Report - ICP Forests

76 3. Intensive Monitoring
the one that leads to the highest increase in explained variance with the constraint that variables
with a correlation |R| > 0.5 with variables already in the model are skipped. Variable selection
was stopped when none remained that could significantly (P < 0.05) improve the fit of the
model. The resulting 'minimal' model explains 12.5% variance which is quite usual in this type
of ecological data. pH is the most important explanatory variable, which is also usual in this
type of data. There is a small, but highly significant effect of deposition quantified as the
EMEP-estimated NO 3 deposition for 2000. The model is summarised in Table 3.4.4-3. The
results strongly agree with those of De Vries at al. (2002) with the traditional factors (in the
order: tree layer, soil, climate) as the most important explanatory variables, and c. 5% of the
variance in the fitted values explained by deposition. However, in contrast to the 2002 analysis
this effect of deposition is solely due to N-deposition and not partly to e.g. seawater ions.
Table 3.4.4-1: Percentage explained variance due to countries and 'real' environmental variables
source
TMV
uniquely due to countries 5,0%
uniquely due to environmental variables 14,1%
undetermined 6,3%
total variance explained 25,4%
Table 3.4.4-2: Result of forward selection of environmental variables to explain the vegetation of the last relevé of
each plot, using EMEP estimates to quantify deposition and using the countries as covariables. Eigenvalues: λ 1 =
0.259, λ 2 = 0.24, λ 3 = 0.188, λ 4 = 0.125, Σλ = 11.739, Number of plots = 477, number of species = 170. Rare species
are downweighted. F = (regression mean square with this term - regression mean square without this term) / error
mean square; P = probability of this, or a higher F-value under the null hypothesis as determined on the basis of 999
bootstrap samples.
Variable
compartment
F P percentage
explained
variance
pH organic 8,1 0,001 1,70%
mediterr. oak tree 6,79 0,001 1,45%
temperate oak tree 5,95 0,001 1,28%
Pinus sylv+nigra tree 4,83 0,001 1,02%
Fagus tree 4,42 0,001 0,94%
CEC mineral 2,84 0,001 0,60%
N/C organic 2,46 0,005 0,51%
Latitude climate 2,44 0,001 0,43%
NO3 (2000) deposition 2,36 0,001 0,51%
Longitude climate 2,28 0,001 0,51%
coniferous 'other' tree 2,23 0,003 0,43%
deciduous 'other' tree 2,13 0,061 0,43%
Ca organic 2,1 0,002 0,43%
Atlantic South climate 1,96 0,008 0,43%
Age tree 1,89 0,003 0,34%
Atlantic North climate 1,85 0,005 0,34%
K organic 1,87 0,011 0,43%
Boreal climate 1,72 0,012 0,34%
P organic 1,7 0,007 0,34%
Altitude 1,36 0,092 0,26%
N_C_min 1,23 0,173 0,26%
(further terms not given)
SUM if P < 0.05 12,44%