QUANTITATIVE ECOLOGY A N D WOODLAND ECOSYSTEM
177
Ovington, 1959). Whilst Tukey et al. (1958) have demonstrated leaching
of nutrients from the foliage, part of the increase in the nutrient content
of rain-water may be due to the washing off of aerosols and extraneous
matter deposited on the vegetation. The large and complex woodland
canopy forms an effective filter of the lower atmosphere which is becoming increasingly polluted with industrial wastes.
Tamm and Troedsson (1955) have drawn attention to the large
amounts of material blown into woodland ecosystems from dirt roads
resulting in an increase of the nutrient budget. Holstener-Jorgensen
(1960) found that the annual input of wind-borne material into a
spruce woodland in Denmark amounted to 1 000 kg per ha a t a distance
of 80 m from the woodland edge. He attributes this to soil blown from
arable land and points out that, since the smaller, more nutrient-rich
soil particles are carried by the wind, the addition of nutrients in this
way must be considerable.
Additional supplies of nutrients are made available in the soil by
weathering of soil particles and by soil formation from the underlying
rock. The rate at which these two processes take place in woodlands is
not known accurately but Klausing (1956) has suggested that in old
beech woods, erosion of the parent rock may be as much as 1.2 mm a
year for granite and 2.1 mm for diorite. The rate of weathering would
be expected to be fairly high in woodland ecosystems because of the
large annual turnover of organic matter and the huge root systems of
trees which frequently extend deeply into rock fissures. In areas, where
high winds are frequent, trees may be blown down (Lutz, 1040) and the
resultant mass turnover of soil is of considerabie significance in weathering, and in bringing the lower soil horizons to the surface. By this means
compacted horizons may be shattered so that the effective soil volume
and hence amount of nutrients a1:ailable is increased. On steep slopes
soil movement downslope may occur following windblow.
Since the gain of nitrogen by the forest from precipitation is small,
most of the nitrogen must be obtained from the atmosphere. An indication of the magnitude of atmospheric nitrogen fixation by microorganisms is given by the fact that the annual accumulation of nitrogen
in the organic material may be as much as 60 kg per ha and in addition
there tends to be more nitrogen in the mineral soil after afforestation.
Crocker and Major (1955) investigating a natural vegetation sequence,
following glacier retreat, found that the average annual rate of nitrogen
accumulation in the mineral soil under alder amounted to 26 kg per ha
and comparable rates of accumulation have been recorded for forest
plantations (Holmsgaard, 1960). Substantially more nitrogen must be
fixed than is accumulated within the ecosystem. High rates of nitrogen
fixation occur when forest vegetation contains plants, such as alder,
177
Ovington, 1959). Whilst Tukey et al. (1958) have demonstrated leaching
of nutrients from the foliage, part of the increase in the nutrient content
of rain-water may be due to the washing off of aerosols and extraneous
matter deposited on the vegetation. The large and complex woodland
canopy forms an effective filter of the lower atmosphere which is becoming increasingly polluted with industrial wastes.
Tamm and Troedsson (1955) have drawn attention to the large
amounts of material blown into woodland ecosystems from dirt roads
resulting in an increase of the nutrient budget. Holstener-Jorgensen
(1960) found that the annual input of wind-borne material into a
spruce woodland in Denmark amounted to 1 000 kg per ha a t a distance
of 80 m from the woodland edge. He attributes this to soil blown from
arable land and points out that, since the smaller, more nutrient-rich
soil particles are carried by the wind, the addition of nutrients in this
way must be considerable.
Additional supplies of nutrients are made available in the soil by
weathering of soil particles and by soil formation from the underlying
rock. The rate at which these two processes take place in woodlands is
not known accurately but Klausing (1956) has suggested that in old
beech woods, erosion of the parent rock may be as much as 1.2 mm a
year for granite and 2.1 mm for diorite. The rate of weathering would
be expected to be fairly high in woodland ecosystems because of the
large annual turnover of organic matter and the huge root systems of
trees which frequently extend deeply into rock fissures. In areas, where
high winds are frequent, trees may be blown down (Lutz, 1040) and the
resultant mass turnover of soil is of considerabie significance in weathering, and in bringing the lower soil horizons to the surface. By this means
compacted horizons may be shattered so that the effective soil volume
and hence amount of nutrients a1:ailable is increased. On steep slopes
soil movement downslope may occur following windblow.
Since the gain of nitrogen by the forest from precipitation is small,
most of the nitrogen must be obtained from the atmosphere. An indication of the magnitude of atmospheric nitrogen fixation by microorganisms is given by the fact that the annual accumulation of nitrogen
in the organic material may be as much as 60 kg per ha and in addition
there tends to be more nitrogen in the mineral soil after afforestation.
Crocker and Major (1955) investigating a natural vegetation sequence,
following glacier retreat, found that the average annual rate of nitrogen
accumulation in the mineral soil under alder amounted to 26 kg per ha
and comparable rates of accumulation have been recorded for forest
plantations (Holmsgaard, 1960). Substantially more nitrogen must be
fixed than is accumulated within the ecosystem. High rates of nitrogen
fixation occur when forest vegetation contains plants, such as alder,