4. Changes in the state of health of stands in the years 1998-2002

During the last five years, the state of health of stands underwent insignificant changes. The comparison of changes expressed in the values of the defoliation index for all the monitored species altogether using a Tuckey's test showed that the value of the defoliation index in 2002 was significantly higher than that in 1999. In 1999, the defoliation index for all coniferous species taken together was significantly higher than that in 2002. No such differences were found for all broadleaved species together. The reason for the statistically significant differences in defoliation index between 1999 and 2002 for all species and all coniferous species taken together resulted from the clearly higher values of the defoliation index for pine between these two years. Among the monitored broadleaved species no significant differences in individual years of the analysed period were found (Fig. 6).

To find the trend of changes in the state of health of stands in the years 1998-2002, the regression function was applied to determine the trend line for the mean defoliation using the least square method [1]. In the course of the last five years, there was a slight rise in the defoliation trend noted for pine and spruce stands. The more clear trend was found for fir stands. Among the monitored broadleaved species, only birch stands revealed a slightly declining trend in defoliation in the years 1998-2002.

Analysis of changes in the level of damage to stands expressed by the share of damaged and healthy trees over the last five years revealed an impairment of the health of forest stands in 2002. The share of healthy trees for all species together (defoliation below 10%) decreased from 31.37% in 1996 to 22.24% in 2000 and to 15.86% in 2002. A simultaneous increase in the share of damaged trees (defoliation greater than 25%) was noted: from 13.47% in 1996 to 20.02% in 2000 and to 23.15% in 2002. The increase concerned the younger stands of both coniferous and broadleaved species. In 2002, this trend of changes was found in four natural-forest regions. However, a decrease in the share of damaged trees in the younger stands in this year was found in the Carpathian and Silesian natural-forest regions and, to a lesser extent, in the Sudeten and Baltic natural-forest regions.

The increase in the level of damage to the older stands was not so distinct. The analysis of the health state of the monitored stands revealed a decrease in the share of healthy trees for all species together from 9.94% in 2001 to 8.81% in 2002 and an increase in damaged trees from 30.28% to 32.28%. The changes in the level of damage for coniferous species were reflected in a decrease in the share of healthy trees from 9.07% in 2001 to 7.79% in 2002 and in an increase in the share of damaged trees from 30.05% to 32.14%. Similar tendency was found for broadleaved species. The year 2002 showed a decrease in the share of healthy trees to 12.10% and an increase in the share of damaged trees to 32.71%. Over the last five years, the increase in the share of damaged trees higher than that in 2002 was recorded only in 1998.

All monitored species revealed the same tendency with the exception of fir stands in which the share of healthy trees increased from 3.72% in 2001 to 8.04% in 2002 and the share of healthy trees decreased from 66.51% in 2001 to 58.04% in 2002. Nonetheless, among all the monitored stands they still were the most damaged ones.

The 2002 saw an increase in the level of damage to forest stands when compared to 2001; it mostly occurred in central and northern Poland in the Mazowsze-Podlasie, Mazury-Podlasie, Wielkopolska-Pomerania and Małopolska natural-forest regions. A decrease in the share of damaged trees was noted in the Silesian and, to a lesser extent, in the Sudeten, Carpathian and Baltic natural-forest regions - see Ryc. 7.