The rate of defoliation and forest decline in Slovenia was relatively stable until 2020, with a slightly negative trend. After 2020, a deterioration of tree defoliation was observed.

Increased defoliation was detected in 2014 and 2015 due to ice storm and windthrows, followed by stable defoliation status for both conifers and broadleaves until 2020. After 2020, defoliation was observed to worsen in both conifers and broadleaves. This could be due to the effects of climate change, bark beetles, and, especially for deciduous trees, the effects of secondary damage following the 2014 ice storm and the 2022 summer drought. Average defoliation of conifers has been lower than average defoliation of deciduous trees since 2020.

In general, the average defoliation in Slovenian forests is 31.6% (2022) - still higher than the average of other European countries where defoliation has been accessed, where it was 23.5% in 2021. The trend of increasing defoliation can also be observed at the European level.

The most damaged tree species are black pine, sweet chestnut, oak (especially pedunculate oak), common beech and Norway spruce. According to recent findings, the annual variation in estimates is mainly a combination of unpredictable weather conditions between years and other biotic and abiotic causes.