, leads to negative consequences: the morphological structure of plants changes (the growth of the apical and side shoots slows down, the number of needles increases, the lower parts of the trunks become bared, the length and volume of the crown are reduced), also physiological processes are disturbed – moisture deficiency increases, water holding capacity decreases, the pigment composition of needles changes towards decreasing the amount of chlorophylls (a + b) and carotenoids. A decrease in the vitality level of the study plants with a positive temperature gradient was confirmed by the vitality indices Rfd established by means of the fluorescence method (negative gradient 0.64; positive gradient 0.33).