12. Analysis of the relationship between the health condition of forest and environmental factors in 2002, as well as directions to predicted changes

During the last five years, the health state of forests experienced a distinct stability in all regions of the country. The defoliation index for all monitored species together ranged between 2.58 in 1999 to 2.69 in 2002. Only between these two years, the differences in the values of this index proved statistically significant. The elevated value of the defoliation index in 2002 when compared to the values in 1999 resulted from the deterioration of the health condition of pine forests in 2002. The other monitored species did not show any significant differences in the defoliation index over the last five years.

The differences in the level of damage to stands enabled to distinguish two groups of the natural-forest regions: the more damaged stands of southern Poland being particularly numerous in the Carpathian natural-forest region and the less damaged stands of northern Poland most frequently occurring in the Baltic and Mazury-Podlasie natural-forest regions. However in 2002, the differences in the level of damage between the southern and northern regions of the country were smaller than in previous years. This was caused by an increase in the forest health condition in southern Poland on the one hand, and the worsening of the health condition of forests in north-eastern Poland, on the other. The 2002 saw an increase in the share of damaged trees in the Mazury-Podlasie and Mazowsze-Podlasie natural-forest regions when compared to 2001. The reason of the deterioration of the health state of forests in the region where the level of healthiness of stands was so far the highest was a negative impact of unfavourable weather conditions, and the deficit of water during the 2002 growing season. In the Regional Directorates of the State Forests in Białystok, Olsztyn and Warszawa, the mean level of atmospheric precipitation in the period April-September amounted to 61% to 76% of the long-term mean. The low level of precipitation occurred with a simultaneous rapid increase in mean temperatures in May and June.

Air pollution and atmospheric deposition are one of the biotic factors, impacting markedly upon the health condition of stands. However, in 2002 the concentrations of SO2 in the atmosphere were lower than in 2001 in all natural-forest regions with the exception to the Mazury-Podlasie natural-forest region. This tendency continued for the last five years. With the declining concentrations of SO2 in the successive years, the differences in SO2 concentrations between the natural-forest regions were maintained being higher in southern Poland (Silesian and Małopolska natural-forest regions) and lower in northern Poland (Baltic and Mazury-Podlasie natural-forest regions). Marked differences in SO2 concentrations were found between the winter (higher concentrations) and the summer (lower concentrations).

In 2002, concentrations of NO2 were higher than in 2001 over most forest areas in Poland with the exception to the Baltic and Wielkopolska-Pomerania natural-forest regions. Since 2002, a steady increase in NO2 concentrations was recorded in the southern regions of Poland, particularly in the Małopolska and Carpathian natural-forest regions. At the same time, higher NO2 concentrations were noted in the winter than in the summer period.

The level of SO2 and NO2 concentrations over the forested areas was distinctly below the standards set out in the regulation of the Minister of Environment [16].

The level of SO2 and NO2 concentrations is, to a great extent, connected with the pH of atmospheric precipitation. Markedly higher pHs were noted in southern Poland than in northern Poland, and higher in the winter than in the summer. In the last five years a constant decrease in the pH of atmospheric precipitation was recorded in all natural-forest regions. This indicates the minimising effect of sulphur dioxide and nitrogen dioxide concentrations, as well as pH of atmospheric precipitation on the health status of forests in Poland. This effect can be of local importance for the forest areas situated in the vicinity of major sources of emission.

The chemical composition of atmospheric precipitation can greatly affect the functioning of forest ecosystems. The concentration values of a majority of ions were at the similar level as in previous years. During the winter period, the concentrations of majority of ions were higher than those during the summer with the exception to K+ and N-NH4+. The 2002 saw a continuing tendency towards decrease in sulphate-sulphur deposition, which was especially high in the Carpathian natural-forest region. The concentrations of heavy metals: lead and cadmium in atmospheric precipitation were found to be the highest in southern Poland in the Małopolska and Carpathian natural-forest regions, while the lowest ones in northern and central-eastern Poland in the Baltic and Mazowsze-Podlasie natural-forest regions.

The chemical properties of soils and the deposition of biogenic compounds influence the chemical composition of needles. Chemical analyses of pine and spruce needles revealed an increase in the content of nitrogen and phosphorus in pine and spruce needles between the years 1997 and 2001. A decrease in sulphur concentration in spruce needles might result from decrease in SO2 concentration in air, as well as in the declining sulphur deposition as observed during recent years. The chemical composition of pine needles collected from POPs located in the northern regions of the country proved that concentrations of most analysed elements were lower than those from the southern regions.

The health of forests is greatly influenced by the biotic factors. The population density of folivorous insects and the threats posed by pathogenic fungi to stands were evaluated on permanent observation plots under the monitoring system.

The 2000 saw further increase in the number of pine sawflies in the Mazowsze-Podlasie, Małopolska, Wielkopolska-Pomerania and Baltic natural-forest regions. The abundances of pine hawk moth and pine looper moth on POPs in all natural-forest regions were higher than in the previous year. The abundance of nun moth in 2002 remained on the high level, however, in most natural-forest regions it was higher than in 2001.

The 2002 saw no increase in the biotic threat to forests evaluated by means of intensity of pine shoots dieback and occurrence of root pathogens, as well as abundances of folivorous insect pests. The level of threat posed by pathogenic fungi and folivorous insect pests remained stable and low during recent years and no circumstances predicting a sudden change were observed.

The health state of forests in Poland is determined by a configuration of environmental factors, site conditions, species composition and age structure of stands which are relatively stable factors, slowly changing over time and by varying factors like weather conditions, air pollution, abundance levels of pest insects and occurrence of pathogenic fungi. The recent years saw the eutrophication process resulting from the elevated deposition of biogenic elements, and particularly NO2 concentrations, what might cause eutrophication of forest sites. The phenomenon could attributable to an increase in soil fertility and greater share of eutrophic species in the composition of the forest floor. An increased fertility of forest sites enhances the competitiveness of broadleaved species, which in a long run can affect the species composition of stands. Climate warming can accelerate this process. Due to reductions in harvesting of timber in forests, the age structure in recent years has been disturbed. The area of older stands and the mean age of stands [13] show a rising tendency. It is likely that an increase in susceptibility of forest ecosystems to unfavourable changes in weather conditions, attacks of pest insects and infections by pathogenic fungi will follow.

There was a stabilisation of the state of forest health in 2002. The environmental factor, which most adversely affected the forest health status, was water deficit during the growing season in north-eastern Poland. However, this increase in the damage to stands in this region is not permanent and intensive. The improvement of the state of health of forest stands can be quick provided the weather conditions improve, and especially when the amount of precipitation in the following year approaches the long-term mean. The stability of forest health in the upcoming years will to a high degree depend on weather conditions. Shortages of precipitation and quick increase of mean air temperature in the growing season and the observed eutrophication process of forest sites may result in incidental and short-term disturbances in forest ecosystems in some regions of Poland. These disturbances may be followed by a rapid increase in population abundance of pest insects or the occurrence of extreme weather phenomena causing windbreaks, flood or fires.