Report - ICP Forests

3. Intensive Monitoring 71
Relevés
The analysis was based on a single relevé for each combination of plot and year. If there were
data from more than one subplot (called 'survey' in the database) at a given date, the subplots
were combined by taking the average cover per species. If there were data from more than one
date within a given year, these were combined by taking the maximum cover per species over all
these dates. Relevés taken in fenced plots were not used. The tree layer was not considered as a
part of the spontaneous vegetation and not used in the analysis. The moss and lichen layer was
left out of consideration because it was not recorded by all countries. For technical reasons
species with cover percentages < 0.005% (possibly after averaging over subplots) were left out
of consideration. Relevés without any species (possibly after the above operations) were also
left out of consideration for technical reasons. For the multivariate analyses cover percentages
were ln(X+1) transformed.
Soil data
The database contains a wide range of chemical data from a wide range of horizons. Missing
values occur for some variables. In principle, chemical values have been computed as the
average over the complete organic layer, and as the average over 0 - 20 cm depth in the mineral
layer. The following soil chemical variables were used:
- in the organic layer: pH(CaCl 2 ); extractable (mostly by extraction with 0.1 M BaCl 2 ) Ca, K,
Mg, P; and C-total, N-total;
- in the mineral layer: pH(CaCl 2 ), CEC, base saturation, C-total, N-total.
If there were data from more than one year of a given plot, only the last year was used. pH
values were averaged after exponentiation. N/C ratios were computed as (N-total) / (C-total) for
both the organic and the mineral layer. Missing values have been treated as follows:
- records with missing values for pH(CaCl 2 ) in either the mineral or the organic layer were not
used;
- records with missing values for more than one variable were not used;
- occasional missing values in other variables were estimated on the basis of their correlation
with a single other variable (see Table 3.4.2-4 for details).
The chemical data were checked for normality and outliers. Normality was achieved by
logarithmisation, occasional outliers were corrected to their nearest value (max. one per
variable). Details are given in Table 3.4.2-4A.