Q U A N T I T A T I V E ECOLOGY A N D W O O D L A N D ECOSYSTEM
141
this is equivalent t o about 2.5% of the annual incident radiation occurring within the radiation wavelengths known t o be utilized in photosynthesis (4 000 t o 7 000 A). Wassink (1959) has concluded that, when
incident radiation values are limited t o the wavelengths available for
photosynthesis, the overall annual photosynthetic efficiency for field
crops in Holland lies between 1 and 27;, although over periods of a few
weeks efficiencies of 7 t o 9% were attained. Scots pine is not the most
productive of tree species and the site conditions are not of the best, so
that other woodlands elsewhere in Britain attain higher rates of production and of energy transformation, with an annual efficiency of
energy fixation probably just over 3%. Tropical forests a t least equal
temperate forests in photosynthetic efficiency. The estimate of Ogawa
et a.1. (1961) of a net annual production of dry matter of 40 x lo3 kg per
ha for tropical forest gives an annual fixation of about 18 x 1010 cal per
ha in a region where the annual incident radiation within the range for
photosynthesis is about 600 x 10'0 cal per ha, thus photosynthetic
efficiency is around 3%. This compares well with 1.9% for sugar-cane
in Java given by Hellmers and Bonner (1959). Monteith (1959) has
suggested that forests may absorb more radiation than agricultural
crops because of their darker colour and the trapping of radiation
between individual tree crowns and conifer needles so that reflection
of radiation is at a minimum. Thus a pine forest in northern Scotland
may absorb more radiation than a pasture in southern England, even
though the incoming radiation is less, because of the lower amount of
reflection from the pine canopy.
Of the sunlight penetrating through the earth's atmosphere, only a
small portion is converted to chemical energy by photosynthesis. This
is not surprising for solar energy is dissipated in various other
ways, and the photosynthetic process is not l OOyo efficient. Energy
capture by the faster-growing woodland ecosystems a t their most
productive period may therefore be approaching the maximum
possible under natural conditions and so provides a means of determining maximum site potential, a value of considerable interest to ecologists and of some application to land use projects aimed a t improving
productivity.
B. ENERGY ACCUMULATION A N D RELEASE
The storage of energy in the whole woodland ecosystem is considerable. When forest fires break out, this energy is released rapidly as heat
and very high temperatures result, despite the rapid dissipation of heat
upwards. Davis (1959) states that in hot forest fires, the above-ground
temperatures in the organic mass may exceed 800" C. Temperatures of
200" C and higher are common at the ground surface and, with much
141
this is equivalent t o about 2.5% of the annual incident radiation occurring within the radiation wavelengths known t o be utilized in photosynthesis (4 000 t o 7 000 A). Wassink (1959) has concluded that, when
incident radiation values are limited t o the wavelengths available for
photosynthesis, the overall annual photosynthetic efficiency for field
crops in Holland lies between 1 and 27;, although over periods of a few
weeks efficiencies of 7 t o 9% were attained. Scots pine is not the most
productive of tree species and the site conditions are not of the best, so
that other woodlands elsewhere in Britain attain higher rates of production and of energy transformation, with an annual efficiency of
energy fixation probably just over 3%. Tropical forests a t least equal
temperate forests in photosynthetic efficiency. The estimate of Ogawa
et a.1. (1961) of a net annual production of dry matter of 40 x lo3 kg per
ha for tropical forest gives an annual fixation of about 18 x 1010 cal per
ha in a region where the annual incident radiation within the range for
photosynthesis is about 600 x 10'0 cal per ha, thus photosynthetic
efficiency is around 3%. This compares well with 1.9% for sugar-cane
in Java given by Hellmers and Bonner (1959). Monteith (1959) has
suggested that forests may absorb more radiation than agricultural
crops because of their darker colour and the trapping of radiation
between individual tree crowns and conifer needles so that reflection
of radiation is at a minimum. Thus a pine forest in northern Scotland
may absorb more radiation than a pasture in southern England, even
though the incoming radiation is less, because of the lower amount of
reflection from the pine canopy.
Of the sunlight penetrating through the earth's atmosphere, only a
small portion is converted to chemical energy by photosynthesis. This
is not surprising for solar energy is dissipated in various other
ways, and the photosynthetic process is not l OOyo efficient. Energy
capture by the faster-growing woodland ecosystems a t their most
productive period may therefore be approaching the maximum
possible under natural conditions and so provides a means of determining maximum site potential, a value of considerable interest to ecologists and of some application to land use projects aimed a t improving
productivity.
B. ENERGY ACCUMULATION A N D RELEASE
The storage of energy in the whole woodland ecosystem is considerable. When forest fires break out, this energy is released rapidly as heat
and very high temperatures result, despite the rapid dissipation of heat
upwards. Davis (1959) states that in hot forest fires, the above-ground
temperatures in the organic mass may exceed 800" C. Temperatures of
200" C and higher are common at the ground surface and, with much