FutMon logoPhenology

In 5 Intensive Monitoring plots we collect information about the timing and the duration of certain events in the life of trees (phenology): when do the trees flush in spring, how about the extent of seed production, when do leaves start to fall in autumn, ...
In addition to the crown condition assessments, these phenological observations provide supplementary information on the status and development of forest trees during the year.

Time series of phenological data will enable us to detect changes in the timing of annual development stages of forest trees in relation to environmental factors of natural and/or anthropogenic origin, such as climate change and air pollution. Do trees flush earlier under the influence of global warming? And do they produce seeds more frequently?

In each plot we collect data on leaf unfolding or needle appearance, flowering, fruiting, autumn colouring and leaf abscission. These data are collected on a weekly basis in spring and in autumn, by means of visual observations with a binocular.
In spring and in summer damage to the trees by insects, fungi, drought and other biotic and abiotic factors is also assessed.
Data from the litterfall collectors in the plots provide complementary and quantitative information on flowering, fruiting and leaf fall.


Sampling and analysis of needles and leaves

Trees show typical symptoms under the influence of an inadequate nutrient supply. Discolouration of leaves, either yellowing or another colour deviating from the normal colour of the living foliage of the tree, is one of the more important visible symptoms. Growth is also influenced by the nutritional status of the trees.
An inadequate or imbalanced nutrient supply may be the result of high air pollution levels or unfavourable conditions in the rooting zone of trees. It may be a direct cause of low tree vitality or a factor which increases adverse air pollution effects.
The leaves/needles of the trees in the Intensive Monitoring plots are sampled and analysed on a regular basis, in order to describe the nutritional status of trees, to detect time trends and to investigate the impact of air pollution.

In each plot trees are sampled biannually. Treeworkers climb 5 trees per plot and collect leaf/needle samples according to a standard protocol: leaves are collected from the top third of the crown and in equal proportions from the four cardinal directions. After pre-treatment of the samples the concentration of N, S, P, Ca, Mg, K, C, Na, Al, Fe, Zn, Mn, Pb, Cu and Cd is determined in the INBO laboratory. In conifers the needles of the current needle year (N) and older needles (N+1) are analysed separately. Analysis methods are in line with the ICP Forests manual.

Foliar sampling in a Beech plot in the forest of Wijnendale  (Photo L. De Geest).

Nitrogen is one of the key elements in the nutritional status of trees. It used to be the main limiting factor for forest growth in our region. However due to the high nitrogen supply through atmospheric deposition that is no longer the case.

In beech the highest nitrogen leaf concentrations are found in the forests of Wijnendale and Buggenhout. Trends are comparable in all beech plots, with an increasing N concentration until 2001 followed by a decrease in 2003 and 2007. In 2009 N concentrations have increased again.

Evolution of mean N concentration in Beech leaves in 1995 – 2009 (limits according to van den Burg, J. & Schaap, W., 1995).


Litterfall is sampled in each Level II plot by means of 10 litterfall collectors. The litterfall collectors are installed in the plot in a systematic way: each collector is placed at 1 m distance from one of the throughfall collectors as shown in the scheme below.

Litterfall schematic

A litterfall collector consists of a litterbag, made from a chemically inert fabric with small pores to allow drainage of throughfall water and prevent decay of the sample. The collector has a circular opening of 60 cm diameter on top. On top the collector is attached to a plastic ring, which is mounted horizontally on three wooden poles at 1 m above ground level. Below the litterfall bag has an opening of 20 cm diameter, which can be opened and closed manually with a rope so as to collect the samples.

Litterfall collector

Litterfall is collected once per month from January till August and twice per month from September through December. After each sampling the total volume of litterfall collected by the 10 collectors is combined to a mixed sample for that period, which is transported entirely to the laboratory. In the laboratory, samples are first weighed (fresh weight), then dried (40°C) for 2-3 days and weighed again (dry weight). The material is then sorted manually into different fractions:

  • leaves and/or needles (main tree species is always separated from other species)
  • woody debris
  • fruits and seeds (main tree species is always separated from other species)
  • bud scales
  • rest

After sorting the dry weight of every fraction is determined.

At the end of each year a mixed sample of all samples per fraction is composed and transported to the laboratory for chemical analysis.

Litterfall analysis in laboratory


The growth of trees is an important indicator of forest condition, since it reflects changes in site and environmental conditions.

In the Intensive Monitoring plots tree growth is measured as the periodic increment of all trees on the plot and as permanent diameter change of selected trees.

For the periodic increment the total height and the diameter at 1.3 m of all the trees on the plot with a minimum diameter of 5 cm are measured with a five-yearly interval.

Permanent determination of the stem diameter is performed on a subsample of trees using permanent girth bands. The data of these girth bands are collected on an annual basis.

Permanent girth band (photo L. De Geest)


Crown condition

In the Intensive Monitoring programme (level II) information on the condition of trees is collected for the study of cause – effect mechanisms.
Observations are carried out in 5 Intensive Monitoring plots in Beech (Fagus sylvatica), European Oak (Quercus robur), Scots pine (Pinus sylvestris) and Corsican pine (Pinus nigra laricio).

Besides defoliation, discolouration and other tree vitality characteristics the assessments also include information on damage causes like pests and diseases and their impact on crown condition. Regular crown condition assessments are carried out in summer. Additional observations are carried out in spring in order to collect information on defoliators.
The methodology of the crown condition assessments is the same as in the Level I grid.
In the frame of the Life+ programme FutMon the condition of Beech in the Intensive Monitoring plots is also assessed in winter according to the Roloff method.

Crown condition of Beech according to the Roloff method is assessed in winter