Agriculture Reference
In-Depth Information
during senescence and prior to leaf abscission, partly through their translocation to other
parts of the plant.
The tanning of cytoplasmic proteins by polyphenols released from vacuoles at cell
death is a further process of particular importance for decomposition (see Chapter IV).
This results in the formation of highly decomposition-resistant tannin-protein complexes
(Handley, 1954; Toutain, 1987a). The accessibility of remaining water-soluble sugars
may be further reduced by the presence of such soluble toxic components as polyphenols,
waxes and resins (Mangenot and Toutain, 1980).
Litters may thus be classified into the three following broad groups:
Litters with high concentrations of soluble carbohydrate whose exploitation is not
limited by extreme physical or chemical characteristics. Examples include the litters
of hornbeam ( Carpinus spp.) and birch ( Betula spp.) in temperate areas and those of
such tropical legumes as Sesbania or Erythrina;,
Litters with high concentrations of soluble carbohydrates, waxes and/or polyphenols
including those of conifers, European oaks ( Quercus spp.), chestnut ( Castariea sativa )
and the tropical leguminous shrub Inga edulis (Palm and Sánchez, 1991);
Litters with low concentrations of soluble carbohydrate such as European beech
( Fagus sylvatica ) .
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2.
3.
The chemical quality of fresh leaf litter thus depends on the compositions of
the leaves of the individual plant species present and their relative contributions to
the overall mass of litter present.
Many plants accumulate quite specific sugars and a wide range of waxy materials,
terpenes and alkaloids, especially those of tropical areas (Waterman and McKey, 1989;
Baas, 1989). The chemical variability of leaf litter in tropical forests may well exceed
that of temperate forests because of the greater species diversity and complexity of
tropical plant material. However, in a tropical Bornean rainforest, only 4 of 306 species
present contributed more than 5 % to litterfall, the maximum proportion being 10 %
(Burghouts, 1993). Similarly, at a single location at the foot of an individual tree in
a French Guyanese rainforest, only 16 to 21 % of the litter sampled came from the tree
above and more than 20 different species contributed significantly to the liner collected
at this location throughout a yearly cycle (Leroy, unpublished data).
The quality of leaf material also changes with its position on the tree ('light' as
opposed to 'shade' leaves) and the nutrient status of the soil (Heath et al., 1966;
Dickinson and Pugh, 1974; Ellsworth and Reisch, 1993).
Woody litter
Wood has more structural supporting tissue implying higher cellulose and lignin and
substantially-lower nutrient element concentrations than foliage. The accessibility of
woody litter is reduced by its hardness and the low surface to volume ratio it offers to
potential consumers; its accessibility is therefore substantially lower than that of leaf
litter (Table I.25).
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