Agriculture Reference
In-Depth Information
The profile sketched in Figure IV.21 (top), was observed in a grass unit (regeneration
phase). These units have a mull litter system in which litter is rapidly exported to the
drilosphere within the casts of the endogeic and anecic earthworms. These casts form most
of the macro-aggregate structure of the underlying Al horizon (5 to 12 cm). Bark found at
3-4 cm depth and dead coniferous roots at 6-12 cm indicated that this gap had only recent-
ly been formed. In a 30-year Spruce
(Picea abies)
stand (early aggradation phase, Figure
IV.21 right), only mixed Spruce and Larch
(Larix decidua)
litters and mosses were present in
the upper part of the litter system. At 10-15 cm depth, fragments of dead Spruce roots
comprised two thirds of the organic fragments and the remaining third consisted of dead grass
components (
Deschampsia flexuosa
and
Luzula sylvatica
) and mosses. At 8-12 cm depth,
bark residues were abundant. Above this deposit, large amounts of dead mosses occured.
From 3 to 6 cm depth, decomposing Spruce needles were derived from the present plant
community.
This sequence suggests that before the present canopy of young Spruce trees closed, the
soil was first covered by grasses and then by a thick moss cover. At the same time, large
inputs of bark and woody material indicated gap formation within a mature Spruce stand. The
presence of stumps of old (200-year) Spruce trees and a few living individuals confirm these
observations.
As the Spruce stands grow, the canopy progressively closes, the density of endogeic and
anecic earthworms decreases sharply and the litter system shifts to a dysmoder (
i.e.,
inter-
mediate between moder and mor) in the 60-year old Spruce stand (Fig. IV.21, bottom). The
H layer between 4.5 and 7.5 cm depth mainly comprises holorganic invertebrate
faecal pellets. At 6-7 cm depth, large amounts of woody materials and bark fragments in the
soil evidence the collapse of a previous Spruce stand. Below these organic deposits a 1 cm
thick bleached eluvial horizon is present (Bernier and Ponge, 1994) indicating podzolisation.
In 160-190 year old Spruce stands (biostatic phase Figure IV.21 left), small populations of
anecic earthworms (
Lumbricus terrestris)
are restored and re-incorporate organic matter from
the micro-podzol created in the previous phase (Figure IV.21 bottom). The litter system is a
dysmull, an intermediate stage between the moder and the mull that will develop later in the
grasseco-unit.
At different elevations, the correspondence between phases of the vegetation succession
and the appearance and disappearance of anecic earthworms or the time of their peak
density is slightly different. At 900 m elevation, the peak density of anecic and endogeic
earthworms occurred in the old Spruce stand, whereas it was delayed up to the phase when
grass covers the soil at 1550 m, and to an even later phase at 1700 m,
i.e.,
that of young
(30-50 year) Spruce stands (Bernier, 1996). Differences in the quantity and quality of
organic matter accumulated in the litter system may explain these differences.
Actively growing Spruce stands produce large quantities of low quality litter that
discourage drilosphere activities and result in the formation of moder litter systems.
In mature stands, the return of anecic and endogeic earthworms may be attributed to an
overall change in the quantity and quality of the organic matter available in the environment.
Old trees may release large amounts of twig material and organic matter may become more
palatable to the earthworms after approximately a century of maturation. Anecic earthworms
may thus return and their communities become fully mature at the next stage, in the grassy
vegetation where Spruce also re-establishes itself. At 1700 m elevation, temperatures are
