TOWARD UNDERSTANDING ECOSYSTEMS
25
specific environmental factor. At the same time it is often not clear a t
all which factor or factors may be limiting because of the complexity
of the plant community and the simultaneity of action of all factors.
If the light intensity within a well-watered forest is low then the
young seedlings of the forest floor may not photosynthesize at a rate
sufficiently greater than the rate of respiration to result in much growth
and light is the limiting factor. Because of the generally temperate
environment of the forest floor it is clear that the temperature of the
seedlinga is unlikely to be too warm. There is very little forced convection, for lack of wind, but the plants do not need the ventilation. It is
likely that the carbon dioxide concentrations of the air near the forest
floor bcild up considerably during the night time and thereby facilitate
even the weak level of photosynthesis at low light intensity during the
day time. From direct measurements of the assimilation rates of the
seedlings at the light levels and air temperatures encountered it is possible to estimate quantitatively the amount of growth and the marginal
level by which these seedlings continue to live. Although light is clearly
the limiting factor here it is only in the context of simultaneity of other
factors that light is limiting, e.g. that the wind is negligible, the air
temperature is neither too high nor too low and the relative humidity
makes little difference.
1. Water Usage
For many places on the earth’s surface it is obvious that water is the
limiting factor. The more difficult question to answer is precisely to
what exknt is the water limiting. What is the water usage by a stand
of vegetation of known composition? If there ia a shortage of soil water
when doos it become a stressed situation for certain species and, if so,
for which ones and why? Tho question concerning water aa a limiting
factor is particularly dramatic from the standpoint of simultaneity, for
the water usage is a strong function of wind, air temperature, relative
humidit-, and radiation intensity. This is illustrated in Fig. 8 which
shows the transpiration rate per unit leaf area from a single leaf of
0.6 cm width as a function of the wind speed a t a fixed amount of
absorbed radiation equal to 1.0 cal cm-2 min-l at, a diffusion resistance
of 2 sec cm-1 for various air temperatures and relative humidities.
These are specifications which are appropriate for some grasses and for
wheat end oats. It is seen that with cool (10°C) air of any moisture
content an increase of wind speed from still air conditions resulte in 8
strong reduction in transpiration rate. Transpiration into warm (30°C)
air increases with wind speed, except at high relative humidities, for
the small leaf size and low diffusion resistance given here. Change tt:
leaf size to 5 cm width and the internal diffusion resistance t o 10 sec cm-l
25
specific environmental factor. At the same time it is often not clear a t
all which factor or factors may be limiting because of the complexity
of the plant community and the simultaneity of action of all factors.
If the light intensity within a well-watered forest is low then the
young seedlings of the forest floor may not photosynthesize at a rate
sufficiently greater than the rate of respiration to result in much growth
and light is the limiting factor. Because of the generally temperate
environment of the forest floor it is clear that the temperature of the
seedlinga is unlikely to be too warm. There is very little forced convection, for lack of wind, but the plants do not need the ventilation. It is
likely that the carbon dioxide concentrations of the air near the forest
floor bcild up considerably during the night time and thereby facilitate
even the weak level of photosynthesis at low light intensity during the
day time. From direct measurements of the assimilation rates of the
seedlings at the light levels and air temperatures encountered it is possible to estimate quantitatively the amount of growth and the marginal
level by which these seedlings continue to live. Although light is clearly
the limiting factor here it is only in the context of simultaneity of other
factors that light is limiting, e.g. that the wind is negligible, the air
temperature is neither too high nor too low and the relative humidity
makes little difference.
1. Water Usage
For many places on the earth’s surface it is obvious that water is the
limiting factor. The more difficult question to answer is precisely to
what exknt is the water limiting. What is the water usage by a stand
of vegetation of known composition? If there ia a shortage of soil water
when doos it become a stressed situation for certain species and, if so,
for which ones and why? Tho question concerning water aa a limiting
factor is particularly dramatic from the standpoint of simultaneity, for
the water usage is a strong function of wind, air temperature, relative
humidit-, and radiation intensity. This is illustrated in Fig. 8 which
shows the transpiration rate per unit leaf area from a single leaf of
0.6 cm width as a function of the wind speed a t a fixed amount of
absorbed radiation equal to 1.0 cal cm-2 min-l at, a diffusion resistance
of 2 sec cm-1 for various air temperatures and relative humidities.
These are specifications which are appropriate for some grasses and for
wheat end oats. It is seen that with cool (10°C) air of any moisture
content an increase of wind speed from still air conditions resulte in 8
strong reduction in transpiration rate. Transpiration into warm (30°C)
air increases with wind speed, except at high relative humidities, for
the small leaf size and low diffusion resistance given here. Change tt:
leaf size to 5 cm width and the internal diffusion resistance t o 10 sec cm-l