L.A. Vierling, C.A. Wessman Agricultural and Forest Meteorology 103 2000 265–278 269
with actual measurements of sunfleck activity using photodiodes Chazdon and Field, 1987.
3. Results
3.1. PAR transmission, vertical LAI profile, and foliar photosynthesis
Photosynthetically active radiation PAR is highly depleted by the dense, continuous overstory foliage
characteristic of monodominant G. dewevrei canopies Louis, 1947. On a mostly clear day Julian Day
345, 10 December 1996, the PPFD incident upon the three ceptometers registered 5.1, 5.1, and 2.0 of
above-canopy PPFD at 34, 24, and 3 m AGL, respec- tively Table 1. The low transmission of PAR even at
the level of the top ceptometer directly relates to the fact that more than half of the G. dewevrei canopy LAI
is contained in the upper 20 of the canopy height Fig. 1. Thus, LAI≈4 even at the topmost ceptome-
ter placement of 34 m AGL. The mean total LAI at the site, derived from five hemispherical photographs
acquired at 1 m AGL, was 7.2. Over the entire 12-day measurement period, there was an average transmis-
sion of only 1.2 to a canopy height of 1 m AGL. As a result, light-demanding lianas are sparse within the
canopy except near forest gaps.
Rates of steady state net photosynthesis de- creased with increasing LAI depth, with average
PPFD-saturated CO
2
fixation rates of approximately 4.5, 3.5, and 1.5 mmol CO
2
m
− 2
s
− 1
at heights of 30, 20, and 1.5 m, respectively see Vierling, 1999. The
point at which photosynthesis saturated with respect
Table 1 PPFD values above and at three levels within the Gilbertiodendron
dewevrei forest canopy on a mostly clear day Julian Day 345; 10 December 1996
a
Canopy height Energy received mol
Transmission m AGL
photons m
− 2
per day Above canopy
31.7 100
34 1.62 0.16
5.1 24
1.63 0.29 5.1
3 0.63 0.06
2.0
a
Measurements were successfully recorded during 98.6 of the time between sunrise and sunset on this day. Values within
parentheses represent standard deviations from the mean. Fig. 1. Vertical distribution of LAI in G. dewevrei canopy. Cep-
tometer placement heights within the canopy are denoted by the letter ‘X’. Frror bars depict one standard deviation of the mea-
surements; the shaded zone shows the LAI range falling within one standard deviation as interpolated through the canopy.
to PPFD also varied with canopy height. By evaluat- ing the average photosynthetic light saturation points
near the height of each ceptometer, we assigned sun- fleck PPFD threshold values to be 200 mmol photons
m
− 2
s
− 1
at the top and middle levels and 100 mmol photons m
− 2
s
− 1
in at the bottom level. 3.2. PPFD heterogeneity
PPFD was very heterogeneous within the canopy over both the course of a day Fig. 2 and over the
course of the 12-day sample period. Autocorrelation calculations reveal that the spatial PPFD heterogene-
ity within this canopy varied with respect to both sky diffuse fraction and canopy depth Fig. 3. At
34 m AGL, there was a distinct difference between the spatial correlation of sensor measurements dur-
ing times characterized by higher SDF ≥0.8 and those during low SDF 0.8; Fig. 3A. While under
high SDF conditions the PPFD measurements at 34 m AGL remained highly correlated across a 90 cm sen-
sor separation distance, during clearer skies the mean correlation coefficient dropped to 0.1–0.2 for sensors
spaced only 18 cm apart. In general, the mean spatial correlation coefficients did not decrease as abruptly
in the lower portion of the canopy compared to higher within the canopy Fig. 3A–C.
