13. The influence of weather conditions on the health of stands over the last five years

Jerzy Wawrzoniak, Jadwiga Małachowska

Fig. 15. Damage to stands and precipitation sum in the growing season (June-September) in the period 1999-2003 (mean values for natural-forest regions) - part one

Weather conditions during the 2003 growing season negatively influenced the health of forests all over the country. In 2003, the mean sum of atmospheric precipitation during the growing season calculated on the basis of data obtained from more than 50 IMGW synoptic stations reached the lowest level in the last five year period 1999-2003 amounting to 307 mm, or 79% of the long-term mean. The year 2003 was the subsequent year when precipitation sum during the growing season was below the long-term mean. In 2002, the mean sum of atmospheric precipitation during that time was 95% of the long-term mean.

The greatest water deficit in 2003 was noted in the Wielkopolska-Pomerania natural-forest region (65%), the greatest abundance - in the Baltic natural-forest region (86% of the long-term mean) - Fig. 15 and 16. Precipitation sum in the natural-forest regions oscillated between 226 mm in the Wielkopolska-Pomerania region to 480 mm in the Carpathian region.

While analysing the atmospheric precipitation distribution pattern in individual months of the growing season of 2003, it should be remembered that June was the month characterized by particularly low precipitation and high mean air temperature exceeding 18oC. This situation occurred in the Baltic, Mazury-Podlasie, Silesian, Małopolska and Sudeten natural-forest regions.

In the majority of natural-forest regions water shortages were manifested in the increased share of damaged trees in stands. This was particularly distinct in the Baltic, Mazury-Podlasie, Silesian, Małopolska and Carpathian natural-forest regions. The increase in the share of damaged trees in the stands of those regions ranged from 4.08% to 5.16%. This tendency was less clear in the Wielkopolska-Pomerania natural-forest region while the decline in the share of damaged trees was noted in the Sudeten and Silesian natural-forest regions.

Fig. 16. Damage to stands and precipitation sum in the growing seasons (June-September) in the period 1999-2003 (mean values for natural-forest regions) - part two

The opposite tendency was found for the Silesian and Sudeten natural-forest regions which, in spite of the decrease in precipitation sum during the 2003 growing season, can be associated with relatively high precipitation in 2002 being considerably higher than the long-term means for the both natural-forest regions and especially for the Sudeten region. The water deficit level in soils was not high when compared to other natural-forest regions. Moisture conditions in the stands were also affected by the amounts of snowfall in a given year which has not been taken into account in this report.

The year 2003 saw an increased insufficiency of atmospheric precipitation during the growing season on a majority of the country. The mean precipitation sum for the country was the lowest over the last five years and was 307 mm (or 79% of the long-term mean). The largest shortages of atmospheric precipitation occurred in the Wielkopolska-Pomerania natural-forest region where the level of precipitation approached 65% of the long-term mean. June was the month featuring particularly low precipitation accompanied by high mean air temperature exceeding 18oC. This situation occurred in the Baltic, Mazury-Podlasie, Silesian, Małopolska and Sudeten natural-forest regions.
  Raport 2003