BGBiogeosciencesBGBiogeosciences1726-4189Copernicus GmbHGöttingen, Germany10.5194/bg-12-4195-2015Transmissivity of solar radiation within a Picea sitchensis stand under various sky conditionsDengelS.sigrid.dengel@helsinki.fiGraceJ.MacArthurA.School of GeoSciences, Crew Building, University of
Edinburgh, EH9 3FF, UKNERC Field Spectroscopy Facility (FSF), School of
GeoSciences, The Grant Institute, University of Edinburgh, EH9 3JW, UKnow at: Department of Physics, University of Helsinki, P.O. Box 48, FI-00014 Helsinki, FinlandS. Dengel (sigrid.dengel@helsinki.fi)16July201512144195420709January201527February201517June201503July2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://bg.copernicus.org/articles/12/4195/2015/bg-12-4195-2015.htmlThe full text article is available as a PDF file from https://bg.copernicus.org/articles/12/4195/2015/bg-12-4195-2015.pdf
We tested the hypothesis that diffuse radiation from cloudy and overcast
skies penetrates the canopy more effectively than direct radiation from clear
skies. We compared the flux density and spectral properties of direct and
diffuse radiation (around solar noon (±1 h)) above, within and below a
forest stand under sunny, cloudy and overcast conditions in a thinned Sitka
spruce (Picea sitchensis (Bong.) Carr.) forest (28 years old, with a
leaf area index of approximately 5.2 m2 m-2). We recorded
vertical profiles of radiation penetration (from 350 to 1050 nm), and we
also explored the horizontal pattern of radiation along a 115 m transect.
We showed that in “clear sky” conditions, the photosynthetically active
radiation in the lower parts of the canopy was substantially attenuated, more
so than under cloudy and overcast skies. It was particularly depleted in the
blue part of the spectrum, but only slightly blue-depleted when the sky was
overcast or cloudy. Moreover, the red : far-red ratio under clear skies fell to
values less than 0.3 but only to 0.6 under cloudy or overcast skies. Near the
ground, the light climate was strongly influenced by the thinning pattern
(carried out in accordance with standard forestry management practice).
Introduction
The solar radiation reaching the Earth's surface is influenced by the
absorption, transmission and reflection of light by the aerosol and water
vapour constituents of the atmosphere. The extent of cloud cover affects the
intensity and proportions of “direct” and “diffuse” radiation reaching
the Earth's surface. While diffuse radiation is thought to enhance
photosynthesis of terrestrial vegetation (Gu et al., 1999, 2002; Urban et
al., 2007; Dengel and Grace, 2010), direct solar radiation can cause
saturation of photosynthesis at the top of the canopy and possibly
photo-inhibition (Powles, 1984; Krause, 1988; Long et al., 1994).
Furthermore, unsaturated photosynthesis during direct solar radiation is
possibly occurring within the canopy and understorey region as a result of
shading (Kanniah et al., 2012). Urban et al. (2007, 2012) hypothesised that
optimal photosynthetic activity of the canopy is achieved under diffuse
radiation (cloudy) conditions, when scattered light penetrates throughout the
canopy, illuminating all the leaves to some extent and providing a more
uniform distribution of light between the leaves. However, the spectral
properties of the diffuse component inside the canopy have only been
investigated in a Norway spruce (Picea abies L. Karst)–European
beech (Fagus sylvatica L.) forest stand in Southern Germany
(Leuchner et al., 2007; Hertel et al., 2011) and a Norway spruce stand by
Navrátil et al. (2007) and Urban et
al. (2007, 2012) in the Czech Republic. No measurements are known from higher
latitudes. Here, we introduce a study carried out at one out of only two
Sitka spruce (Picea sitchensis (Bong.) Carr.) forest research sites
in the UK and Europe where long-term forest growth and CO2 exchange
measurements are carried out (Clement et al., 2003; Dengel and Grace, 2010). This species is a
non-native species to the UK/Europe, but highly valued for its fast growth
and timber quality. In the UK and Ireland it is the most frequently planted
commercial tree species.
The vertical profile of irradiance through a plant canopy is often
approximated by the Beer–Lambert equation of light extinction first
introduced by Monsi and Saeki (1953), and subsequently serves as the base of
many canopy transmission studies (Grace and Woolhouse, 1973; Norman and
Jarvis, 1974; Lewandowska et al., 1977; Hale, 2003; Sonohat et al., 2004).