Analysis of the hemispherical photographs indi- cates that during the sample period the daily irradiance
a function of the angular position, size, and number of canopy gaps; Chazdon and Field, 1987 received
270 L.A. Vierling, C.A. Wessman Agricultural and Forest Meteorology 103 2000 265–278
Fig. 2. Daily tracks of PPFD above the canopy and at the three measurement heights within the canopy on 10 December 1996, a mostly sunny day. Plots at each level within the canopy show data from three arbitrarily chosen sensors positioned at 60 cm intervals along each
ceptometer.
Fig. 3. Spatial auto-correlation of PPFD measurements with respect to sky diffuse fraction at A 34 m; B 24 m; and C 3 m AGL
within the G. dewevrei canopy. Sensors were spaced 6 cm apart at each canopy height.
at each measurement height was comparable to that expected during other times of the year Fig. 4. Of
particular note is that at 24 m AGL, daily irradiance is relatively high when the sun tracks through the
southern sky including during the measurement time period, and then reaches relative minima during pe-
riods when the sun tracks across the northern sky. This is due to the impact of a tree fall gap approxi-
mately 100 m SW of the measurement location on the afternoon radiation regime at 24 m AGL.
In sum, 539, 3161 and 2910 sunflecks occurred at the respective heights of 3, 24 and 34 m AGL during
the observation period. The overall length and PAR energy attributes of the sunflecks varied greatly with
both canopy height and sample day Fig. 5. Sensors recorded sunflecks during 0.7 of the total sample
time, and the amount of energy contained within these sunflecks averaged 9.5 of the total recorded energy.
At the height of the top ceptometer, values ranged from no sunfleck activity on three overcast days to a
maximum on Julian Day 345 when sunflecks lasted 1.6 of the total time and contributed 14 of the to-
tal PPFD. At the middle ceptometer, sunfleck activity ranged from zero on the overcast days to maxima of
L.A. Vierling, C.A. Wessman Agricultural and Forest Meteorology 103 2000 265–278 271
Fig. 4. Annual course of irradiance reaching each of the three canopy layers. Values at each layer are scaled such that the max-
imum value during the year equals 1. The open symbols denote the estimated daily irradiance at the midpoint of the 12-day sam-
ple period. Dashed lines represent 2nd degree polynomial fits to the data.
Fig. 5. Percent time and percent energy represented by sunflecks during the field experiment. ‘Average’ column is the average for
all days at each level, with the exception of the ceptometer at 34 m, which was positioned below a tree branch at 29 m during the
first 5 days of measurements denoted by shading. The ‘Average’ column at 34 m shows the average of all days after Julian Day 339.
Fig. 6. Frequency distributions of a average PPFD and b peak PPFD reached within sunflecks at each of the three canopy heights.
Numbers of sunflecks at each level denoted in figure legends. Insets show the low end i.e. 100–200 mmol photons m
− 2
s
− 1
of the frequency distribution at 3 m in more detail.
2.4 of the time and 26 of the energy on Julian Day 345, while at the bottom ceptometer sunfleck activity
ranged from zero to maxima of 0.9 of the time and 22 of the total PPFD on Julian Day 335.
The average PPFD contained in sunflecks also var- ied with canopy height Fig. 6a. At 3 m AGL, 94 of
all sunflecks contained an average PPFD between 100 and 200 mmol m
− 2
s
− 1
. At 24 and 34 m where PPFD values below 200 mmol m
− 2
s
− 1
were not classified as sunflecks, 45 and 52 of all sunflecks contained
average PPFDs between 200 and 225 mmol m
− 2
s
− 1
. Peak PPFD values during sunflecks showed similar
trends with respect to canopy height and exhibited a slight shift towards higher PPFD classes Fig. 6b rel-
ative to average sunfleck PPFD values. At 3 m AGL, 90 of sunflecks peaked at a PPFD between 100 and
200 mmol m
− 2
s
− 1
while at 24 and 34 m, 36 and 44
272 L.A. Vierling, C.A. Wessman Agricultural and Forest Meteorology 103 2000 265–278
Fig. 7. Frequency distributions of sunfleck characteristics deter- mined from 30 sensors placed at 34 and 24 m AGL, and 29 sensors
placed at 3 m AGL in a Gilbertiodendron dewevrei forest from the period 30 November–11 December 1996.
