Light-transmission Profiles in anOld-growth Forest Canopy:

Simulations of PhotosyntheticallyActive Radiation by Using SpatiallyExplicit Radiative Transfer Models

Maria J. Mariscal,1 Scott N. Martens,1 Susan L. Ustin,1* Jiquan Chen,2

Stuart B. Weiss,3 and Dar A. Roberts4

1Center for Spatial Technologies and Remote Sensing, Department of Land, Air and Water Resources, University of California, Davis,California 95616, USA; 2School of Forestry and Wood Products, Michigan Technological University, Houghton, Michigan 49931,

USA; 3Center for Conservation Biology, Department of Biological Sciences, Stanford University, Stanford, California 94305, USA;4Geography Department, University of California, Santa Barbara, California 93106-4060, USA

ABSTRACT

Light interception is a driving variable for many key

ecosystem processes in forests. Canopy gaps, as

natural irregularities, are common features of Pa-

cific Northwest conifer forests and have profound

importance on the within-canopy light environ-

ment. We used two spatially explicit radiative

transfer models (OLTREE2 and SolTran2,3 ) to under-

stand better the vertical profile distribution of light

penetration in an old-growth forest. Canopy access

at the Wind River Canopy Crane Research Facility

provided an opportunity to apply these models in a

tall, old-growth, Douglas-fir–western hemlock for-

est. Both models required three-dimensional de-

scriptions for every crown (location, orientation,

and size) in a 4-ha area. Crowns were then simu-

lated as foliage-filled ellipsoids through which light

is attenuated following Beer’s law. We simulated

vertical profiles (2-m height intervals) of transmit-

ted photosynthetically active radiation (PAR) in 16

gaps previously measured by Parker (1997). Point-

by-point comparisons (n = 480) between measured

and modeled results showed little agreement be-

cause small errors in crown location yielded large

local differences in PAR transmittance. However,

average gap profiles (n = 16) of PAR transmittance

showed excellent agreement (r2 = 0.94) between

simulated and measured values. SolTran was used

to simulate vertical profiles of daily PAR flux at

different seasons for the whole 4-ha canopy, not

just gaps. Overall, our results show that both models

produced excellent simulations of spatially aver-

aged vertical profiles of PAR transmission in the old-

growth forest and are suitable for further investi-

gations at other space and time scales.

Key words: light profiles; photosynthetically ac-

tive radiation (PAR) irradiance distribution; radia-

tive transfer models; old-growth conifer forests;

forest gap structure; Wind River Canopy Crane

Research Facility (WRCCRF).

INTRODUCTION

The distribution of light in forests has long been of

interest to ecologists because light interception is a

driving variable for many key ecosystem processes.

Received 15 February 2002;1 accepted 4 September 2002; published online

12 May 2004.

*Corresponding author; e-mail: slustin@ucdavis.edu

Ecosystems (2004) 7: 454–467DOI: 10.1007/s10021-004-0137-4

454

Light interception controls energy balance (Gates

1980), net ecosystem exchange of carbon dioxide

and water (Hollinger and others 1994; Paw U and

others 2004; Unsworth and others 2004; Winner

and others 2004), and canopy reflectance (Sellers

1985; Roberts and others 2004). Quantifying ab-

sorbed photosynthetically active radiation (PAR) is

essential to bottom–up models of canopy photo-

synthesis and transpiration (Winner and others

2004). Thus, the spatial and temporal distribution

of light in forest canopies is important for under-

standing forest processes and ecosystem functions.

However, the structural complexity of forest can-

opies makes such quantification difficult.

Figure 1. A45 map of the Wind River Canopy Crane Research Facility. A: Canopy crane circle (UW Plot) used by Parker

(1997) for measurements of radiation in vertical transects. B: Location of trees in 400 · 200-m area and location of area

used for simulated radiation measurements. Data collected by University of Washington (UW), Michigan Technological

University, or Earthwatch groups46 .

