The Consequences of Sunftecks for Photosynthesis
353
calculated value is the effect of the source air. The value of the source air
was thus -9.0%0, which is slightly higher than values reported for tropical
forest understories (Sternberg et aI., 1989). The respiration rates may be
lower because of cooler temperatures and slower decomposition, or the
mixing rates of the understory air with the bulk atmosphere greater in the
redwood forest than in the tropical forests. Using the corrected value for
the source air the Ol3C expected during sunflecks can be obtained from the
pJPa and is equal to - 29.1. Thus the carbon gained during sunflecks should
be 4.50/00 lighter than the carbon gained during diffuse light.
The values for carbon fixed during sunflecks and diffuse light can be used
with Eq. (3) to calculate the fractional contribution of sunflecks. For 5 mol
photons m 2 day-I, which is representative for the brightest microsites sampled, Eq. (3) predicts that 46% of the carbon gain that went to production
of the biomass of these plants was due to sunflecks. By contrast, at 1 mol
photons m- 2 day-t, which is characteristic of the microsites with the smallest amount of direct PFD, only about 9% of the carbon gain was due to
sunflecks. Thus utilization of sunflecks was responsible for a substantial
fraction of the total annual carbon gain in the growing season but, as already
shown with the daily gas exchange measurements (Fig. 17.1), it varied
greatly from one microsite to another as a function of the direct PFD.
Comparison of the long-term fraction of assimilation due to sunflecks as
estimated from carbon isotope ratios with the short-term ratios from gas
exchange reveals that the latter are substantially greater. This is to be
expected since the diurnal measurements were done on mostly clear days
when sunflecks would obviously be most abundant whereas the carbon
isotope measurements integrate over both clear and cloudy days. The
brightest microsites used for the carbon isotope studies were brighter than
the brightest of those for the diurnal run, so that in fact the difference may
be even larger than indicated by the maximum values.
17.6 Consequences of Sunftecks for Growth and Reproduction
The studies with gas exchange and carbon isotopes both indicate that utilization of sunflecks contributes a significant fraction of the carbon gain in the
brightest microsites but much less in the most shaded sites. The obvious
question is how does this translate into performance in the understory in
terms of growth and reproduction and to the variablity of these parameters?
To examine this question, the shadow band system was used and growth and
allocation were compared between plants that received sunflecks and those
from which sunflecks were excluded. Aboveground growth was measured
nondestructively via allometric techniques for 2 years, followed by harvest
of both the above- and belowground parts. Since the plants were in micro-
353
calculated value is the effect of the source air. The value of the source air
was thus -9.0%0, which is slightly higher than values reported for tropical
forest understories (Sternberg et aI., 1989). The respiration rates may be
lower because of cooler temperatures and slower decomposition, or the
mixing rates of the understory air with the bulk atmosphere greater in the
redwood forest than in the tropical forests. Using the corrected value for
the source air the Ol3C expected during sunflecks can be obtained from the
pJPa and is equal to - 29.1. Thus the carbon gained during sunflecks should
be 4.50/00 lighter than the carbon gained during diffuse light.
The values for carbon fixed during sunflecks and diffuse light can be used
with Eq. (3) to calculate the fractional contribution of sunflecks. For 5 mol
photons m 2 day-I, which is representative for the brightest microsites sampled, Eq. (3) predicts that 46% of the carbon gain that went to production
of the biomass of these plants was due to sunflecks. By contrast, at 1 mol
photons m- 2 day-t, which is characteristic of the microsites with the smallest amount of direct PFD, only about 9% of the carbon gain was due to
sunflecks. Thus utilization of sunflecks was responsible for a substantial
fraction of the total annual carbon gain in the growing season but, as already
shown with the daily gas exchange measurements (Fig. 17.1), it varied
greatly from one microsite to another as a function of the direct PFD.
Comparison of the long-term fraction of assimilation due to sunflecks as
estimated from carbon isotope ratios with the short-term ratios from gas
exchange reveals that the latter are substantially greater. This is to be
expected since the diurnal measurements were done on mostly clear days
when sunflecks would obviously be most abundant whereas the carbon
isotope measurements integrate over both clear and cloudy days. The
brightest microsites used for the carbon isotope studies were brighter than
the brightest of those for the diurnal run, so that in fact the difference may
be even larger than indicated by the maximum values.
17.6 Consequences of Sunftecks for Growth and Reproduction
The studies with gas exchange and carbon isotopes both indicate that utilization of sunflecks contributes a significant fraction of the carbon gain in the
brightest microsites but much less in the most shaded sites. The obvious
question is how does this translate into performance in the understory in
terms of growth and reproduction and to the variablity of these parameters?
To examine this question, the shadow band system was used and growth and
allocation were compared between plants that received sunflecks and those
from which sunflecks were excluded. Aboveground growth was measured
nondestructively via allometric techniques for 2 years, followed by harvest
of both the above- and belowground parts. Since the plants were in micro-