344
R.W. Pearcy and W.A. Ptitsch
moist redwood forst understories of northern California, as the experimental
plant. This perennial species forms a basal rosette of typically 6 to 20 leaves
in March and may remain vegetatively active until September. Although
sexual reproduction is common there is no apparent vegetative reproduction.
17.2 Sunftecks in Forest Understories
The light regimes of forest understories exhibit tremendous spatial and
temporal variation largely because of the occurence of sunflecks resulting
from canopy gaps of varying size. When these gaps are aligned with the
solar path, sunflecks occur that may reach PFDs up to 20-fold greater than
the shade light. In total, they typically are present at a given location <10%
of the day but typically contribute 35 to 80% of the daily PFD on clear days
(Bjorkman and Ludlow 1972; Pearcy 1983, 1987; Chazdon and Fetcher
1984). The redwood forest understory where Adenocaulon was abundant
received on average on clear days 214 sunflecks that contributed 69% of the
PFD and were present for 12% of the time (Pfitsch and Pearcy 1989a). The
earth's rotation and canopy movement cause the duration of sunflecks to be
quite brief. Penumbral effects caused by the finite (1/2°) angular diameter of
the solar disc result in a reduction of the maximum PFD in small sunflecks in
tall canopies and a spreading of the sunfleck so that the edges are indistinct
(Miller and Norman 1971). While the consequences of penumbral spreading
of sunflecks has been investigated in tree canopies where the effect is small
(Oker-Blom 1984), no work has been done in forest understories where a
larger effect could be expected.
Canopy structure is the dominant factor influencing the temporal characteristics of sunflecks. Overstory canopies generally are clumped so that when
viewed from the understory there are areas with more gaps while others
have few gaps. As a result, sunflecks tend to occur in clusters of several to
100 or more, separated by periods when there are few or none. In total,
there may be >300 sunflecks recorded at a single site during a day. Therefore most are brief, resulting from canopy movement and lasting less than
10 s. However, longer sunflecks have been shown to contribute a disproportionate share of the PFD in redwood forest understories (Pfitsch and Pearcy
1989a). Open, flexible canopies such as those of quaking aspen stands
have many more sunflecks and proportionately more of the PFD is contributed by brief sunflecks (Roden and Pearcy 1993). Cloudiness is also
important in determining the frequency of sunflecks. In climates where
clouds build in the afternoon, for example, sunflecks will be restricted to
the morning.
R.W. Pearcy and W.A. Ptitsch
moist redwood forst understories of northern California, as the experimental
plant. This perennial species forms a basal rosette of typically 6 to 20 leaves
in March and may remain vegetatively active until September. Although
sexual reproduction is common there is no apparent vegetative reproduction.
17.2 Sunftecks in Forest Understories
The light regimes of forest understories exhibit tremendous spatial and
temporal variation largely because of the occurence of sunflecks resulting
from canopy gaps of varying size. When these gaps are aligned with the
solar path, sunflecks occur that may reach PFDs up to 20-fold greater than
the shade light. In total, they typically are present at a given location <10%
of the day but typically contribute 35 to 80% of the daily PFD on clear days
(Bjorkman and Ludlow 1972; Pearcy 1983, 1987; Chazdon and Fetcher
1984). The redwood forest understory where Adenocaulon was abundant
received on average on clear days 214 sunflecks that contributed 69% of the
PFD and were present for 12% of the time (Pfitsch and Pearcy 1989a). The
earth's rotation and canopy movement cause the duration of sunflecks to be
quite brief. Penumbral effects caused by the finite (1/2°) angular diameter of
the solar disc result in a reduction of the maximum PFD in small sunflecks in
tall canopies and a spreading of the sunfleck so that the edges are indistinct
(Miller and Norman 1971). While the consequences of penumbral spreading
of sunflecks has been investigated in tree canopies where the effect is small
(Oker-Blom 1984), no work has been done in forest understories where a
larger effect could be expected.
Canopy structure is the dominant factor influencing the temporal characteristics of sunflecks. Overstory canopies generally are clumped so that when
viewed from the understory there are areas with more gaps while others
have few gaps. As a result, sunflecks tend to occur in clusters of several to
100 or more, separated by periods when there are few or none. In total,
there may be >300 sunflecks recorded at a single site during a day. Therefore most are brief, resulting from canopy movement and lasting less than
10 s. However, longer sunflecks have been shown to contribute a disproportionate share of the PFD in redwood forest understories (Pfitsch and Pearcy
1989a). Open, flexible canopies such as those of quaking aspen stands
have many more sunflecks and proportionately more of the PFD is contributed by brief sunflecks (Roden and Pearcy 1993). Cloudiness is also
important in determining the frequency of sunflecks. In climates where
clouds build in the afternoon, for example, sunflecks will be restricted to
the morning.