Agriculture Reference
In-Depth Information
IV.2
THE LITTER SYSTEM
The litter system is that part of the ecosystem within which above-ground litter
accumulates and decomposes. Most studies of decomposition and nutrient cycling have
been conducted in this specific environment and the basic decomposition processes
described in Section IV.1 are directly relevant to the functioning of this system. Litter sys-
tems differ widely in their morphologies and in their community compositions. This is
reflected in the modal types recognised in past classifications of humus types and by the
distinct microbial and invertebrate communities that characterise litters of different qual-
ity. In this section‚ particular emphasis is placed on: (i) the relationships between biotic
communities and the morphology and structure of the litter system; and (ii) on the
spatial and temporal patterns of the processes taking place within them.
2.1
Composition
The litter system includes the above-ground litter which serves as the energy source‚
a rich microflora dominated by fungi and the epigeic invertebrates and surface roots that
act as regulatory macro-organisms. As seen in Chapter I (Section I.3.2.2) above-ground
litter is an heterogeneous resource. It comprises a mixture of relatively high quality
resources such as fresh leaf-litter‚ flowers‚ fruits‚ seeds‚ dead micro-organisms and
animals and structures of lower quality‚ mainly woody materials. As decomposition
proceeds‚ fresh litter falls to cover the previously deposited materials; these older
materials have greater resistance to further decomposition and the litters of certain plant
species accumulate in deep layers forming a gradient of substrates of diminishing
quality with increasing depth. However‚ transfers of organic materials and bioturbation
processes may alter this distribution.
Litter inputs are also highly variable in time (seasonal variations) and they change
with climate resulting in clear patterns in variation of inputs along a thermo-latitudinal
gradient. At individual sites‚ trees of different ages (Bernier and Ponge‚ 1994) and
different species (Boettcher and Kalisz‚ 1992; Grandval‚ 1993) may produce litters of
contrasting qualities thereby inducing heterogeneity in the litter system at scales that
may range from metres to decameters.
Readily-assimilable carbohydrates may be released from decomposing litter and
percolate through the underlying litter layers (see‚
e.g
.‚ Carlisle
et al.‚
1966) to prime
local microbial activity. The microflora is dominated by the fungi which possess a
pseudo-mobility resulting from their capacity for rapid growth and an ability to translo-
cate cytoplasm. As shown in Chapter III‚ their ability to perforate cell-walls and feed
preferentially on carbohydrates makes them efficient early litter colonisers.
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