TOWARD UNDERSTANDINO ECOSYSTEMS
17
function of sunlight intensity incident on a leaf and as a function of leaf
temperature. These curves are somewhat idealized and partially hypothetical, but are based on observational data reported by Bjorkman
and Holmgren (1 '363), Strain and Chase (I90tl), and Scott and Billings
(1964). For other amount8 of carbon dioxide concentration a different
set of curves will apply. In fact, oiic should consider that R third axis
exists representing carbon dioxide concentration and these curves are
only one section of photosynthetic or respiration surfaces in a threedimensional space. Furthermore, arctic and alpine plants have temperature optima near 12 or 15"C, see Scott and Billings (1964), rather
than near 25°C as shown here. Hence, a warm day for arctic and alpine
plants may be an optimum day for plants of more temperate habitats.
B. PHYSIOLOGICAL CONSEQUENCES O F DIURNAL CLIMATE
An hypothetical example, based on observations, is now described to
illustrate one method for understanding the influence on a plant of the
environment during a diurnal cycle of events. It is emphasized that
this is a somewhat simplified example and what, in fact, actually happens to a plant under the conditions described here may be a little
different. Yet this is a reasonable first approximation to reality and
illustrates the quantitative and analytical methodology which is necessary for ecology. The diurnal variations of sunlight on a leaf and air
temperature near a leaf are given in Fig. 3 for a warm, clear summer
day at about 50" latitude and near the summer solstice. Two types of
leaves are considered first: a fully exposed sunlit leaf with horieontal
surface at all times, and a horizontal shaded leaf.
If an exposed, horizontal leaf is receiving sunlight incident on its
surface approximating the diurnal curve shown in Fig. 3(a), then the
leaf temperature will follow the curve shown in the second graph from
the top when the air temperature is aa shown and the wind speed is
less than 1 m.p.h. For these conditions the exposed, fully sunlit leaf
will transpire according to Fig. 3(c). Tho net photosynthesis and respiration rate for this leaf may be estimated from the curves of Fig. 2 and
the result is given in Fig. 3(d). The abrupt rise of solar intensity in the
early morning will cause a rapid rise of leaf temperature and an abrupt
increaee in assimilation. By 08.00 h the leaf is becoming too warm and
the net photosynthesis for this leaf is diminishing. The leaf becomes
too warm to assimilate and respiration dominates during the period
08.30 to 16.30 h. Only late in the day when leaf temperature diminishes
sufficiently does the net photosynthesia pick up once again and continue
until dark when respiration continues throughout the night. Very few
leaves will ever resemble this case for the fully exposed horizontal leaf
except a few leaves a t the top of a tree. Usually there will be wind
17
function of sunlight intensity incident on a leaf and as a function of leaf
temperature. These curves are somewhat idealized and partially hypothetical, but are based on observational data reported by Bjorkman
and Holmgren (1 '363), Strain and Chase (I90tl), and Scott and Billings
(1964). For other amount8 of carbon dioxide concentration a different
set of curves will apply. In fact, oiic should consider that R third axis
exists representing carbon dioxide concentration and these curves are
only one section of photosynthetic or respiration surfaces in a threedimensional space. Furthermore, arctic and alpine plants have temperature optima near 12 or 15"C, see Scott and Billings (1964), rather
than near 25°C as shown here. Hence, a warm day for arctic and alpine
plants may be an optimum day for plants of more temperate habitats.
B. PHYSIOLOGICAL CONSEQUENCES O F DIURNAL CLIMATE
An hypothetical example, based on observations, is now described to
illustrate one method for understanding the influence on a plant of the
environment during a diurnal cycle of events. It is emphasized that
this is a somewhat simplified example and what, in fact, actually happens to a plant under the conditions described here may be a little
different. Yet this is a reasonable first approximation to reality and
illustrates the quantitative and analytical methodology which is necessary for ecology. The diurnal variations of sunlight on a leaf and air
temperature near a leaf are given in Fig. 3 for a warm, clear summer
day at about 50" latitude and near the summer solstice. Two types of
leaves are considered first: a fully exposed sunlit leaf with horieontal
surface at all times, and a horizontal shaded leaf.
If an exposed, horizontal leaf is receiving sunlight incident on its
surface approximating the diurnal curve shown in Fig. 3(a), then the
leaf temperature will follow the curve shown in the second graph from
the top when the air temperature is aa shown and the wind speed is
less than 1 m.p.h. For these conditions the exposed, fully sunlit leaf
will transpire according to Fig. 3(c). Tho net photosynthesis and respiration rate for this leaf may be estimated from the curves of Fig. 2 and
the result is given in Fig. 3(d). The abrupt rise of solar intensity in the
early morning will cause a rapid rise of leaf temperature and an abrupt
increaee in assimilation. By 08.00 h the leaf is becoming too warm and
the net photosynthesis for this leaf is diminishing. The leaf becomes
too warm to assimilate and respiration dominates during the period
08.30 to 16.30 h. Only late in the day when leaf temperature diminishes
sufficiently does the net photosynthesia pick up once again and continue
until dark when respiration continues throughout the night. Very few
leaves will ever resemble this case for the fully exposed horizontal leaf
except a few leaves a t the top of a tree. Usually there will be wind