Light Profiles in an Old-growth Forest Canopy 455

Among the most complex forest canopies are

those of the old-growth forests of the Pacific

Northwest of the United States. These forests pos-

sess many unique structural features, including

exceptionally high leaf-area index (LAI), large ac-

cumulation of aboveground biomass and coarse

woody debris, and high variability in the microcli-

mate (Franklin and others 1981; Harmon and

others 19864 , 2004; Spies and others 1990; Lertz-

man 1992; Chen and Franklin 1997; Parker 1997;

Parker and others 2004; Paw U and others 2004;

Shaw and others 2004). An important structural

feature of mature forests is the presence of canopy

gaps. Gaps in the forest canopy, which are natural

spacing irregularities formed by fires, wind, insect

damage, and tree mortality, vary in size from tens

to thousands of square meters, normally covering

15%–35% of the forest area (Lertzman and others

1996). Coniferous forests typically have deep,

narrow gaps, which limit direct-beam illumination

near the bottom of gaps (Canham and others 1990;

Spies and others 1990; Easter and Spies 1994). The

low sun angles in northern latitudes also limit5

vertical light penetration. For trees growing in

canopy gaps, Ishii and colleagues (2000b) suggest

that height growth rate should increase as tree

height increases, because of the positive feedback

between increased light availability and accumu-

lation of leaf area. Conversely the presence of gaps

may increase penetration of diffuse light into the

lower canopy.

Despite the controlling role that canopy light

penetration plays in ecosystem processes, knowl-

edge of canopy functions remains incomplete,

partly because of the difficulty of accessing and

measuring tall canopies. Access to the canopy of an

old-growth forest (approximately 500 years) be-

came possible with the establishment of the Wind

River Canopy Crane Research Facility [WRCCRF

(Shaw and others 2004)] in southwestern Wash-

ington. Maximum tree height in this Douglas-fir–

western hemlock (Pseudotsuga menziesii–Tsuga hete-

rophylla) forest is about 62 m.

Crane access enabled Parker (1997) to measure

vertical transects of PAR in 16 canopy gaps at the

WRCCRF. He found evidence for a lumicline in

these canopy gaps: a midcanopy region where a

steep vertical gradient of transmittance exists.

However, the average transmittance profile did not

exhibit a lumicline at a specific height. Parker

(1997) described high spatial heterogeneity in the

light environment of gaps such that at any canopy

height almost any light level was possible. A similar

distribution of within-canopy irradiances is shown

by Winner and colleagues (2004). Despite advances

made possible by better canopy access, simultane-

ous direct-light measurements in the three-di-

mensional canopy space over extended periods are

not practical. However, radiative transfer models

can be used to simulate the spatial and temporal

distributions of light within this forest and examine

the relationship to forest structure.

Numerous studies have examined relationships

between forest structure and radiation absorption,

including the use of models of radiative transfer

and/or canopy photosynthesis (Nilsson and Ecker-

sten 1983; Grace and others 1987; Wang and Jarvis

1990a, 1990b; Tenhunen and others 1994; Bal-

docchi and Harley 1995; Wang and Polglase 1995;

Perttunen and others 1996; Brunner 1998). In

complex, heterogeneous canopies, simple one-di-

mensional models that assume spatial homogeneity

of light-intercepting elements are inadequate

(Norman and Welles 1983). Better estimates can be

Figure 2. Radiative flux (MJ m2 /day) from 1 January

through 31 December 1999. A: Downwelling and up-

welling longwave, net all-wave, and downwelling and

upwelling shortwave radiation. B: Seasonal variation in

the albedo of the old-growth forest.

456 M. J. Mariscal and others

made using an approach where foliage is contained

within ellipsoidal envelopes to simulate the distri-

bution of crowns in the stand (Norman and Welles

1983). Within each envelope, light is exponentially

attenuated following Beer’s law. This spatially ex-

plicit approach allows simulations using a better

representation of stand characteristics (for exam-

ple, crown location and orientation). However, it

requires knowledge of the location, orientation,

and size of each crown in the stand, thus limiting

its general application. Nevertheless, radiative

transfer modeling can assist in developing a better

understanding of the mechanisms that control light

distribution within three-dimensional canopies and

its influence on canopy processes.

We seek to understand better the vertical profile

of light penetration in old-growth forests by using

two spatially explicit radiative transfer models:

OLTREE6 (Mariscal and others 2000) and SolTran6,7

[based on work by Martens and others (2000)].

