Ground Vegetation in Pinus sylvestris Forests at Different Successional Stages following Clear Cuttings: A Case Study.

The impact of intensive forestry on various components of ecosystems has become the main subject of public and scientific debate in many regions in recent years. Forest ground vegetation is considered one of the most consistent and biodiversity-rich indicators of a certain stage of successional forest development. Therefore, changes in this forest component can potentially show the risks of forest damage due to clear-cutting and recovery trends. This study was carried out to identify the ground vegetation species diversity, including species composition and cover, also ground vegetation species relations with organic layer (forest floor) and upper mineral soil parameters at the different successional stages of the Pinus sylvestris L. stand development, including 1-2-year-old clear-cuts, and 6-130 years old stands. This study identified that the herb and dwarf shrub species were more light-demanding in the 2-year-old clear-cuts, as well as in the 6-year and 10-year old P. sylvestris stands compared to the middle-aged and mature forest stands. The dominant ground vegetation species, characteristic for the Pinetum vaccinio-myrtillosum forest type, were negatively dependent on the forest floor mass; they also had negative correlations with the concentrations of total P, K, Ca, and Mg in the forest floor and upper mineral soil but had positive correlations with the soil pH values and total N. The developed regression models of the percentage cover of mosses, herbs and dwarf shrubs according to the P. sylvestris stand age highlight the stabilization of the increase in the moss cover about 30 years after clear-cutting, with no clear trend for vascular species. The herbs and dwarf shrub species were highly variable during the stand rotation due to the species-specific characteristics and random factors rather than due to the influence of stand age. In this study, relatively short-term changes in ground vegetation species composition and percentage cover were determined after clear-cutting, but an important aspect is that new ground vegetation species appeared in the open areas, creating the potential for increasing species diversity. The clear-cutting system supports different species and numbers of herbs and mosses at different stages of stand development, which potentially increases the overall vegetation species diversity of the ecosystem.

Ambient ozone concentration and its impact on forest vegetation in Lithuania.

Continuous ozone concentration measurements at rural sites in Lithuania have been performed since 1982. Long-term ozone data show an increasing trend 0.4 ppb y(-1) in annual mean concentrations during the period 1982-2008, although ozone peak values show an insignificant tendency to decrease. Several studies were performed to evaluate the ozone effect on the main plant species of Lithuanian forests. A strong positive correlation was found between values for the accumulated doses over a threshold of 40 ppb index and the defoliation of Fraxinus excelsior trees in Lithuanian forests during 1991-2008. A strong correlation was found between ozone exposure and the proportion of healthy F. excelsior, Betula sp., and Alnus incana, trees indicating that in Lithuania the deciduous species are more sensitive to ozone than conifers and that healthier trees may be better indicators of the changing environment. The results of the studies suggest that the observed levels of ozone are sufficiently high to have negative effects on the vegetation in Lithuania.

Trends and inter-relationships of ground-level ozone metrics and forest health in Lithuania.

Lithuania is representative of maritime to continental climate, no water limitation, and moderate ground-level ozone (O3) pollution. We investigated the trends of meteorological variables and O3 and how these environmental conditions associate with tree health from 2001 onward. Ozone metrics for forest protection, based on Accumulated O3 exposure Over a Threshold of X ppb (AOTX) or on Phytotoxic O3 Dose over a Y threshold (PODY), were modeled at nine ICP-Forests plots over the time period 2001-2014. Tree-response indicators, i.e. crown defoliation and visible foliar O3 injury, were assessed during annual field surveys carried out at each ICP-Forests plot over the time period 2007-2017. Mann-Kendall and Sen statistical tests were applied to estimate changes over time of meteorological variables, response indicators and O3 metrics. Finally, the O3 metrics were correlated (Spearman test) to the response indicators over the common period 2007-2014. Over this time period, trend analyses revealed an increasingly hotter (+0.27 °C decade-1, on average) and drier climate (rainfall, -48 mm decade-1). A reduction was found for O3 annual mean (-0.28 ppb decade-1, on average) and AOT40 (-2540 ppb·h decade-1, on average) whereas an increase was found for POD0 (+0.40 mmol m-2 decade-1, on average). Visible foliar O3 injury increased (+0.17% decade-1), while an improvement of the crown conditions (-5.0% decade-1) was observed. AOT40 was significantly associated with crown defoliation while PODY and soil water content were correlated with visible foliar O3 injury. As visible foliar O3 injury was negligible in all the studied species, the results suggest that moderate O3 pollution (approximately 30 ppb as annual average) does not induce biologically significant effects on this forest vegetation under the current conditions, however the overall O3 risk (POD0) is expected to increase in the future under a hotter and drier climate.

Meteorological factors and air pollution in Lithuanian forests: possible effects on tree condition.

This study investigates changes in tree condition and environmental factors in Lithuania during the active growing season in 1991-2001. The average crown defoliation and the proportion of healthy trees of Pinus sylvestris, Picea abies, Betula sp., Fraxinus excelsior, Alnus incana, Alnus glutinosa, Populus tremula, and Quercus robur, meteorological (average temperature, amount of precipitation, hydrothermal coefficient) and air pollution data (acidity of precipitation, concentrations of SO2, NO2 and exposure of O3) were analysed. During the period 1991-2001 the condition of Pinus sylvestris, Populus tremula showed a tendency of improvement, while defoliation of Fraxinus excelsior significantly increased. The proportion of healthy trees correlated well with the average temperature and O3 (AOT40), while defoliation correlated well with the acidity of precipitation and the concentrations of SO2 and NO2. Deciduous species appeared to be more sensitive to O3 exposure and conifers to the concentrations of SO2 and NO2.