124
J. D. OVINGTON
A serious source of error in determining net primary productivity
arises through too infrequent sampling so that the plants are not harvested at their maximum weights or some plant parts such as bud
scales of flowers areamissed completely. For example, the annual production of inflorescences in stands of aspen, Populus tremuloides, and
of male cones in stands of white pine, Pinus strobus, amounts to about
230 and 656 kg per ha, equal to 5 and 25% respectively of the annual
production of leaves; if sampling is restricted to summer and autumn
these would not be included in primary production estimates. Estimates
of net primary productivity as root material may be very inaccurate
since they are normally calculated from differences in root biomass from
year t o year but it is known that, just as for the tree leaves, there is an
annual mortality of the smaller roots. According t o Orlov (1955) this
amounts t o about 5% of the total root production. Another source of
error arises through failure t o take into account the amount of primary
production eaten by animals. The fact that plants may show little
evidence of animal cropping at the time of sampling can be misleading;
for instance Tortrix viridana frequently completely defoliates oak trees
in early spring, which later grow a new set of leaves so that leaf production is about twice the weight of leaves recorded by a single summer
sampling. On the whole, errors such as these all lead to an underestimation of net primary productivity and the published figures are likely to
be too small.
ii. Change of Net Primary Productivity with Age. The annual net
primary productivities for the trees and for the ecosystem as a whole
(trees plus ground flora) at different ages in the series of plantationa of
Pinus sylvestris are shown in Fig. 3, the estimates being subject t o some
of the ewors described previously. The mean annual productivity of the
trees increases steadily to just over 12 x lo3 kg per ha at about 35 years
of age, after which productivity decreases slightly. The curve of current
annual productivity for the trees is more irregular than the mean curve
but when smoothed out shows a relatively rapid increase in productivity
to a maximum value of just over 22 x lo3 kg per ha, which is achieved
by about 20 years of age and is maintained for several years before declining fairly rapidly after 35 years.
In the youngest, plantations the trees are small and not contiguous,
whilst in the older plantations thinning is repeatedly creating gaps in
the forest so that the tree canopies and root system do not continually
occupy the available crown and root space. Maximum current annual
productivity by the ecosystem is therefore achieved when the dominance
of the trees, as expressed by canopy and root development is greatest,
so that it is not surprising that the biomass of the understorey vegetation is at a minimum at this stage. When the plantations are 20 years
J. D. OVINGTON
A serious source of error in determining net primary productivity
arises through too infrequent sampling so that the plants are not harvested at their maximum weights or some plant parts such as bud
scales of flowers areamissed completely. For example, the annual production of inflorescences in stands of aspen, Populus tremuloides, and
of male cones in stands of white pine, Pinus strobus, amounts to about
230 and 656 kg per ha, equal to 5 and 25% respectively of the annual
production of leaves; if sampling is restricted to summer and autumn
these would not be included in primary production estimates. Estimates
of net primary productivity as root material may be very inaccurate
since they are normally calculated from differences in root biomass from
year t o year but it is known that, just as for the tree leaves, there is an
annual mortality of the smaller roots. According t o Orlov (1955) this
amounts t o about 5% of the total root production. Another source of
error arises through failure t o take into account the amount of primary
production eaten by animals. The fact that plants may show little
evidence of animal cropping at the time of sampling can be misleading;
for instance Tortrix viridana frequently completely defoliates oak trees
in early spring, which later grow a new set of leaves so that leaf production is about twice the weight of leaves recorded by a single summer
sampling. On the whole, errors such as these all lead to an underestimation of net primary productivity and the published figures are likely to
be too small.
ii. Change of Net Primary Productivity with Age. The annual net
primary productivities for the trees and for the ecosystem as a whole
(trees plus ground flora) at different ages in the series of plantationa of
Pinus sylvestris are shown in Fig. 3, the estimates being subject t o some
of the ewors described previously. The mean annual productivity of the
trees increases steadily to just over 12 x lo3 kg per ha at about 35 years
of age, after which productivity decreases slightly. The curve of current
annual productivity for the trees is more irregular than the mean curve
but when smoothed out shows a relatively rapid increase in productivity
to a maximum value of just over 22 x lo3 kg per ha, which is achieved
by about 20 years of age and is maintained for several years before declining fairly rapidly after 35 years.
In the youngest, plantations the trees are small and not contiguous,
whilst in the older plantations thinning is repeatedly creating gaps in
the forest so that the tree canopies and root system do not continually
occupy the available crown and root space. Maximum current annual
productivity by the ecosystem is therefore achieved when the dominance
of the trees, as expressed by canopy and root development is greatest,
so that it is not surprising that the biomass of the understorey vegetation is at a minimum at this stage. When the plantations are 20 years