NATURAL HABITATS - The Soil - page 149

pillars, c, c' = fungus gnat larvae (Mycetophilidae), d, d' = beetle mite larvae (Phthiracaridae). (From L.
Bal 1970.)
W OODLANDS
The biological importance of litter decay is most conspicuous in woodland. In a de-
tailed study in Holland, L. Bal looked at the sequence of organic horizons formed as
fresh plant litter was progressively attacked by soil organisms. He compared the mod-
er humus profiles developed under red oak Quercus borealis and Douglas fir Pseudot-
suga menziesii when planted on very acid sandy soils. Under both species, the depth
of humus was similar - about six centimetres - but the profiles differed in detail. In
the fir profile ( Fig. 49 ) , there was an approximately equal division into three zones.
The surface litter, or L layer, was composed of only slightly modified fir needles. This
merged into an F layer in which the needles were susceptible to attack by soil anim-
als, and an increasing complement of faunal droppings appeared. The third zone was
a fully humified H layer, dominated by weathered droppings that had passed through
more than one cycle of ingestion and excretion, with only skeletal fragments of the
initial needle litter remaining. The transition between this humus and the underlying
mineral horizon was fairly sharp, there being only minor physical inwash or faunal
transport of the humus particles among the sand grains.
F IG. 50
Leaf discs eaten by various invertebrates. The discs had been cut from oak and beech, placed in nylon
mesh bags of 7 mm, 1 mm and 0.5 mm openings and buried in the soil. (From C. A. Edwards & G. W.
Heath 1963.)
In contrast, the more rapidily decomposed oak litter produced a rather thinner
L layer. This passed again into an F layer of partially decomposed and compacted
leaves mingled with faunal droppings but, below this, the completely humified H ho-
rizon occupied about half the thickness of the whole humus profile, and was more
conspicuously mixed with sand grains from the mineral horizon.