Hemi-boreal forest ecosystem under the pressures of global environmental changes

Ecological and socioeconomic importance of the forest ecosystems is recently becoming of increased concern due to their damages caused by direct effect of climate change and air pollution and their indirect threats, including insect and disease infestation, fragmentation, fire, invasive species and etc. The UNECE Integrated Monitoring Programme, which has been performed since 1994 in Lithuania, provides all the necessary data for solving this problem. Aukstaitija Integrated monitoring station which is included in the eLTER network creates the Long-term forest ecosystem monitoring and modeling platform to meet the recent challenges related to sustainable development of the hemi-boreal forest. 30 years changes in environmental conditions revealed that implementation of international legislation in the area of transboundary air pollution reduction resulted in higher than tenfold reduction of air concentrations of sulphur compounds and their deposition at regional polluted areas between 1994 and 2024. In 2021 and 2022, when precipitation exceeded the value of long term average, sulphur concentration in soil and ground water at Lithuanian Integrated monitoring stations (LT-01 and LT-03) reached the lowest values during whole period of investigation, i.e. by about 0.5 mg/l, and 5 mg/l respectively, following the lowest 7.6 pH values in surface water during the entire study period (Algirdas et al. 2015). These changes in acidifying components promote the regeneration of tree condition expressed by reduction in tree crown defoliation, which occurs most intensively in coniferous tree species. Climate change during the considered period was expressed through the increase in temperature and precipitation amount by 0.05 ˚C per year and 1.35 mm per year respectively. The most significant increase in temperature was recorded in 2024 when mean temperature exceeded 9.0 °C, which was the highest value during the whole period of meteorological investigation (Algirdas et al. 2018). Such increase resulted in outbreaks of Ips typographus damages in mature spruce forest. Therefore, new meteorological threats for forest ecosystems in Lithuania could be hot and dry periods from April up to June. Contrary to that gradual increase in precipitation amount resulted in increase in soil humidity and recovery of the ground water level which resulted in better crown condition and higher annual increment of prevailing in hemi-boreal forest tree species. Only due to damages caused by Ips typographus after drought and heat wave Norway spruce trees crown condition deteriorated significantly. Notwithstanding this the heavy rainfall in 2021-2022 raised the groundwater level, especially during the vegetation period, which could be described as the beginning of the restoration of the geosystem's water resources and regeneration of forest condition. Climate and air pollution changes also resulted in higher annual litterfall production and decrease in mean defoliation at considered sites following by higher tree annual increment (Algirdas et al. 2018; Marius et al. 2021). These changes indicated that increase in litterfall by 100 kg per ha per year results in decrease in crown defoliation by about 0.2 % per year. Chemical analysis of foliage samples of the dominant tree species revealed that, N, K, Ca and especially Fe concentrations significantly increased. Macro and micro elements P, Mn, Mg and Al tended to decrease or remained at a stable level. These changes increase the resistance of the main tree species to unfavorable environmental factors primarily diseases and pests, which is especially important for spruce trees. Correlation analysis of litter and average defoliation and precipitation amount showed that increasing precipitation and rising average temperature increase litter formation, but very slightly and insignificantly. Precipitation also has no significant effect on the change in tree canopy defoliation, while rising air temperature has a positive effect on tree canopy formation, increasing canopy density and foliage amount, which also increases the amount of litter falling on the soil surface. Significantly decreasing air pollution with acidifying components and their concentrations in precipitation and flows with precipitation also have a positive effect on litter formation, reduction in crown defoliation and increasing the productivity of hemi-boreal forest ecosystem. Finally, results obtained in the Aukštaitija and Žemaitija IMS basins revealed that the state of forest ecosystems should improve and resistance to adverse environmental factors, diseases and pests increase. Additionally obtained data revealed that in a forested area, not only does climate warming occur at a lower intensity than in an open non-afforested area, but also the amount of precipitation there is significantly higher. This is the effect of forests on the climate change-mitigation process.