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B h s (Xersonal com~unication) has pointed out that
evidence of effects of compaction on f'oraat pr~~uctfon is
scanty*
Youngberg (1959) examined the growth of Douglm
fir seedlings in soils cornacted by tmctors during
lomng,
Growth was aignific&tly less on coqaoted soils
coqsrutd with that on the apparently undisturbed soils of
clear felled arsa, probably because of poor aeration and
low nitrogen in the former soils. Porest soils in
general are light a d e: aily comgacted (~ull, 1959) but
they are usually little disturbed for long per5ods and are
therefore able to mcwer frw severe compaction or
disturbance w ith the aid of mtural biological and physical
processes.
fi) &awes in the --off
water
In British plantations, 1&5kc:/i of the incident precipitation
fails to reach the ground because of interception by the
tree canopy and only 0.w-0.32"/0 of the t &a1 flows down
tho atems of trees (Ovh&on, 1954). Trees therofom
reduce considerably the amout of water available for soil
leaching and also the direct impact of rain rn the soil.
Appreciable quantities of water which have been taken up
by the roots are lost via the leaves as traspimtion.
As a result. of interoeption plus trznfipimtiorq tree s
8speciUy conifers, tend to dry out the soil. This is
seen ,best in the shrinkage of peat soib aftor aff'orentation
(~inns, 1968). Trees also reduce run-off wter to only
28-5% of incident preoipitat ion, conifers tending t o give
lower f5-s than hardwoods (Ovingt on, t 962).
Compafisons of' total evaporation from forest and grassland
or of water yield from f 0-st
givon variable msult s
and grcsslund catchments have
u utter, I 968). This variEtbility