Effect of climate on extreme radial growth of Scots pine growing on bogs in Latvia

Tree rings are one of the most important indicators for reconstructing past history of climate variability as well as anthropogenic and ecological processes. The climatic signal for the past centuries recorded on tree rings is well known in regions of Europe for Scots pine growing in dry conditions, but little knowledge is available regarding pines growing on peatlands. Extreme changes of environmental conditions limit the radial increment of trees and influence the formation of very wide or very thin tree rings. Usually, these extreme events (pointer years) are best explained by climatic factors. The study was carried out on five bogs with variable evidence of past human disturbance. A large number of pointer years common between study sites were identified (positive 1913, 1916, 1917, 1934-1939, 1957, 1967, 1968, 1972 and negative 1865-1867, 1926, 1928-1931, 1947-1949, 1951, 1990, 2000, 2002). Correlation analysis between climatic variables (maximal, minimal, and mean air temperature and precipitation sum) and pointer years indicated that the response to precipitation sum was mainly local and low compared to the response to air temperature. The main climatic variables influencing the development of pointer years were the mean air temperature in the growing season (May-September) and minimal air temperature in February, as well as the precipitation sum in February (positive response) and in the growing season (negative response). However, pointer year development of Scots pine on bogs is mainly determined by local factors.