However, the equation does not describe the complexity of the radiation field
to which the photosynthesising elements are exposed, neither the spatial,
angular, nor the temporal distribution, because forest canopies are dynamic
and far from homogenous (Gholz et al., 1991; Smith et al., 1991). The diffuse
radiation inside a forest canopy includes the fraction scattered by the
foliage itself as well as radiation transmitted through the leaves and
through the many gaps in the foliage (Muller, 1971; Grant, 1997). Sunflecks
– their size, shape, duration and spectral distribution – depend on the
orientation and inclination of woody and photosynthesising elements within
the forest canopy as well as the position of the sun in the sky (Federer and
Tanner, 1966; Norman and Jarvis, 1974; Pearcy, 1990; Chazdon and Pearcy,
1991; Grant, 1997). The way plants respond to sunflecks may vary, and in some
shade plants this response (saturation of photosynthesis, stomatal regulation
or possibly photo-inhibition) may be crucial to effective gas exchange and
photosynthetic production (Sellers, 1985; Leakey et al., 2003).
Indicators for light quality, in contrary to light quantity, specified for
example as blue light and red : far-red ratio effects are prime factors in
plant functionality. Smith (1982) indicated that the blue-absorbing
photoreceptor present in plants acts to measure light quantity and that the
pigment phytochrome can act to detect the red : far-red ratio as an indicator
of light quality. Blue light may have important implications for stomatal
control, causing stomatal opening (Morison and Jarvis, 1983), while the red : far-red ratio is known to influence photomorphogenesis, heating regulation,
as well as stem elongation and chlorophyll synthesis (Gates,
1965; Smith, 1982; Wherley et al.,
2005; Casal, 2013). Ritchie (1997) reported the ability of
Pseudotsuga menziesii seedlings to detect the presence of nearby
trees via changes in light quality and the ability to adjust their growth by
altering their allometry. Low red : far-red ratio may also have implications
for the adjustment to light and competition, and the optimisation of branch
location in the canopy. Furthermore, Kasperbauer (1971, 1987) showed that row
spacing and orientation (in tobacco plants) are also important regarding
light quality. Leuchner et al. (2007) and Hertel et al. (2011) indicate that
a reduction of the red : far-red ratio is a strong indicator for competition in
Norway spruce.
The observation that diffuse light is utilised in canopy photosynthesis more
effectively than direct sunlight (Urban et al., 2007; Dengel and Grace, 2010)
poses a number of questions to be addressed in the present work. They are (a)
to what extent is it true that light is distributed more evenly throughout
the dense Sitka spruce canopy under cloudy and diffuse conditions; (b) to
what extent are the light climates within the canopy spectrally different
under clear, cloudy and diffuse skies and (c) how much is the light climate
modified by the standard management interventions.
Schematics of the Griffin forest planting and tree distribution
properties, showing the thinning lines (the stumps are illustrated). Also
shown are hemispherical images taken in the un-thinned as well as thinned
area of the forest.
Materials and MethodsSite description
Measurements were carried out in Griffin forest (Clement et al., 2003;
Clement, 2004) in Central Scotland (56∘37′ N, 3∘48′ W;
380 m a.s.l.). This Sitka spruce (Picea sitchensis (Bong.) Carr.)
forest was planted between 1979 and 1983 and row-thinned in 2004 by removing
every fifth row of trees. In addition, trees have been felled selectively,
resulting in a total of 30 % of the forest stand being removed. The
planting distance is around 2 m, with approximately 11 m from any
mid-thinning line to the next. The mean diameter at breast height (DBH) at
the time of measurements was 37 cm, mean canopy height 18.5 m and with an
estimated leaf area index (LAI) of approximately 5.2 m2 m-2. All
meteorological and micrometeorological measurements were carried out on a
walk-up scaffolding tower of 22 m height. Below the forest canopy a
115 m-long transect, crossing 10 sections of 1 thinned and 4 planted rows
and with a North-South alignment, was established in order to measure below
canopy radiation (Fig. 1).
MethodsSpectral flux density
Spectral distribution and flux density were measured using two
spectroradiometers (GER1500, Spectra Vista, New York, USA), fitted with
cosine corrected diffusers (MacArthur et al., 2012), permitting comparison of
the spectral flux density (irradiant energy; units: W m-2 nm-1)
in the canopy with simultaneously measured spectral flux density above the
canopy at 22 m height. The spectral resolution of the GER1500 is 3 nm,
measuring 512 channels, although the post-processing methods interpolate data
to 1 nm intervals (Walker and MacLellan, 2009). The performance of this
instrument declines in the infrared and so we restricted our measurements to
the waveband 350–1050 nm.
