relation between till geochemistry and the occurrence ... - Arkisto.gsf.fi

Geological Survey of Finland, Special Paper 9
Pertti Hari, Alf Björklund, Hannu Rita and Anneli Ylimartimo
samples are from a depth of 1-2
m, which is well
below the sai1 horizon in this area. After drying,
< 0.06 mm fraction was sieved from the samples
and leached at + 80°C for 1 h in 7N aqua regia.
An extract was analyzed by induction-coupled
plasma atomic emission spectrometry (ICP).
An index was developed based on knowledge of
the effects of acid deposition on soil properties and
tree nutrition . The values of the variables describing
the concentrations of K, Mg, Ca, Al, Fe, and
Mn were standardized. The first principal component
coefficients of these variables were roughly
equal. This supported the choice of equal weights
in the definition of the index, which we here call
the GEI (Geochemical Environmental Index):
(K + Mg + Ca)-(Al + Fe + Mn). We use this index
to describe the geochemical properties of till.
Unfortunately, there is no systematic inventory
of the area of damage by scleroderris canker in
Finland; the available records are sporadic and
often based on subjective observations. To obtain
a more quantitative picture of the frequency of
scleroderris canker an experimental survey was ar-
ranged in July, 1986, in southern Finland in the
above-mentioned 100 kmx 300 km area. The
method was to involve a quick-survey to estimate
the disease intensity near by the geochemical survey
points. The most severely damaged young pine
stand not more than in the 0.5 km neighborhood
of each geochemical sampling point was chosen.
In each standthe proportion of all trees having 1)
recovered crowns with one or more changes of a
leading shoot, 2) crowns with recent browning of
shoots, and 3) dead crowns was measured. The
sum of the values of these three response variables
was then calculated for each stand to describe the
long-term situation of damage occurrence. We use
this sum as a disease index.
The 80 pine stands surveyed were ranked according
to the value of the GEI, and 20 stands with
the lowest and 20 stands with the highest GEI
values were chosen for the comparison. Owing to
the strong skewness of the distribution of the disease
index these two chosen pine-stand-groups
were compared using the Mann-Whitney U-test.
RESULTS AND DISCUSSION
It has been normal practice to predict the geochemistry
of glacial overburden on the basis of the
known geology of the bedrock. Accordingly,
predictions of the acidification sensitivity of soil
in an area have been made on the basis of bedrockgeological
maps (Kommitten Miljö 1982, Shilts
1981). Recent geochemical maps over large regions
- e.g. maps of the Nordkalott project (Be~lviken
et al. 1986), maps of the Geochemical Atlas of Finland,
and maps of the Geological Survey of Finland
- show that the areal distribution of the
aqua-regia extractable contents of most elements
correlates with the main bedrock units only to a
limited extent. The reason for the disagreement
and the source of increased contents unrelated to
bedrock geology has not yet been explained. Björklund
(1988a, b), who pointed out this feature, suggests
a relation between past hydrothermal processes,
with related alteration and metal enrichment
in the bedrock, and an increase in the mobile
phases of elements in the overlying glacial drift.
He suggests that deep salt groundwater and
ascending geogas may play an important role in
the transport and enrichment of the mobile phase
of elements in overburden.
The pine stands in the lowest quartile of the GEI
value had a higher disease index mean than the
pine stands in the highest quartile. We used the
Mann-Whitney U-procedure to test the hypothesis
of equal means and were able to reject it with
the p-value of 3.15 %. Areas susceptible to
scleroderris canker can thus be predicted using the
GEI (Figs. 2 and 3). Figure 2 shows a color image
of the moving median (Björklund & Gustavsson,
1987) for GEI based on the data of the Geochemical
Atlas of Finland. The color image of the
moving median of GEI in Fig. 3 is based on the
denser sampling grid of the Geological Survey of
Finland. Because there is no systematic inventory
of the area damaged by scleroderris canker in Finland
the compatibility of the damaged area with
the prediction in the Figs. 2 and 3 cannot be tested.
The disease frequency has been recorded very sporadically
and the records are often based on subjective
observations. A more detailed, long-term
survey of the damage by scleroderris canker in Finland
is needed.
The results of the present study give support to
the hypothesis that till geochemistry and the occurrence
of scleroderris canker are related to each