Geoscience Reference
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The environment should therefore be able to ensure nutrition of the
forest for 1000 years and probably much more, including the elements
lost if the trees are cut periodically and the wood exported. But these
considerable, if not inexhaustible stocks cannot be mobilized at a rate
sufficient to ensure the growth of trees, especially if coniferous trees
with rapid growth are concerned. On the contrary, calculations show
(Marques 1996; Ezzaim 1997) that there is great danger of exhaustion
in a few decades of easily mobilizable reserves (compare lines 3 and 5
in the table).
Table 2.8
Order of magnitude of stocks and flows of bases under forest in
the temperate region.
Broad-leaved trees
Conifers
Stocks
, kg ha
-1
K
Ca
Mg
K
Ca
Mg
1
In plants
200
200
25
200
400
45
2
In litter humus
300
750
250
120-
100-
160-
400
200
200
3
In soil (available)
300
400
100
600
600
140
4
In soil (reserves)
Tens or hundreds of tonnes according to rocks and elements
Flows
, kg ha
-1
5
Total withdrawals
45
30
5
40
35
6
6
Fixation
4
5
1
5-6
5-8
1
7
Total replenishment
40
25
4
36
25
4
8
Atmospheric fallout
4
7
1
3-4
4-7
1
9 Deep leaching
4
12
4
2-8
2-14 2-5
10 Weathering
Kilograms or tens of kilograms according to
rocks and elements
Secondly, withdrawals and replenishments (lines 5 and 7) are five to
ten times greater than fixation in wood (line 6). The forest thus functions
as a sort of pump for bases, drawing them from the soil and bringing
them to the surface every year. In some Podzols, calcium is mostly
present in the upper part of the profile (Langley-Turnbaugh and Evans
2000). One is therefore justified in asking the question: is the annual
recycling of bases (Duchaufour 1983) large enough to compensate for the
natural leaching of these cations by rainwater? In fact, there generally
is a loss (line 9), even without counting the export of wood. The soil
is, therefore, acidified. But in the greatly depleted tropical environment,
under a rapidly growing tree species (eucalyptus), the biological agents