All spectral measurements were carried out around solar noon (±1 h)
during summer of 2008 of which 3 days are shown here as a “snapshot”
(27 May, 22 July and 23 September with max solar angles of 53.5, 53.8 and
33.1∘, respectively). These days were chosen as they show three
distinctive sky conditions and a difference of approximately
600 µmol m-2 s-1 in photosynthetic photon flux density
between the measurements. Adding more days would increase the temporal
distribution of the data, but at the same time it would also add a bias as
measurements could not be taken in exactly the same location under the same
solar radiation intensity. The solar spectrum has a pronounced diurnal
variation and so we carried out measurements at midday. Tower and forest
floor (transect) scans were carried out back-to-back within less than 10 min
of each other. One complete set of measurements including the vertical and
horizontal measurements took around 1 h. Vertical profiles of radiation
penetrating the canopy were made by taking three measurements at 1 m
intervals 1.5 m from the tower (south facing, opposite side of the
artificial gap created during the tower installation), while the scans
recorded for evaluation of the horizontal variation were measured at 1 m
height with 2.5 m intervals along a transect. Three measurements were
carried out at each point of which each measurement represents an average of
10 internally averaged scans. Measurements were carried out under (i) clear
sky conditions (clearness index of around 0.75 over the measurement period),
(ii) cloudy conditions (we selected conditions with altostratus clouds to
guarantee minimal changes in cloudiness over the measurement period
(clearness index of 0.60) and (iii) on a completely overcast day (clearness
index of 0.23). In all cases, light conditions above the forest canopy did not
change significantly over the measuring period. The spectral distribution of
the incoming solar radiation also did not show any significant differences as
can be seen in Fig. 2a and b. To facilitate comparison, data were normalised
to the range 0 to 1 where appropriate. In order to scale the data we have
applied the following scaling method:
NDi=xi-min(x)maxx-min(x),
where x= (x1, …, xn) and NDi is the ith
normalised data.
(a) Spectral distribution of solar radiation above the
canopy for the three conditions, while (b) is the normalised data of
the same data, showing the little variation according to sky condition and
time of year. (c) Visualises the spectrum under clear sky,
in addition to the colour-coded intensity. This visualisation should help
to interpret the spectral distribution in Fig. 3. Furthermore, the main
spectral features together with the wavelength distribution for the visible
and PAR are shown.
Leaf area index
The vertical distribution of leaf area index (LAI) was estimated from
hemispherical images taken every 2 m down the tower using a Nikon digital
camera (Coolpix 4500, Nikon Corporation, Tokyo, Japan) with a fish-eye lens
attachment (Fish-eye converter FC-E8, Nikon Corporation, Tokyo, Japan).
Images were acquired following the protocols established by Chen et
al. (1997a, b) and van Gardingen et al. (1999), and were processed with the
scientific image processing software Gap Light Analyzer (GLA) (Forest Renewal
BC, Frazer et al., 1999). Images were
taken along the same path as the spectral measurements (south-facing,
opposite side from the artificial gap/thinning line), halved and mirrored in
order to avoid tower structural elements being recorded as part of the
canopy. When calculating LAI from hemispherical images in coniferous forests,
a correction value, known as the clumping index (van Gardingen et al., 1999),
is necessary to account for structural aspects of the canopy. The necessary
clumping index value has been calculated from several transect measurements,
using a TRAC (Tracing the Radiation and Architecture of Canopies, Leblanc et
al. (2002) (3rd Wave Engineering, Nepean, Canada) in Griffin forest during
the growing season of 2007 and 2008 and was found to be 0.98. A detailed
explanation on the use of this instrument is given in Sect. 2.2.4.
Photosynthetic photon flux density and transmissivity at various
wavelengths
Values of photosynthetic photon flux density (PPFD) were calculated by
converting irradiant energy (W m-2 nm-1) to quanta
(µmol m-2 s-1) and integrating from 400 to 700 nm (Combes
et al., 2000) (Eq. 1):
PPFD=∫λ=400λ=700Ehυδλ,
where the limits of wavelength (λ) were 400 and 700 nm. E is the
spectral irradiance, h is the Planck constant and υ is frequency,
given by 1/λ. The wavelength increments used for the numerical
integration were 1 nm. Blue light is often indicated as being 400–500 nm
but we chose 430–470 nm as it has been shown that those wavelengths evoke
stomatal opening (Kuiper, 1964; Mansfield and Meidner, 1966; Zeiger and
Field, 1982; Karlsson, 1986).
