[GZ01]
Forest decline and tree defoliation
Assessments published: [ 2010 2009 2008 ]

The basic indicator of the health condition of trees, stands and forests is tree defoliation. It is a visually estimated percentage of missing assimilation tree organs in comparison with normal foliage level in a tree belonging to the same species and same social status and growing in the same site. The assessment of the indicator is rounded to the nearest 5%.

A tree is considered as damaged when its defoliation degree exceeds 25%.

The indicator is contained in the material of MCPFE 2003 (Ministerial Conference on the Protection of the Forests in Europe) under Criterion 2 (Maintenance of Forest Ecosystem Health and Vitality). A proposal has also been made to include it among the FAO/TBFRA indicators.

Image GZ1-1: Trend in shares of damaged trees with defoliation above 25%

Source: Slovenian Forestry Institute, 2009


Image GZ1-2: Share of trees by individual defoliation class in 2008

Source: Slovenian Forestry Institute, 2008


To preserve healthy forests that can permanently and optimally function as an ecosystem and perform ecological, social and productive functions.

The Resolution on the National Environment Protection Programme envisages monitoring of the condition and evaluating the decline of forests.

The data presented in the timeline were collected on sample plots arranged in a 16 x 16 km grid (in 1995 and 2000 data were acquired from a 4 x 4 km grid). The timeline of the entire period shows that in the period 1985-2000, the condition of forests was deteriorating, but from 2000 onwards, it has remained relatively stable. Despite the rise of the value of health indicators in 2008, this should not yet be considered a negative trend.

In the last year, the average defoliation of deciduous trees has been deteriorating and the defoliation of coniferous trees has declined. Greater variation can be seen in the shares of damaged coniferous and deciduous trees. Among the most damaged tree species are fir and oak. The state of forests in Slovenia is comparable to that reported by nearby Central European countries.

How was the indicator measured?
The defoliation assessment process is carried out in two stages. In the first stage, the sample trees are determined based on the statistical selection protocol, and in the second stage, each selected tree is evaluated in terms of defoliation. The indicator is a visually estimated share expressed in the percentage of missing assimilation tree organs (leaves, needles) in comparison with normal foliage level in a tree belonging to the same species and same social status and growing in the same site. The assessment of the indicator is rounded to the nearest 5%.

Data for Slovenia:

• Large-scale inventory of forests and forest ecosystems, Slovenian Forestry Institute.

• Description of the data source:

Methodology: random systematic cluster sampling

Frequency: every year on a 16 x 16 km grid, periodically on a 4 x 4 km grid.

Data manipulation: original data – individual values are calculated to sample units and units of account (=Slovenia)

Quality: subjective, but the only internationally acknowledged and comparable indicator

Reliability: subject to bias because of the constancy of sample plots (avoidance of some plots), but simple. The indicator demands assessment skills that can be acquired annually at calibration courses.

Accuracy: because the indicator is measured based on random sampling, the accuracy may be specified in advance by determining the number of sample units.

30 June 2009
Marko Kovač, Mitja Skudnik, Forestry Institute of Slovenia