134
J. D. OVINQTON
Once true woodland conditions are established, woodland ecosystems
are capable of making fairly full use of the site since the peak annual
biological productivity of a high producing forest type presumably
corresponds closely t o the productive capacity of the site. Evidence t o
support this is found in the interrelationships between the over- and
understorey plants, the extinction of the ground flora at the pole stage
indicating that the production limits of the site have been attained by
the trees alone. At this stage the scope for increasing biological productivity is more restricted, the most promising approach being to try
and improve site conditions.
Some environmental factors can be changed easily. When lack of
mineral nutrients limits productivity, various improvement techniques
may be adopted, e.g. the application of fertilizers, the breaking of impenetrable soil layers by ploughing in order to make a larger volume of
soil available and the use of appropriate mixtures of complementary tree
species (Pogrebnyak, 1960). Similarly soil moisture can be regulated by
irrigation or drainage and pest control attempted by biological or
chemical means.
Other environmental factors such as the carbon dioxide concentration
of the air, temperature and light cannot be changed readily over large
areas, and when these factors are limiting it seems that biological productivity can only be increased greatly by breeding plant varieties
having a greater photosynthetic efficiency. Nevertheless, Dadykin
(1960) has suggested that some improvement in production can be
obtained by running the tree rows from north t o south and using certain
combinations of tree species with canopy characteristics giving the
maximum absorption of the incident solar energy.
Whilst it is generally accepted that sylvicultural operations such as
thinning and pruning result in an improvement of timber quality, their
effect on productivity has been the subject of some controversy. After a
detailed examination of some of the experimental dataMoller (1954,1960)
concluded that within wide limits of thinning intensity, the average increment of the trees over long periods is not affected by thinning and that
the long-established maxim that removing the lower third of the live
crown of trees does not reduce the increment is incorrect. However,
Mbller is primarily concerned with trees, particularly tree boles, and
not with whole ecosystems. In a thinning experiment with Scots pine in
Britain, at 29 years of age no ground flora was present in the lightly
thinned plot but in the heaviest thinned plot the ground flora had an
oven-dry weight of 0.6 x lo3 kg per ha, so that if the productivity of the
trees is constant the productivity of the ecosystem must have been
changed by thinning. Haberland and Wilde (1961) have shown that
heavy thinning in dense red pine plantations in Wisconsin induces un-
J. D. OVINQTON
Once true woodland conditions are established, woodland ecosystems
are capable of making fairly full use of the site since the peak annual
biological productivity of a high producing forest type presumably
corresponds closely t o the productive capacity of the site. Evidence t o
support this is found in the interrelationships between the over- and
understorey plants, the extinction of the ground flora at the pole stage
indicating that the production limits of the site have been attained by
the trees alone. At this stage the scope for increasing biological productivity is more restricted, the most promising approach being to try
and improve site conditions.
Some environmental factors can be changed easily. When lack of
mineral nutrients limits productivity, various improvement techniques
may be adopted, e.g. the application of fertilizers, the breaking of impenetrable soil layers by ploughing in order to make a larger volume of
soil available and the use of appropriate mixtures of complementary tree
species (Pogrebnyak, 1960). Similarly soil moisture can be regulated by
irrigation or drainage and pest control attempted by biological or
chemical means.
Other environmental factors such as the carbon dioxide concentration
of the air, temperature and light cannot be changed readily over large
areas, and when these factors are limiting it seems that biological productivity can only be increased greatly by breeding plant varieties
having a greater photosynthetic efficiency. Nevertheless, Dadykin
(1960) has suggested that some improvement in production can be
obtained by running the tree rows from north t o south and using certain
combinations of tree species with canopy characteristics giving the
maximum absorption of the incident solar energy.
Whilst it is generally accepted that sylvicultural operations such as
thinning and pruning result in an improvement of timber quality, their
effect on productivity has been the subject of some controversy. After a
detailed examination of some of the experimental dataMoller (1954,1960)
concluded that within wide limits of thinning intensity, the average increment of the trees over long periods is not affected by thinning and that
the long-established maxim that removing the lower third of the live
crown of trees does not reduce the increment is incorrect. However,
Mbller is primarily concerned with trees, particularly tree boles, and
not with whole ecosystems. In a thinning experiment with Scots pine in
Britain, at 29 years of age no ground flora was present in the lightly
thinned plot but in the heaviest thinned plot the ground flora had an
oven-dry weight of 0.6 x lo3 kg per ha, so that if the productivity of the
trees is constant the productivity of the ecosystem must have been
changed by thinning. Haberland and Wilde (1961) have shown that
heavy thinning in dense red pine plantations in Wisconsin induces un-