Dendroecological Researches On The Urbanization Effect On The Growth Of Black Locust In Lanzhou City

Accompanied with the urbanization process, the climate of the Lanzhou City has changed to some extent. The temperature elevated gradually and steadily. Patent heat island effect and "dry land" effect appeared. Moreover, Lanzhou is severely polluted. All these factors may affect the plant growth.Black locust (Robinia pseudoacaci L.) was chosen as the material in the dendroclimatological, dendroecological and dendrochemical researches. In addition to the result of some researches on the climate and pollution in Lanzhou, these researches give proof of the influence of the urbanization effect on the growth of the black locust. Five chronologies, three of which (LD, QL and AN) have acceptable quality, were obtained from five sites in Lanzhou City, including ST, LD, QL, AN and XG.Drought became the key factor affecting the growth of the black locust as the temperature elevated. Dividing the whole chronologies into two parts by the year 1980 when the average temperature exceeded 9.5℃, comparing with the early part of the chronologies, the latter part of the chronologies correlate more significantly with the climatic elements, indicating that the effect of drought on the growth of black locust after 1980 is more severe than that before the year 1980. As to chronologies after 1980, QL and AN chronologies significantly positively correlate with the precipitation in the growing seasons of the current year and the previous year;LD chronology significantly negatively correlate with the temperature in the growing seasons of the current year and the previous year. The fact that drought is the key limiting factor for black locust growth was further proved in the correlation analysis between the chronologies and the humidity index of Lanzhou city.XG chronology significantly negatively correlates with the May to October NOx and SO₂ concentration, indicating the probable damage of the pollutant on the growth of black locust, while LD chronology significantly positively correlate with the May to October, November to April, and all-year NOx concentration. Further investigation reveals that the cause of the positive correlation between black locust growth and the NOx concentration is not the direct N fertilization effect on the plant. Because of the influence of the mixing layer, the pollutant concentration negatively correlates with the temperature. Plus, the black locust growth negatively correlates with the temperature too. These two negative correlations make pollutant concentration and black locust growth correlate positively with each other. No proof is found in this research that the pollutant deteriorate the growth of the black locust.Obvious elementary distribution pattern was found in the black locust tree-ring disc in the dendrochemical experiment. The concentrations of K, Zn, Mg, Cu, Fe and Mn increase markedly in the outer part of the tree ring. Concentrations of some elements correlated well with each other, including (1) K, Mn, Zn, and (2) Fe, Mg, Mn, Zn. This may suggest similar transporting andbinding behavior of these elements. Moreover, the concentrations of the elements in the growth ring significantly positively correlate with the ring width index, except Cu and K.From the experiment, we can conclude that in the Lanzhou city, accompanying with the development of the city and the change of the climate, drought become the key factor affecting the black locust growth. Furthermore, in site XG, pollutant may affect the growth of the black locust, while in LD, no proof is proved for the same conclusion.