The transmissivity of PPFD was calculated as the quotient of PPFD at the
height h and the simultaneous measured PPFD at 22 m (top of tower), while
the transmissivity of blue light was calculated as the quotient of blue light
at the height h and the simultaneously recorded blue part of the irradiance
spectrum at 22 m. Hereafter the blue transmissivity of the two diffuse
conditions and the blue light transmissivity on the clear day can be visually
compared. For an indication of possible photomorphogenetic response, light
quality may also be stated as the red : far-red (R : FR) ratio of incident
radiation and expressed as follows (Hayward, 1984; Holmes and Smith, 1977):
R:FR=∫655nm665nmE(λ)∫725nm735nmE(λ),
where E is the spectral irradiance. Holmes and Smith (1977) note that red : far-red ratio remains more or less constant over the year and during the
day, whereas within the canopy it is additionally dependent on the
interaction of the incident light with phytoelements.
Vertical profile of spectral distribution of solar radiation
traversing the forest canopy on a clear, cloudy and overcast day.
(a) vertical profile on the clear day; (b) vertical profile
on the cloudy day; (c) vertical profile on the overcast day. For the
visualisations the data are normalised on a scale from 0 to 1. (d),
(e) and (f) show the spectral distribution at 2 m above
ground in absolute terms, showing the exact energy distribution across all
wavelength.
Below-canopy PPFD
High resolution below-canopy photosynthentic photon flux density was measured
with a mobile handheld TRAC (Tracing Radiation and the Architecture of
Canopies – Leblanc et al. (2002) (3rd Wave Engineering, Nepean, Canada),
recording continuously at 32 Hz along the same transect, resulting in a high
resolution data set of total incident (global) and diffuse (through the use of
a shading strip) PPFD values. In addition, the TRAC software also estimated
LAI, the fraction of absorbed photosynthetically active radiation (fAPAR),
gap fraction, gap dimension and the clumping factor. These measurements were
carried out immediately after the spectral flux density measurements at solar
noon. The TRAC sensor was manually moved along the same transect as used for
the spectral irradiance measurements. The standard walking pace along the
transect was 0.3 m s-1 (while continuously recording), following
markers at 5 m intervals to ensure a consistent high-resolution data set. As
data were recorded at 32 Hz not all segments have the identical number of
data points. Raw data were logged internally inside the instrument and
downloaded after each run before converting and processing them using its own
TRAC software. Exact details on theory description, the calculations of gap
fraction and dimension, as well as the clumping factor can be found in Chen
and Cihlar (1995), Leblanc et al. (2002) and Leblanc (2008).
Canopy CO2 exchange under such conditions
As mentioned above, there are only two Sitka spruce forest sites with
long-term canopy CO2 exchange measurements: Griffin and Harwood forest
(in Northern England). CO2 exchange at canopy scale were carried out in
Griffin forest as described in Clement et al. (2007) and Dengel and
Grace (2010). Corrections and quality control are applied to data including
exclusion of data recorded at low turbulence, reducing the data availability
(also for the 3 represented days). Therefore data from the other Sitka
spruce forest (250 km away, same age, spacing, plantation, etc.) were
included in the current study in order to provide a “big picture” on how
canopy CO2 exchange of forests are affected by changes in sky conditions
and hence light distribution within the forest canopy. Eight consecutive days
were included here, as already introduced in Dengel and Grace (2010).
Spectral distribution of solar radiation at 5 m above the ground
for the wavelength range of PAR, showing the various distributions of blue
and in particular the fraction of blue wavelength (430–470) that does evoke
stomatal opening (sensu Kuiper, 1964; Mansfield and Meidner, 1966).
(a) vertical distribution of leaf area index (LAI) and
cumulative leaf area index (cLAI); (b) vertical distribution of
photosynthetic flux density (PPFD) on a clear day (black line), cloudy day
(pecked black line), overcast day (grey line). The insert is a magnification
of the lowest 5 m above the forest floor, showing the forest floor
illumination caused by sunflecks; (c) the same as (b) but
normalised as transmissivity; (d) transmissivity of blue light;
(e) vertical profile of the red : far-red ratio (R : FR).
