This paper presents research results on forest decline in Serbia. The results were obtained through monitoring defoliation of 34 tree species at 130 sample plots during the period from 2004 to 2018. This research aimed to determine whether the occurrence of defoliation and tree mortality were caused by drought. Defoliation was assessed in 5% steps according to the International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (ICP Forests) methodology. All the trees recorded as dead were singled out, and annual mortality rates were calculated. To determine changes in air temperature and precipitation regimes during the study period, we processed and analysed climatic data related to air temperature and precipitation throughout the year and in the growing season at 28 main weather stations in Serbia. Tree mortality patterns were established by classifying trees into three groups. The first group of trees exhibited a gradual increase in defoliation during the last few years of monitoring, with dying as the final outcome. The second group was characterised by sudden death of trees. The third group of trees reached a higher degree of defoliation immediately after the first monitoring year, and the trees died after several years. Tree mortality rates were compared between years using the Standardised Precipitation Evaporation Index (SPI) and the Standardised Precipitation Evapotranspiration Index (SPEI), the most common methods used to monitor drought. The most intensive forest decline was recorded during the period from 2013 to 2016, when the largest percentage of the total number of all trees died. According to the annual mortality rates calculated for the three observation periods (2004–2008, 2009–2013, and 2014–2018) the highest forest decline rate was recorded in the period from 2014 to 2018, with no statistically significant difference between broadleaved and coniferous tree species. As the sample of coniferous species was small, the number of sample plots should be increased in order to achieve better systematic forest condition monitoring in Serbia. The analysis of the relationship between defoliation and climatic parameters proved the correlation between them. It was noted that the forest decline in Serbia was preceded by an extremely dry period with high temperatures from 2011 to 2013, supporting the hypothesis that it was caused by drought. We therefore conclude that these unfavourable climatic conditions had serious and long-term consequences on forest ecosystems in Serbia.

The aim of this research was to examine the influence of acorn size on morphological characteristics of one-year-old Northern red oak (Quercus rubra L.) seedlings. The quality of seedlings correlates with the quality of the seed they are derived from. In species with large seeds, as in the case of Northern red oak, the seedling growth in the first growing season is closely related to seed size or the amount of reserve nutrients that are stored in the seed. The height and root collar diameter of one-year-old Northern red oak seedlings increases with increase in acorn size. The results obtained in this research can be used as a guideline for acorn grading, because they show that improved Northern red oak seedlings quality can be achieved using seeds of appropriate size.

To address the issues of taxonomic delimitation and to select the material with desirable traits for horticultural use, a comprehensive study of the total variability of morphological characters of two naturally occurring snowdrop species ( Galanthus nivalis L. and Galanthus elwesii Hook.) in the central part of the Balkan Peninsula was conducted. The analyses were performed for 16 quantitative characters of aboveground parts of 630 plants collected at 21 geographical points in the territory of the Republic of Serbia. To identify the trends in morphological differentiation, descriptive, univariate and multivariate statistical procedures were employed. Most of the examined characters exhibited moderate degrees of variability ( cv from 10% to 20%) and the values of several characters exceeded the limits of variation stated in literature. Samples were mostly morphometrically homogeneous, with comparatively high similarity; only four populations of G. nivalis from the southern and southeastern part of the country were discrete. It is questionable whether these populations merit formal taxonomic recognition at any infraspecific level, but given their decorative qualities, the selected plants can be introduced into cultivation as horticultural plants.

Spatial and urban plans have recently given an increased focus to the sensitive issue of planning the urban resilience of existing and establishing new urban forests. Urban forests are recognised as an extremely important factor of environmental quality, while scientific studies indicate that the health condition of urban forests is threatened by present-day climate change. There have been relatively few systematic assessments of the causes of tree die-back in urban areas in Serbia. Research, forest establishment planning and tree monitoring to estimate possible consequences to existing trees will enable city authorities and managing bodies to raise the issue of urban forest resilience to climate change to a higher level and incorporate it into urban planning. Future scenarios in which urban forests are resistant to potential adverse climate change will depend on the scope of activities of adaptive planning of urban forest establishment and adaptive management of existing ones. Therefore, it is necessary to conduct long-term studies of monitoring the condition of urban forests in the large cities of Serbia (Belgrade, Novi Sad, Niš, Subotica, etc.) in the coming period in order to identify specific problems and remediate the consequences such as deteriorated tree health condition and die-back.

The afforestation of bare lands, sandstone, and skeletal terrains are one of the biggest challenges forestry scientists face. These terrains are characterized by specific ecological conditions that are generally unfavorable for the growth of woody species. These are usually shallow soils, unstable, and poor in nutrients and moisture. The characteristics of these habitats make said terrains unfavorable for the regeneration of forest vegetation. It is therefore crucial for the success of afforestation to gain detailed knowledge and understanding of environmental conditions. Only after the detailed research and study of field conditions can the selection of species for afforestation, including selection of species characteristics and technology of planting, begin. Mistakes made during previous establishments of green areas are one of the main reasons some species of vascular flora have disappeared. This alone expresses the undeniable importance of knowledge on habitat specifics, work schedule, and selection of species for afforestation. With the aim to implement the future afforestation within the planned scope, it is necessary to organize an effective nursery production of seedlings with characteristics that will suit the environmental conditions of bare lands, sandstones, and skeletal terrains whose afforestation is planned.