Forest vitality in 2025
In Flanders, we monitor forest health using a forest vitality monitoring network. This network comprises 78 sample plots, and we examine 1,474 sample trees annually. According to international guidelines, a tree is considered damaged when more than a quarter of its leaves or needles are missing. This loss of leaves or needles is the most important criterion for determining the health status of forest trees.
The Flanders Forest Vitality Inventory shows that in 2025, one in five trees was damaged (20.3%). Just over one per cent of the trees were severely damaged (>60% leaf loss) or had recently died. Pedunculate oak and Corsican pine accounted for the highest proportion of damaged trees. Overall, the difference compared to the previous inventory is small. We did, however, note a decline in conifers and the ‘other deciduous trees’ category: this includes all deciduous tree species with the exception of pedunculate oak, beech and American oak. This also includes the ash. The health of ash trees continues to deteriorate due to ash dieback, a fungal disease caused by Hymenoscyphus fraxineus.
We observed little storm damage in the monitoring network. Severe leaf damage caused by insects and leaf discolouration caused by fungi also remained limited. The drought did cause leaf discolouration and early leaf or needle drop, but mainly towards the end of the summer. The good seed production in pedunculate oak was striking.
Viewed over a long period, the health status is evolving negatively. Leaf or needle loss is increasing in Flanders during the period 2006–2025, just as it is in the rest of Europe.
Since monitoring began, acid rain caused by sulphur emissions has disappeared, but other problems have persisted or worsened, such as eutrophication, drought, storms and forest fires. Climate change is affecting tree diseases and pests: weakened trees fall prey more easily, and new diseases and infestations are emerging. Monitoring the health of trees and forests therefore remains essential.
Geert Sioen, Pieter Verschelde
Image above: Forest vitality trial plot in Kluisbergen (June 2025)