Transmissivity and attenuation curves according to the Monsi and
Saeki (1953) method. Transmissivity and light attenuation through the forest
canopy after applying the Beer–Lambert attenuation law. Stars and dotted
lines represent clear sky, grey solid circles and the grey line represent cloudy and
solid black circles and the black line represent the overcast conditions.
Results
Figure 1 illustrates the schematics of the forest, along with visual
impressions of canopy structure and fish-eye photographs of the canopy and
sky taken during the measurements. This forest structure is typical of many
commercial coniferous plantations. All above-canopy irradiance spectra
display the expected features (Fig. 2c): they have their peak spectral
irradiances in the blue region at around 480 nm; both oxygen absorption
bands are clearly seen (687 and 761 nm), as are the water absorption bands
at around 730 and 940 nm.
Below the canopy the absorption pattern is changing, showing high absorption
in the PPFD region while little absorption occurs from 700 nm onwards.
Figure 3a visualises the spectral/energy change that occurs once radiation
penetrates the forest canopy on the clear sky day. An abrupt shift is
observed at the height where the canopy is closed, with a large sunfleck
becoming visible at the heights of around 11 m above ground in this data set.
Hereafter the majority of the energy appears in the infrared region. Once
radiation reaches the forest floor which is illuminated partly by sunflecks
and by large open parts of the canopy this shift reverses to similar
distributions as seen above and close to the top of the canopy. Figure 3b and
c represent the spectral flux density recorded in cloudy and overcast
conditions, respectively. Here, under both sky conditions, high energy levels
within the blue region of the spectra remain conserved much lower/deeper into
the canopy, with overcast conditions showing a more even distribution. Within
the canopy (around 8 m above ground) some important differences can be noted
between the sky types. Under both cloudy and overcast conditions there is
relatively more blue radiation although in absolute irradiances this isn't
always true. Much less of the incoming radiation is in the photosynthetically
– active part of the spectrum (400–700 nm) in the case of the clear sky
compared with the cloudy/overcast conditions. Figure 3d, e and f show the
energy distribution at 2 m above the forest floor in absolute irradiances
showing very similar values for clear and overcast. Higher in the canopy, at
5 m above the ground (Fig. 4) this pattern is changing with less energy in
the blue fraction under the clear sky.
Normalised spectra along the 115 m transect on the clear
(a) and on the overcast day (b) respectively, showing
clearly the distribution of sunflecks and open spaces (on the clear day) and
the thinning lines on the overcast day (b).
Photosynthetic photon flux density (PPFD) distribution below the
forest canopy under clear sky (a) and under overcast
(b) conditions. Total (global) PPFD is marked as a solid grey line,
while diffuse PPFD measured simultaneously (using a shading strip) is marked
as a solid black line. Thinning lines are every 11 m.
Light use efficiency (LUE) curves for 8 consecutive days
previously introduced in Dengel and Grace (2010). These show the day-to-day
changes in light use efficiency when sky conditions change from overcast to
cloudy to clear sky conditions. After 4 consecutive clear days (lowest
light use efficiency) these are followed again by a cloudy and an overcast
day. The scales represent the gross primary productivity (GPP) estimated for
these days together with the corresponding photosynthetic photon flux density
(PPFD). The insert is a modified reproduction from Dengel and Grace (2010,
Fig. 2c), representing global radiation in black and diffuse radiation in
grey.
The spectra were re-expressed as quanta and numerically integrated between
400 and 700 nm to yield values of PPFD (Eq. 2). Above the canopy on top of
the 22 m tall tower PPFD was approximately 1600 on the clear day, 1000 under
cloudy conditions and 400 µmol m-2 s-1 when overcast,
representing three distinctive sky/clearness conditions separated by approx.
600 µmol m-2 s-1 between conditions. The mean and
cumulative LAI (Fig. 5a) and PPFD distributions (Fig. 5b) down the vertical
profile, and the transmissivity values associated with PPFD (Fig. 5c) and
blue light (Fig. 5d) are shown as attenuation curves in Fig. 5, respectively.
The attenuation of direct radiation (“clear”) is abrupt in the top-most
part of the canopy (14–15 m) (Fig. 5b, c). The canopy is inhomogeneous and
at around 11 m above the ground the sensor encountered a large sunfleck,
which has produced a very high signal. Under cloudy and overcast conditions
the curves are relatively smooth, showing gradual attenuation on passing
through the canopy (Fig. 5b, c). These data may also be presented as a
classical Beer–Lambert log-plot (Fig. 6), wherein the slope may be used to
yield an estimate of the attenuation coefficient (k). The classical
Beer–Lambert approach applied to diffuse conditions (Fig. 6 – solid grey and
black line) yields k values of 0.79 and 0.81, respectively. However, under
clear sky conditions, this approach is unreliable and cannot be used here, due
to the inhomogeneous vertical distribution of foliage, and the presence of a
large gap. Overall, the result shows that under sunny conditions a very high
fraction of PPFD is absorbed or reflected at the top of the canopy, and
therefore much less remains after a leaf area index of 1.5 (in the main
canopy).
