A needle trace method was used to reveal the chronology of needle retention and growth rates on Scots pine in a stand near a power plant operating on oil shale. During the period of 1964-97, the height increment varied between 31 and 64 cm yr (-1), and the radial increment ranged from 1.06 to 3.71 mm yr (-1). The average needle density on the newly sprouted shoots of the main stem was 5.9 needles cm (-1) and the mean needle production on the same shoots was 266. The average number of needle sets along the main stem ranged from 2.2 to 3.1, the mean age of needles varied between 1.3 and 2.3 years. The short-term maximum concentrations of oil shale fly ash and SO2 ranged from 55 to 2510, and from 75 to 270 micro g m (-3), respectively. The annual average concentrations of fly ash varied between 6 and 95 micro g m (-3), and of SO2 between 2.5 and 10 micro g m (-3). Changes in neither the concentrations of fly ash nor SO2 were associated with the variation in needle retention of Scots pine. In the case of height increment, however, the correlation with the annual average concentration of oil shale fly ash was significant (r=0.83; p0.05)

The research was conducted in 1990-99 in forest observation sites in the neighbourhoods of industrial enterprises of Kohtla-Jaerve-Johvi and the cement plant of Kunda and in the control area in Lahemaa National Park. Differences between the radial increments of conifers growing under pollution and in a relatively unpolluted area were estimated. The stands selected for investigation were similar as to their edaphic conditions and forest survey indicators. The high sensitivity of conifers to long term emissions of pollutants into the atmosphere from oil shale based power engineering and chemical industry, production of building materials and fertilisers and other industrial enterprises in the investigation area is expressed as changes in the radial increment of trees with the changes depending on the pollution load and alterations in precipitation and soil