QUANTITATIVE ECOLOGY A N D WOODLAND ECOSYSTEM
153
matter has the largest weights present with values for K, Ca, Mg, P and
N of about 900, 2 700, 390, 150 and 2 050 kg per ha; in the temperate
woodlands the maximum recorded values are Na 50, K 470, Ca 1 350, Mg
150, P 130 and N 1 150 per kg ha. The rate of build up of chemicals into
the organic part of the ecosystem varies for different woodlands, the
greatest mean annual accretion for the lifetime of the existing trees
obtained from the tables being 1 kg per ha of Na, 23 for K, 53 for Ca,
8 for Mg, 6 for P, and 60 for N. Few data are available of the carbon
contained within forest ecosystems but the organic mass of 350 x lo3
kg per ha recorded for the 50-year-old woodlands gives a carbon content
of about 175 x lo3 kg per ha with an average annual accretion of 3 500
kg of C per ha.
The amounts of the various elements in the mineral soil given in the
Tables are undoubtedly too small since it seems likely that the tree
roots penetrate through a greater soil depth than has been sampled and
it should be emphasized that except for N these are exchangeable, not
total values. The amounts of exchangeable chemical elements present
in the soil are not greatIy in excess of the weights accumulated in the
organic matter of woodland ecosystems.
B. REMOVAL O F CHEMICAL ELEMENTS BY
HARVESTING O F T R E E TRUNKS
Large volumes of timber have been extracted from some of the older
woodlands in Tables X t o XV. At each harvesting occasion considerable
amounts of chemicals are removed from the ecosystem in the harvested
tree trunks but the overall average annual loss is small. If the standing
trees were felled and all boles harvested, the average annual loss,
inclusive of past harvestings, would not exceed 0.2 kg per ha of Na, 7
of I<, 15 of Ca, 2 of Mg, 14 of P and 14 of N. In fact, smaller amounts
would normally be removed since the nutrient-rich upper parts of tree
trunks and the bark are frequently left within the ecosystem and some
tree boles are not harvested. The total weight of elements removed in
the forest crop is relatively small compared with that of agriculture,
Russell (1956) gives annual removal values for typical English crops of
up to 99 kg per ha for Na, 279 for K, 72 for Ca, 29 for Mg, 26 for P and
167 for N. Similar figures for the annual nutrient uptake of field crops in
Germany are given by Ehwald (1957), i.e. K 167, Ca 63, P 24 and N 164
kg per ha.
c. NUTRIENT UPTAKE, RETENTION A N D RELEASE
The weight of chemical elements in the organic matter of woodland
ecosystems does not represent the total uptake by the plants since,
apart from losses due to harvesting, chemical elements are continually
153
matter has the largest weights present with values for K, Ca, Mg, P and
N of about 900, 2 700, 390, 150 and 2 050 kg per ha; in the temperate
woodlands the maximum recorded values are Na 50, K 470, Ca 1 350, Mg
150, P 130 and N 1 150 per kg ha. The rate of build up of chemicals into
the organic part of the ecosystem varies for different woodlands, the
greatest mean annual accretion for the lifetime of the existing trees
obtained from the tables being 1 kg per ha of Na, 23 for K, 53 for Ca,
8 for Mg, 6 for P, and 60 for N. Few data are available of the carbon
contained within forest ecosystems but the organic mass of 350 x lo3
kg per ha recorded for the 50-year-old woodlands gives a carbon content
of about 175 x lo3 kg per ha with an average annual accretion of 3 500
kg of C per ha.
The amounts of the various elements in the mineral soil given in the
Tables are undoubtedly too small since it seems likely that the tree
roots penetrate through a greater soil depth than has been sampled and
it should be emphasized that except for N these are exchangeable, not
total values. The amounts of exchangeable chemical elements present
in the soil are not greatIy in excess of the weights accumulated in the
organic matter of woodland ecosystems.
B. REMOVAL O F CHEMICAL ELEMENTS BY
HARVESTING O F T R E E TRUNKS
Large volumes of timber have been extracted from some of the older
woodlands in Tables X t o XV. At each harvesting occasion considerable
amounts of chemicals are removed from the ecosystem in the harvested
tree trunks but the overall average annual loss is small. If the standing
trees were felled and all boles harvested, the average annual loss,
inclusive of past harvestings, would not exceed 0.2 kg per ha of Na, 7
of I<, 15 of Ca, 2 of Mg, 14 of P and 14 of N. In fact, smaller amounts
would normally be removed since the nutrient-rich upper parts of tree
trunks and the bark are frequently left within the ecosystem and some
tree boles are not harvested. The total weight of elements removed in
the forest crop is relatively small compared with that of agriculture,
Russell (1956) gives annual removal values for typical English crops of
up to 99 kg per ha for Na, 279 for K, 72 for Ca, 29 for Mg, 26 for P and
167 for N. Similar figures for the annual nutrient uptake of field crops in
Germany are given by Ehwald (1957), i.e. K 167, Ca 63, P 24 and N 164
kg per ha.
c. NUTRIENT UPTAKE, RETENTION A N D RELEASE
The weight of chemical elements in the organic matter of woodland
ecosystems does not represent the total uptake by the plants since,
apart from losses due to harvesting, chemical elements are continually