What is meant by healthy forests?

There is no consensus on a clear definition of ‘Forest Health’. The origin of the term Forest Health is strongly connected to the concern of acid rain and forest decline (‘Waldsterben’) in the 1980s. Since then, the condition of the air quality and the forest evolved and Forest Health became relevant in a broader area of interest e.g. climate change, biodiversity, resilience, sustainable forest management. Forest Health is one of the criteria for sustainable forest management (SFM) used by Forest Europe (FE). European member states record forest health indicators in a standardised way in the ICP Forest monitoring network.

Forest health indicators

Important indicators for forest health are tree crown condition, soil condition and foliar nutrient status. The forest health indicators are collected in a standardised way by the ICP Forest monitoring network and reported to the Forest Europe process.

Tree crown condition

Tree crown condition is expressed here as degree of defoliation. A defoliation of more than 25% means moderately to severely defoliated trees.

Percentage of defoliated trees, in percentage


Defoliation increased for broadleaved trees and conifers in the last twenty years for the EU27 member states (source ICP Forests).

In 2019, damage cause assessments by ICP forests were carried out on 103,297 trees in 5,654 plots and 26 countries. Most of the damage was reported on broadleaved trees, mostly by insects.

Foliar nutrient content

Foliar nutrient ratio’s below or above the critical limits are related to decreased resilience and susceptibility of trees to plagues and diseases and to decreased growth levels.

Percentage of ICP-Level II plots below, above the critical limit for the N:P ratio for 1995 and 2017

Source: Adapted from Temporal trends and spatial variability of foliar nutrients in Europe Maarten van Doorn, 2020

The percentage of plots with a foliar N:P ratio above the critical limit increased for Pinus sylvestris, Picea abies and Fagus sylvatica. For Quercus spp. it decreased.

Soil condition

Forest soils are the foundation of forest ecosystem functions and services. They affect all parts of the environment: water, atmosphere, animals, vegetation, and climate.

Main soil chemical properties include pH, soil organic carbon, total nitrogen, CEC, exchangeable potassium (K), available phosphorous (P) and the derived C:N ratio

Monitoring forest soils helps better understand soil function and the influence of soil structure and nutrients on forest conditions and dynamics.

Soil acidity (pH)

Soil acidity (pH) is an important soil property that influences concentrations of important nutrients and minerals. Low soil acidity may cause unfavorable growing conditions for forests. The soil acidity in European forests stayed more or less at the same level over the period 2009-2015 (approximately 0.1 unit change pH CaCl2 ).


Soil organic carbon in 2020, g kg-1

Forest nutrition, atmospheric Nitrogen deposition, climate change


Total nitrogen in 2020, g N kg-1

Forest nutrition, atmospheric Nitrogen deposition, climate change


Soluble phosphorus in 2020, mg P kg-1


Extractable potassium in 2020, mg kg-1


CEC, meq/100g

The cation exchange capacity (CEC) has relevance for the buffering acid input and the state of acidification


C:N ratio

The C:N ratio is considered as an indicator of nitrate leaching in response to high atmospheric nitrogen (N) deposition.