Figure 5d shows the profile of blue-light irradiance. In clear conditions, it
is attenuated substantially, but only slightly attenuated in cloudy or
overcast conditions. Close to ground level, blue light increases which we
attribute to lateral illumination within the trunk space.
In clear conditions, there was a region of the canopy with a very low red : far-red ratio, usually indicative of deep shade (Fig. 5e – black line).
However, there was considerable spatial variation. In large gaps the
clear-sky red : far-red ratio is high, reaching near-above canopy values
visible in Fig. 5e. Usually, however, the red : far-red ratio is lower, below
0.75.
The horizontal heterogeneity at the forest floor was surveyed, first by using
the spectroradiometer (Fig. 7) and then with the TRAC device (Fig. 8). The
spectral flux density, illustrated in Fig. 7, shows the thinning lines clearly. There are distinctive differences within the photosynthetically active
part of the spectrum, with higher energy levels in the photosynthetically
active part of the spectrum under overcast conditions. Figure 7a and b
illustrate the spectral flux density along the entire 115 m long transect
(2.5 m measurement interval) for the clear and overcast day, respectively
(cloudy conditions not shown here). Under both conditions the thinning lines
become visible, though the irradiance levels shift (also Fig. 8), depending
on light regime. Under clear conditions distinctive sunflecks are visible
with high energy (similar to above canopy levels) in the photosynthetically
active part of the spectrum. Under overcast conditions high energy levels
within the photosynthetically active part of the spectrum are sustained and
more evenly distributed along the forest floor. Energy levels within the
far-red and infrared regions remain high under both conditions.
In clear-sky conditions the huge variation caused by sunflecks is seen
(Fig. 8a), often reaching photon flux values of several hundred
µmol m-2 s-1, superimposed on a background that varies
systematically with the presence of thinning rows, from a minimum of about 3
to a maximum of about 20 µmol m-2 s-1. Overcast
conditions (Fig. 8b) show highly regular behaviour, closely resembling the
`background' values shown in Fig. 8a, although much higher.
As a “big picture” overview on how canopy CO2 exchange and the light
use efficiency (LUE) in Sitka spruce is behaving under such conditions we
included data from 8 consecutive days from Harwood forest. Canopy
CO2 fluxes from Harwood forest show generally the same flux variability
and range throughout the year as in Griffin forest (data not shown here).
These 8 days show the day-to-day changes in light use efficiency when sky
conditions change from overcast to cloudy to clear sky conditions. After 4
consecutive clear days (lowest light use efficiency) these are followed again
by a cloudy and an overcast day and are evident in Fig. 9.
Discussion
The study introduced here, carried out in Griffin forest, is the first to
report on both the vertical and an extensive horizontal transect through a
forest plantation.
Spectral effects
The spectral distribution of radiation is very important for plant growth and
morphogenesis (Endler, 1993; Escobar-Gutuérrez et al., 2009). The
spectral distribution of incoming solar radiation was similar under all three
sky conditions. However, substantially more energy in the photosynthetically
active wavebands penetrated the canopy in the case of diffuse skies. There
was significantly more blue light within the canopy under cloudy skies,
possibly a result of multiple reflections and scattering involving the waxy
abaxial surfaces of needles (Jeffree et al., 1971; Reicosky and Hanover,
1978; Cape and Percy, 1993). Differences in the directional properties of
direct vs. diffuse radiation may also have a role in explaining this
difference. Blue-enrichment may have important implications for stomatal
control of photosynthesis and water use. For Scots pine and Sitka spruce,
Morison and Jarvis (1983) reported that blue wavelengths are more effective
in causing stomatal opening than red wavelengths. Smith (1982) reported that
at low PPFD stomata open only in response to blue light, red light being
ineffective; thus, if Smith's is a general result, we may conclude that the
conditions of diffuse radiation in the present case are especially conducive
to stomatal opening in the lower regions of the canopy, where PPFD is low in
all three conditions.
