Geoscience Reference
In-Depth Information
A forest floor has the greatest variety of plant litter - annual sheddings from the
tree canopy with occasional branches and logs, all in varying stages of disintegration
try is around 2.5 kg a square metre (10 tons an acre). Seventy-five percent of this is
leaf litter, the rest being made up of twigs, bark and seeds etc. The detailed character
of this litter is important in providing habitats and food sources for different organ-
isms. In a particular study of woodland litter, H. Heatwole decided there were three
main categories. His first class consisted of leaves that roll or bend when they fall
to the ground, thus producing large, round or angular spaces between them. Class 2
consisted of leaves that remain flat and so have small, narrow interspaces, and class 3
consisted of solid, woody objects. Each class was subdivided, for example to distin-
guish thick leathery leaves from thin papery ones and conifer needles, or accumula-
tions of twigs from large logs. Some of these types are characteristic of natural, mixed
deciduous woodland, beech hangers or pine woods in this country. A few are perhaps
associated more with parks and gardens, where exotic trees and shrubs like rhododen-
dron have been planted.
There is an enormous difference in the persistence and smothering effect of dif-
ferent kinds of leaves, ranging from ash and apple, which disappear in weeks, to
beech, holly, rhododendron and conifer needles which may last for years. This dif-
ference is partly due to their size and thickness, and partly to their palatability to soil
animals and susceptibility to fungal attack. The depth of litter can vary greatly de-
pending on the density of trees, the time of year, the properties of the underlying soil,
and the micro-relief of the ground. Mounds and convex surfaces may remain largely
bare of litter while leaves and twigs accumulate in hollows which therefore act as foci
for litter-seeking invertebrates. The student of these groups soon gets an eye for such
'hot spots'; with experience, he can judge very accurately what species to expect.
Where litter persists for several months or years, one can usually see three dis-
tinct organic layers above the mineral soil itself. The uppermost layer of curled and
uncompressed leaves has a great deal of interstitial space. This is the zone favoured
by large, active springtails which grow to 5-6mm in size, and which form the prey of
many beetles and spiders. Both hunting and web-building spiders exploit this open-
textured but sheltered environment. The webs may be fairly simple arrangements of
criss-cross threads spun across the ends of the rolled leaves, but these suffice to en-
tangle or delay weak prey. Experiments have shown that leaf characteristics influence
the numbers of spiders in woodland litter: curled leaf litter tends to support higher
densities and a greater assortment of spiders.
Beneath this layer of relatively unaltered plant remains comes a zone of partly
decomposed but still clearly recognizable plant fragments, and below this a zone of
