The Consequences of Sunflecks for Photosynthesis
355
inflorescences. There were no differences in the rate of growth during the
rapid vegetative phase, which may have depended on reserves in the roots.
However, by the end of the second year, vegetative biomass was significantly greater for plants receiving sunflecks than for those from which
sunflecks had been excluded. Large differences in reproductive allocation
were evident with the infloresence mass reduced by 83% in the plants from
which sunflecks had been excluded. Overall, plants from which sun flecks
had been excluded were 51 % smaller than those receiving sunflecks. These
results show that the fraction of carbon gain contributed by utilization of
sunflecks takes on an even greater importance when growth is considered.
This is because the initial growth and maintenance costs must be borne
before resources can be allocated to reproduction. When the light is reduced
by removal of sunflecks, the effect is largest for reproductive output. Some
caution must be expressed, however, because it was not possible to rule out
a role for variation in diffuse light. Performance of the banded plants was
significantly correlated with estimates from fish eye photographs of the diffuse
light they received, as would be expected. As will be discussed below, there
was also evidence for an important role for diffuse light for the plants
receiving sunflecks.
Examination of the growth and reproductive performance in microsites
differing in the direct PFD received revealed some at first surprising patterns. First, there was, with one exception, no correlation between direct
PFD and any measure of growth or reproductive performance in the different microsites. The one exception was that leaf area per plant was negatively
correlated with direct PFD. Plant size and reproductive effort were, however, positively related to estimates of diffuse PFD, both for plants under
the shadow bands and for plants also receiving sunflecks. A greater sensitivity to a given amount of diffuse PFD could be expected since it is in the
quantum yield-dependent region of the light response where the efficiency
of use is greatest. Many sunflecks, particularly those from canopy gaps that
contribute the most to estimates from fisheye photographs, are, on the other
hand, more than saturating.
It might be expected on the basis of the positive relationship between
carbon gain and direct PFD and the effects observed when sunflecks were
"removed", that more sunflecks would cause an increase in vegetative
growth or reproductive output. However, plants under the shadow bands
shrank as a result of a reduced carbon balance rather than the plants
receiving sunflecks growing more. Other studies with tree seedlings and
saplings (Pearcy 1983; Oberbauer et al. 1988) have shown a positive correlation between direct PFD and growth. However, tree seedlings have
considerable potential for additional growth, whereas most Adenocaulon
plants that were receiving sunflecks were not increasing in size. Possibly
Adenocaulon plants had already reached a size that reflected some other
limitations. There may be additional competition for nutrients or water in
areas receiving more sunflecks. In addition, sunflecks themselves may cause
355
inflorescences. There were no differences in the rate of growth during the
rapid vegetative phase, which may have depended on reserves in the roots.
However, by the end of the second year, vegetative biomass was significantly greater for plants receiving sunflecks than for those from which
sunflecks had been excluded. Large differences in reproductive allocation
were evident with the infloresence mass reduced by 83% in the plants from
which sunflecks had been excluded. Overall, plants from which sun flecks
had been excluded were 51 % smaller than those receiving sunflecks. These
results show that the fraction of carbon gain contributed by utilization of
sunflecks takes on an even greater importance when growth is considered.
This is because the initial growth and maintenance costs must be borne
before resources can be allocated to reproduction. When the light is reduced
by removal of sunflecks, the effect is largest for reproductive output. Some
caution must be expressed, however, because it was not possible to rule out
a role for variation in diffuse light. Performance of the banded plants was
significantly correlated with estimates from fish eye photographs of the diffuse
light they received, as would be expected. As will be discussed below, there
was also evidence for an important role for diffuse light for the plants
receiving sunflecks.
Examination of the growth and reproductive performance in microsites
differing in the direct PFD received revealed some at first surprising patterns. First, there was, with one exception, no correlation between direct
PFD and any measure of growth or reproductive performance in the different microsites. The one exception was that leaf area per plant was negatively
correlated with direct PFD. Plant size and reproductive effort were, however, positively related to estimates of diffuse PFD, both for plants under
the shadow bands and for plants also receiving sunflecks. A greater sensitivity to a given amount of diffuse PFD could be expected since it is in the
quantum yield-dependent region of the light response where the efficiency
of use is greatest. Many sunflecks, particularly those from canopy gaps that
contribute the most to estimates from fisheye photographs, are, on the other
hand, more than saturating.
It might be expected on the basis of the positive relationship between
carbon gain and direct PFD and the effects observed when sunflecks were
"removed", that more sunflecks would cause an increase in vegetative
growth or reproductive output. However, plants under the shadow bands
shrank as a result of a reduced carbon balance rather than the plants
receiving sunflecks growing more. Other studies with tree seedlings and
saplings (Pearcy 1983; Oberbauer et al. 1988) have shown a positive correlation between direct PFD and growth. However, tree seedlings have
considerable potential for additional growth, whereas most Adenocaulon
plants that were receiving sunflecks were not increasing in size. Possibly
Adenocaulon plants had already reached a size that reflected some other
limitations. There may be additional competition for nutrients or water in
areas receiving more sunflecks. In addition, sunflecks themselves may cause