Agriculture Reference
In-Depth Information
lower and maturation of the organic matter may be delayed up to the stage when the
Spruce forest first matures.
This example illustrates the importance of carbon cycling and energy fluxes as
determinants of succession in the above-ground vegetation and litter system. It also demon-
strates the importance of climatic factors in the synchronisation of plant successions with
changes in the organic matter status of the system. Considerable energy flows occur
through the litter system. Ovington (1961) estimated that the energy contained in the needle
litter of a 55-year-old
Pinus sylvestris
stand was
ca.
251 GJ and that which had
already been released was 3.8 TJ. Part of this energy may be used by anecic earthworms
and other 'ecosystem engineers' to fuel bioturbation processes (Villenave
et al.‚
1999).
Accumulation of litter and single-tree effects
Macro- and microtopographic features and the disposition of trees may lead to considerable
variation in the thickness of litter cover‚ especially of the L and F layers. In a French
temperate-climate forest with a moder litter system‚ the thickness of the litter layer (L+ H)
was significantly correlated with microtopographic features. Litter was unexpectedly less
abundant in small depressions than between them‚ perhaps due to the effects of the white-rot
fungi that develop preferentially in thick F layers (Garay‚ 1989) (see Section IV.2.5.4).
The distributions of trees‚ and differences in the quality of the litters that they produce
locally determine the litter system. Zinke (1962) proposed the concept of the 'single-tree
circle of influence' to substantiate the idea that forests are a mosaic of distinct soil and litter
systems that are associated with individual trees. In a Bornean tropical forest‚ Burghouts
(1993) reported large variation in the thickness of the L and F layers. Surprisingly‚ positive
correlations were found among nutrient contents‚ litter inputs‚ and the thickness of the litter
system. In that situation‚ litter accumulation was not due to slow decomposition rates‚ but to
the rapid cycling of nutrients which favours the production of large quantities of relatively
high quality materials.
In a tropical forest in French Guyana‚ Leroy
et al.
(1992) and Grandval (1993) described
the patterns of distribution of leaf-fall around the trunk of a single tree. Due to the high
diversity of tree species in the rainforest‚ the proportion of leaves deposited close to and
derived from this particular tree only accounted for 18-22 % of the overall litter collected; the
remaining 80 % comprised leaves from the surrounding trees but belonging to different
species. The computation of semivariograms based on an intensive sampling showed that the
mass of leaves deposited at the foot of the tree decreased regularly with increasing distance
away from the trunk and became negligible at approximately 30 m from the trunk.
In systems where litter quality differs among species‚ a mosaic of litter patches of different
quality may form (Figure IV.22).
Such 'single-tree influence circles' have been found to affect soil properties and decom-
poser communities in temperate-climate forests where trees produce litters of
contrasting quality (Boettcher and Kalisz‚ 1991‚ 1992). Similarly‚ the introduction
of conifers or
Eucalyptus
spp into broadleaf deciduous or conifers into
Eucalyptus
spp.
forests may alter the litter system so that it shifts from a mull to a moder humus type (
e.g.‚
Turner and Franz‚ 1985; Garay‚ 1989).
Finally‚ differences in litter production by individual trees‚ and especially‚ asynchrony
in leaf fall at the level of populations or individual trees may reinforce the patchiness of
