Identification of meteorological conditions in the growth of Robinia pseudoacacia on the basis of pointer years in urban conditions

This study assessed changes in air temperature and rainfall during the growth of black locusts (Robinia pseudoacacia L.) in urban conditions in the city of Wroclaw based on pointer years. Materials consisted of 54 wooden discs taken from felled straight-trunked trees at a height of 1.3 m from the ground at four sampling sites in the area of a defunct garden established at the turn of the 20th century. Meteorological indicators were calculated based on monthly data obtained from the Wroclaw meteorological station, part of the national atmospheric monitoring network. Tree ring widths were determined using LINTABTM 6 and TSAP-Win software. A set of meteorological elements describing temperature and rainfall conditions during the growth of the black locusts significantly affecting the tree ring widths, were selected using cluster analysis. Pointer years were defined as the years when a unidirectional change (e.g. an increase) in tree ring width compared to a previous year was observed in at least 75% of the trees. In the multiannual period 1955–2014 we found seven pointer years, comprising three positive years (tree ring wider than in the year before): 1986, 1989, 1996, and four negative years (tree ring narrower than in the year before): 1963, 1983, 1990, 1991. The width of the tree rings were formed under the influence of different unique thermal and rainfall systems each year. Within the groups of positive and negative pointer years, individual months differed in the impact of air temperature and rainfall conditions on tree ring widths.