Agriculture Reference
In-Depth Information
between the diversity of the environment (as measured by the Shannon diversity index)
and that of the oribatid communities. The F layers were found to be the most diverse
followed by the L and H layers (Figure III.3).
Using a similar approach but employing a much larger number of characteristics
(see
e.g.‚
Figure IV. 17)‚ Ponge (1985‚ 1988) made a detailed description of the successive
layers of a moder litter developed in a
Pinus sylvestris
forest. The proportions of different
items were measured and the casts and excrements of different invertebrates were
accurately located and identified under a light microscope. A combination of these
methods was later used to describe the vertical structure of humus layers of Spruce
(
Picea abies
) forests in the northern French Alps (Bernier and Ponge‚ 1994) and
permitted reconstruction of the vegetation successions that had occurred on the site.
2.3.2
HORIZONTAL AND TEMPORAL PATTERNS
Litter systems are highly heterogeneous in the horizontal dimension. At scales between
those of a watershed catchment and a hectare‚ forests consist of 'eco-units'
i.e.‚
component
elements of a mosaic at different stages of regeneration after gap formation. At a smaller
scale‚ wind and slope may contribute to the creation of specific areas with accumulations
or deficits of litter. Finally‚ in multispecies communities‚ each tree creates a gradient of
heterogeneity referred to as a 'single-tree effect' (Zinke‚ 1962; Spain‚ 1973). In conse-
quence‚ the three types of litter systems may coexist‚ and form a mosaic in the forest
floor (Figure IV.18).
Litter systems vary also in time‚ at scales differing from a few months (seasons) through
decades to centuries (successions). Significant changes in the seasonal thickness of litter
systems have been reported in both temperate (Mollon‚ 1982) and tropical systems (Healey
and Swift unpublished in Swift
et al.‚
1979; Spain‚ 1984)(Section IV.2.6.1). At the broader
scale of vegetation successions‚ changes in litter systems parallel those of soil organic matter
status‚ communities of decomposers and vegetation. Changes are not necessarily
synchronous: in the alpine forest considered below‚ differences in litter palatability determine
the presence or absence of anecic and endogeic earthworms‚ and the formation of a mull