Within the canopy there is a very high proportion of near-infrared under all
three sky conditions. This is not surprising, as leaves generally transmit as
much as 50 % of incident radiation at this waveband and reflect much of
the remaining (Middleton and Walter-Shea, 1995; Middleton et al., 1997; Knapp
and Carter, 1998; Combes et al., 2000; Carter and Knapp, 2001). On the other
hand, in the chlorophyll-absorbing region of the red, leaves transmit rather
little energy; therefore, the ratio of red to far red is dictated by the
presence of leaves. This aspect of light quality has received much attention.
The decline in the red : far-red ratio has long been known and has been linked
in numerous studies to aspects of photomorphogenesis (see reviews by Federer
and Tanner, 1966; Smith, 1982; Woodward, 1983; Morgan et al., 1985; Endler,
1993). In the present study, we have found that the red : far-red ratio in the
canopy is much lower under clear skies (Hertel et al., 2011), indicating a
lower photomorphogenical “light quality” (sensu Smith, 1982) than under
diffuse conditions.
Contrasting light attenuation under cloudy vs. clear skies
It is evident that there are profound differences in the transmissivity of
solar radiation under the different sky conditions. The most important of
these differences is the extent to which the direct sunlight is absorbed or
reflected near the top of the canopy, shown by the attenuation patterns. This
energy is therefore not available for photosynthesis lower down in the
canopy. It is also shown, quite independently, by the extent to which the
diffuse irradiation is relatively higher at the forest floor and by the
distribution of ground-level data between transmission classes (data not
shown here). The same phenomenon was shown by Morgan et al. (1985) for pine
canopies and by Leuchner et al. (2005), Navratil et al. (2007) and Urban et
al. (2007, 2012) for Norway spruce.
The vertical profile under sunny conditions demonstrated only a poor fit to
the Beer–Lambert law because of the canopy's inhomogeneity. Further data
integrating spatially and temporally would of course reduce the uncertainty
in our estimated k value. There was marked variation in the attenuation
coefficient k, also in the data shown by Norman and Jarvis (1974) and
Lewandowska et al. (1977), who obtained similar k values to those reported
here for the same species. Smith (1993) also stated that a single extinction
coefficient using the Beer's law model cannot be used effectively to predict
the light penetration in Douglas fir (Pseudotsuga menziesii). We
presume that part of the explanation of this variation lies in the variable
structure as one proceeds from the top to the bottom of the canopy: near the
top the leaves are densely crowded on the stems, whereas near the bottom
leaves are thin, sparse and attenuation is dominated by branches and stems
(Norman and Jarvis, 1974; Schulze et al., 1977; Ford, 1982; Leverenz et al.,
1982; Stenberg et al., 1998).
One obvious difference between clear sky radiation and overcast skies is the
directional distribution of the radiation. From a general consideration of
the angular distribution of brightness of an overcast sky (Grace, 1971), it
is apparent that proportionately more energy from low-angle rays of skylight
will penetrate the canopy. Such low-angle light may be important in the
photosynthesis of vertically aligned leaves but this effect will be
underestimated by a cosine-corrected horizontal sensor. For this reason,
spherical sensors have sometimes been advocated for in-canopy use (Biggs,
1986), as they more closely resemble the near-spherical distribution of leaf
angles in a forest canopy.
Gaps and sunflecks determine spatial patterns
There are two types of gaps that can occur in forest stands, firstly, natural
gaps as the result of the clumping of leaves and stems i.e. the structure and
orientation of the coniferous shoot and the needles they hold (Norman and
Jarvis, 1974; Leverenz et al., 1982). The second type of gap is artificial,
created through forest management (planting design and thinning regime).
Under clear skies the occurrences of gaps in the crown, which are sometimes
short-lived (seconds to minutes) and wind-dependent (Federer and Tanner,
1966; Pearcy, 1990; Chazdon and Pearcy, 1991), are spots where the direct
radiation beam, or some fraction of it, penetrates into the canopy (Fig. 1,
lower schematics), sometimes as far as the forest floor (Stenberg, 1995).
They create highly illuminated areas where the incident light can in
extremis reach higher values than above the canopy itself due to lateral
illumination in the trunk space and a high proportion of scattering of
radiation on the surrounding branches (Muller, 1971). Sunfleck spectra are
similar to incident radiation (Endler, 1993; Combes et al., 2000; Leuchner et
al., 2012) and may also be areas with transient higher temperatures, which in
some cases may have physiological significance. Sunflecks also have red : far-red ratios (Fig. 3e) close to those measured above the canopy (Reitmayer
et al., 2001; Leuchner et al., 2012).