of sunflecks, respectively, peaked between 200 and 225 mmol m
− 2
s
− 1
. A greater percentage of sunflecks comprised longer
duration classes with increasing canopy depth Fig. 7a. While these data suggest that a large fraction of all
sunflecks are short in duration, these short sunflecks, in sum, deliver a small proportion of total sunfleck
PPFD to the forest understory Fig. 7b. For example, at 34 m AGL sunflecks equal to or shorter than 12 s in
duration accounted for only 6.4 of the total sunfleck PPFD, while those longer than 48 s accounted for 81.
Fig. 8. Histograms depicting three time scales of the low-light period occurring prior to sunflecks at three levels within the G. dewevrei canopy.
In addition to sunfleck duration, the duration of the low-light interval between sunflecks can also substan-
tially affect foliar carbon gain via mediation of the photosynthetic induction state of a leaf Kirschbaum
and Pearcy, 1988a; Sassenrath-Cole and Pearcy, 1994; Valladares et al., 1997. Histograms depicting the time
between sunflecks in the G. dewevrei canopy show that sunflecks mostly occur either quite close to one an-
other or are spaced out over long time periods Fig. 8. The bimodal character of these data becomes more
pronounced with depth in the canopy.
At levels higher in the canopy, progressively larger proportions of all sunflecks form clusters Fig. 9a.
The percentage of sunflecks contained within clus- ters reaches a plateau between the low-light interval
thresholds LLITs of 15 and 30 s Fig. 9a. To assess how efficiently leaves might utilize light within these
sunfleck clusters for photosynthesis, we calculated the amount of time within each cluster that occurred as
sunfleck. For all three layers, the mean amount of clus- ter time as sunfleck was quite high ∼97 with an
LLIT of 1.5 s Fig. 9b. In general, at LLITs longer than 30 s, then the higher in the canopy, the less the
time within each cluster is comprised of sunfleck. The mean duration of clusters defined by various LLITs is
depicted in Fig. 9c.
Because photosynthesis would likely respond to clusters as if they were single sunflecks when a large
percentage of the cluster time is made up of sunflecks see Pearcy et al., 1990; Valladares et al., 1997, we
also calculated histograms of cluster duration for each cluster class in order to compare their statistics to the
raw sunfleck duration histogram shown in Fig. 7a. The duration of clusters defined by even a short LLIT
greatly affect the frequency distributions Fig. 10 relative to raw sunfleck duration. For example, at all
L.A. Vierling, C.A. Wessman Agricultural and Forest Meteorology 103 2000 265–278 273
Fig. 9. Plots of a the percentage of all sunflecks falling within sunfleck clusters; b the percentage of cluster time that is repre-
sented by sunflecks; and c the mean cluster duration, each with respect to the maximum duration of the low-light interval between
sunflecks used to group sunflecks into clusters. All measurements were made from 30 November–11 December 1996.
Fig. 10. Histogram of sunfleck cluster duration within the G. dewevrei canopy. A low light interval threshold LLIT of 4.5 s was
used to define the clusters for which statistics are presented here.
three canopy heights there were more raw sunflecks of 1.5 s in duration than in any other duration class
Fig. 7a. Maximum frequencies for sunfleck clusters defined by an LLIT of only 4.5 s, however, occur at
the 48–96 s class at the top and bottom ceptometer placements and at the 96–192 s class at the middle
ceptometer Fig. 10. The fact that this large shift in the frequency distribution occurs even at short LLIT
durations where, in the case of this example, with an LLIT of 4.5 s sunflecks comprise ∼95 of the
cluster time; see Fig. 9b suggests that the clustered occurrence of sunflecks is likely to significantly affect
carbon gain within the canopy.
4. Discussion
