Agriculture Reference
In-Depth Information
In a French temperate-climate forest with a mixed vegetation cover of
Quercus
sessiliflora
and
Carpinus betulux,
white-rot fungi developed preferentially in the thick
F layers and the structure of the acarine communities present appeared to depend on the
occurence of these fungi (Garay, 1986b). Overall population density of mites in the
F horizon of sites where litter had accumulated was 100 500 in patches colonised by
white-rot fungi compared with 60 800 in non-infected areas; such population density
increases were unevenly distributed among taxonomic groups. In a French Guyanese
tropical rain forest, the occurrence of white-rot fungi may be closely related to litter
quality; again, different invertebrate communities have been found in litters colonised
and not colonised by white-rot fungi: earthworms colonised patches with no white rot
fungi whereas Phalangidae (Opiliones) and Coleoptera tended to have an opposite dis-
tribution (Leroy
et al.,
1992; Grandval, 1993). In tropical forests and tree plantations
from Congo and French Guyana, the dependence of white-rot fungi on litter quality has
been confirmed by Bernhard-Reversat and Schwartz (1997) and Leroy
et al.
(1992).
2.5.5
ROOT ACTIVITIES
Roots act as sinks for nutrients and water. As a result, the quantities of nutrients leached
from the litter system are significantly diminished in microcosms with roots in comparison
with those where roots are absent (Figure IV.26) (Anderson
et al.,
1983a). In litterbag
experiments conducted in Central Amazonia, roots colonised the decomposing litter and
had no significant effect on the rate of decomposition (Luizao, 1994). Roots may be
colonised by a wide range of different mycorrhizal fungi that interact
with the non-symbiotic microflora. Through their effects on soil physical conditions
(reductions in moisture and oxygen contents), and biological interactions with both the
free-living and symbiotic microflora, roots may have a positive (priming) or negative
(conserving) effect on decomposition.
Studies in which roots have been isolated from the litter system by trenching, have
demonstrated the substantial capacity for tree roots to absorb soil moisture (
e.g.,
Slavikova,
1958; Staff, 1988) and to take up nutrients from the litter system (Aber
et al.,
1983;
Harmer and Alexander, 1985). Gadgil and Gadgil (1971) hypothesised that mycorrhizal
tree roots suppress the decomposition of forest litter. In a New Zealand
Pinus radiata
plantation, in which root activity - and thus mycorrhizal activity - were reduced by
trenching the soil to a depth of 30 cm to isolate blocks of soil and litter from root
activity, litter decomposition rates were significantly increased. This increase was
attributed to the suppression of competition for nutrients between mycorrhizal fungi
and saprotrophic micro-organisms. However, subsequent similar experiments have
not confirmed this effect (Staff, 1988). This may be due to the complex synergistic or
competitive interactions that can take place between tree roots, mycorrhizal and sapro-
trophic fungi (Dighton
et al.,
1987). The factors determining the direction and intensity
of these interactions are far from being clearly understood.