At the forest floor a complex spatial pattern of sunflecks is generally seen.
The intensity of the sunflecks shows that almost always they contain
substantial penumbral components (Stenberg, 1995). They appear not in the
thinning lines but below the trees themselves: under clear sky conditions
there is a lateral shift in the total penetrated radiation compared with the
diffuse skies. This phenomenon is visible because the tree planting lines in
this forest happen to be oriented east–west, and at the prevailing solar
angles the beam must pass through a large thickness of canopy in order to
reach the ground. However, under overcast conditions solar radiation
distribution follows the thinning pattern with highest radiation values
recorded inside the thinning lines.
As these measurements were carried out around solar noon in summer, the path
through the canopy was minimal and radiation values below the canopy are
likely to be near their maximum. This high insolation distribution does not
remain constant during the day due to the planting orientation and thinning
pattern. These radiation distributions do of course change over the course of
a clear day with highest values within the thinning lines early and later in
the day, respectively (Reifsnyder, 1989; Leuchner et al., 2012). An aspect
not investigated within the frame of the current study is the below canopy
vegetation, which is also influenced by the type of forest management. At
this site, the below canopy vegetation is much more pronounced in the
thinning lines than below the canopy itself, as it is visible in the sidewise
taken hemispherical image in Fig. 1.
Implications for CO2 exchange under such conditions
As shown in many studies (Gu et al., 1999, 2002; Urban et al., 2007, 2012;
Dengel and Grace, 2010) diffuse radiation enhances photosynthesis in
terrestrial vegetation. Urban et al. (2007, 2012) and Dengel and Grace (2010)
hypothesised that optimal photosynthetic activity of the canopy is achieved
under diffuse radiation (cloudy and overcast) conditions, when scattered
light penetrates throughout the canopy, illuminating all the leaves to some
extent and providing a more uniform distribution of light between the leaves.
Leverenz and Jarvis (1979, 1980) determined light response curves of this
Picea species under controlled conditions and found light saturation
at around 500 µmol m-2 s-1, a value which is often
exceeded at the top of the canopy. Similar over-saturation values are visible
in the current study. If the uppermost level of a canopy is experiencing an
over-saturation of light and also encountering the highest shoot temperatures
in the forest, it is possible that stress responses such as closure of
stomata may occur (in this species stomata tend to close at high leaf-to-air
vapour pressure difference, Grace et al., 1975; Neilson and Jarvis, 1975;
Alton et al., 2007). Other stress responses such as photoinhibition are also
possible (Powles, 1984; Krause, 1988; Long et al., 1994). Thus, along the
sunfleck pathway, such effects may contribute to under-activity of
photosynthesis in relation to the level of incident radiation (Pearcy, 1990;
Kanniah et al., 2012).
Given the poor penetration of direct radiation into the canopy, and the
possible stress effects of PPFD values in excess of
500 µmol m-2 s-1, we can now ask: what influence do sky
conditions have on the photosynthesis of the canopy? In an earlier study on a
very similar canopy we showed that light was used more efficiently under
diffuse irradiance (see Fig. 9, insert is modified from Dengel and Grace
2010). In that study we found the quantum efficiency under direct radiation
to be 28.6, but 41.0/50.1 under cloudy and overcast conditions,
respectively. Moreover, tree ring analysis showed that diffuse radiation does
not only influence gas exchange in the short-term (hourly, daily, monthly),
but also influences long-term forest growth (Dengel et al., 2009).
S. Dengel has designed and carried out the experiment, processed the data and
written the manuscript. J. Grace has contributed to the design of the
experiment, the data interpretation and actively contributed to the
manuscript writing. A. MacArthur has taken part in the training and
experiment itself and has contributed to the data processing and manuscript
writing.
Acknowledgements
This study was part of a PhD study funded by the Torrance Bequest and a
School of GeoSciences scholarship, University of Edinburgh. The study
introduced in the current work is part of a mini project approval by NERC
(National Environmental Research Council, UK) and is assisted by the loan of
the paired spectrometers by NERC FSF (Edinburgh). The authors would like to
thank Terry Dawson for providing the TRAC system and the staff at the NERC
Field Spectroscopy Facility (FSF) (Edinburgh) for their support. Furthermore
the authors would like to thank Anitra Fraser for her support in the field.
Edited by: G. Wohlfahrt
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