QUANTITATIVE ECOLOQY A N D WOODLAND ECOSYSTEM
173
being released by the decomposition of organic matter. Russian
scientists have been particularly interested in the determination of
nutrient uptake, retention and release by trees and some typical results
are given in Table XVI. The large differences between nutrient uptake
and accretion are probably underestimates since in measuring uptake,
usually no account is taken of nutrients leached out of leaves, or root
exudates (Vinokurov and Tyurmenko, 1960), aiid of the annual inortality of fine roots. As a woodland matures, the magnitude of nutrielit
exchange between the trees and soil changes and is greatest about the
time when organic matter production is at a maximum. Smirnova and
Gorodentseva (1958) found the annual uptake, retention and release of
nitrogen by the trees in a 30-year-old birch wood to be 11 1, 45 and 66
kg per ha respectively, whilst the corresponding values in a 67-year-old
birch wood are 51, 3 and 48. In young birchwoods from 30 to 500/, of
the nutrient uptake is retained in the tree biomass whilst in old birchwoo& only 10 to 20y0 is so immobilized. Mina (1955) reports that in
mixed oakwoods of the Russian forest steppe region, uptake and retention of nutrients reach a maximum when the trees are about 60
years of age. Vinokurov and Tyurinenko (1958) have analysed forest
soils throughout the year arid suggest that nutrient uptake varies
seasonally, reaching a peak at the end of June or beginning of July.
The part played by understorey vegetation in nutrient circulation
must not be overlooked since, at certain stages in the life history of a
woodland, a greater weight of chemical elements may be circulating
through the understorey than through the trees. P’Yavchenko (1960)
found this to be the case in old spruce and pine forests in the Vologod
region of U.S.S.R. I n an 120-year-old pine forest the annual productioii
of dry matter by the understorey vegetation was sixteen tinies greater
than that by the trees and the relative amounts of nutrients absorbed
were even greater. A comparable situation occurs in young aiid old
forest plantations in Britain where the ground flora is very luxuriant.
Nutrient uptake by woodland plants and nutrient release by decomposition are rarely in balance so that changes in the nutrient capital of
the organic matter of the ecosystem must be taken into account in preparing balance tables (Table XVII). Trees, established in formerly treeless areas, represent a positive gain but long-term changes in the
nutrient content of the understorey vegetation and litter layers may be
either positive or negative. Finally, changes in the exchangeable and
total amounts of nutrients in the mineral soil resulting from woodland
growth, may be very important.
173
being released by the decomposition of organic matter. Russian
scientists have been particularly interested in the determination of
nutrient uptake, retention and release by trees and some typical results
are given in Table XVI. The large differences between nutrient uptake
and accretion are probably underestimates since in measuring uptake,
usually no account is taken of nutrients leached out of leaves, or root
exudates (Vinokurov and Tyurmenko, 1960), aiid of the annual inortality of fine roots. As a woodland matures, the magnitude of nutrielit
exchange between the trees and soil changes and is greatest about the
time when organic matter production is at a maximum. Smirnova and
Gorodentseva (1958) found the annual uptake, retention and release of
nitrogen by the trees in a 30-year-old birch wood to be 11 1, 45 and 66
kg per ha respectively, whilst the corresponding values in a 67-year-old
birch wood are 51, 3 and 48. In young birchwoods from 30 to 500/, of
the nutrient uptake is retained in the tree biomass whilst in old birchwoo& only 10 to 20y0 is so immobilized. Mina (1955) reports that in
mixed oakwoods of the Russian forest steppe region, uptake and retention of nutrients reach a maximum when the trees are about 60
years of age. Vinokurov and Tyurinenko (1958) have analysed forest
soils throughout the year arid suggest that nutrient uptake varies
seasonally, reaching a peak at the end of June or beginning of July.
The part played by understorey vegetation in nutrient circulation
must not be overlooked since, at certain stages in the life history of a
woodland, a greater weight of chemical elements may be circulating
through the understorey than through the trees. P’Yavchenko (1960)
found this to be the case in old spruce and pine forests in the Vologod
region of U.S.S.R. I n an 120-year-old pine forest the annual productioii
of dry matter by the understorey vegetation was sixteen tinies greater
than that by the trees and the relative amounts of nutrients absorbed
were even greater. A comparable situation occurs in young aiid old
forest plantations in Britain where the ground flora is very luxuriant.
Nutrient uptake by woodland plants and nutrient release by decomposition are rarely in balance so that changes in the nutrient capital of
the organic matter of the ecosystem must be taken into account in preparing balance tables (Table XVII). Trees, established in formerly treeless areas, represent a positive gain but long-term changes in the
nutrient content of the understorey vegetation and litter layers may be
either positive or negative. Finally, changes in the exchangeable and
total amounts of nutrients in the mineral soil resulting from woodland
growth, may be very important.