Can the desiccation of forests in Tara National Park (Serbia) be attributed to the effects of a drought period?

Forest ecosystems within national parks are threatened by various biotic and abiotic factors. To determine the causes of the desiccation and death of trees in mixed coniferous and deciduous forests of Tara National Park (TNP), Serbia, we monitored defoliation and mortality of individual trees in permanent experimental plots. Data on the desiccation of a large number of trees were gathered by determining the total volume of dry trees and areas of forests under drying stress. The two sets of data were combined to determine the impact of climatic events, primarily drought periods, on the desiccation of forests. Combining data from the International Co-operative Program on Assessment and Monitoring of Air Pollution Effects on Forests (ICP Forests) with TNP data helped relate forest desiccation to climate events. Key climate signals were identified by monitoring tree defoliation changes in two permanent experimental plots, and then assessed for their influence on tree desiccation in the entire national park. The standardized precipitation evapotranspiration index (SPEI) was used for a more detailed analysis of the drought period. Despite the lack of climate data for a certain period, the SPEI index revealed a link between climate variables and the defoliation and desiccation of forests. Furthermore, the desiccation of trees was preceded by a long drought period. Although mixed coniferous-deciduous forests are often considered less vulnerable to natural influences, this study suggests that forest ecosystems can become vulnerable regardless of tree species composition due to multi-year droughts. These findings contribute to a better understanding of important clues for predicting possible future desiccation of forests. Continuous monitoring of the state of forests and of more permanent experimental plots in national parks could provide better quality data and timely responses to stressful situations.