Both models use an approach similar to that of

Norman and Welles (1983). The two models differ

in several ways (see Methods), but largely in that

OLTREE explicitly includes calculation of within-

canopy scattering whereas SolTran does not. OL-

TREE was developed for olive tree orchard canopies

(Mariscal and others 2000) and has not been pre-

viously applied to natural forests. SolTran was de-

veloped for simulating the light environment in

semiarid woodlands along the grassland–forest

continuum (Martens and others 2000). Neither

model has been used for tall, old-growth conifer

forests, but both have formulations that are ap-

propriate for such an application.

Our first objective was to test and corroborate

model predictions of light transmission for a com-

plex, old-growth conifer canopy. To achieve this

objective, we compared model estimates against

instantaneous PAR measurements made by Parker

(1997) for canopy gaps at the WRCCRF. Our sec-

ond objective was to use the validated models to

demonstrate the utility of spatially explicit radiative

transfer modeling for extrapolating beyond meas-

urements in both time and space. We simulated

PAR transmittance profiles for the 4-ha stand (not

just gaps) for daily radiation at different seasons.

Overall, our results show that both models pro-

duced excellent simulations of spatially averaged

vertical profiles of PAR transmission in the old-

growth forest.

METHODS

The WRCCRF is located in the T. T. Munger

Research Natural Area of the Gifford Pinchot Na-

tional Forest (Franklin and DeBell 1988; Shaw and

others 2004). The crane allows direct access to

about 2.3 ha of forest from a suspended gondola.

Beginning in 1995, a map of tree locations was

constructed in the 12-ha area surrounding the

crane. The locations of the 16 gaps measured by

Parker (1997) are shown in Figure 1. The site

contains an average of 443 living trees and 97 dead

trees per ha (Shaw and others 2004). Tree height,

crown ratio, and average crown radius were cal-

culated using the empirical models developed by

Song (1998).

Vertical transects of PAR in 16 gaps (Parker 1997)

provided a basis to compare our model results.

Parker’s measurements were made within 2.5 h of

solar noon on 27 and 28 July 1995, corresponding

to a range of sun elevation angles from 51.7� to

63.5�. PAR measurements were made every 10 s as

the gondola was lifted from the ground, yielding a

mean vertical position spacing of about 1.9 m.

Vertical light PAR and near infrared (NIR)8 trans-

mission profiles were available from hemispheric

photographs measured in nine canopy gaps at 5-m

intervals from the canopy crane (Figure 1B).

Figure 3.47 Characteristics of the simulated canopy. A:

Vertical profile of ellipsoid volume (m3) per meters of

height summed at 1-m height intervals over all 1761

ellipsoidal crowns simulated in the 4-ha plot. B: Cumu-

lative upward leaf-area index (LAI) (m2 m)2) derived by

assuming a leaf-area density of 0.38 m2 m)3 for each

ellipsoid and integrating at 1-m intervals from the ground

upward (for total LAI = 9.11).

Light Profiles in an Old-growth Forest Canopy 457

Hemispherical photography used standard proce-

dures (Rich 1989, 1990), Kodak TriX PAN 400 ASA

film with a Nikkor 8-mm lens, and a red filter to

enhance contrast between foliage and sky. Photo-

graphs were digitized and analyzed with the

CANOPY 2.1 program (Rich 1989, 1990) to esti-

mate direct and diffuse radiation by month. The

proportion of diffuse and direct radiation at the

WRCCRF was estimated using data from the long-

term (1961–90) shortwave insolation9 data base

provided by the National Solar Radiation Data Base

(NREL) for Portland, OR (approximately 65 km

west of the WRCCRF) and from two solarimeters

(Zipp and Zonner), measuring incoming and out-

going solar radiation at 80-m height from the crane

at the WRCCRF. Measurements were made

throughout the day every 30 min for 1998, the

year the photography was acquired.

The transmission of a light beam through an el-

lipsoid follows Beer’s law in the OLTREE and Sol-

Tran models. Specifically, the transmittance (T) of a

beam of radiation in the canopy is described by

T ¼ eð�G�LAD�SÞ ð1Þ

where G is the G function10 , LAD is leaf area density

(m2 leaf area m)3 canopy volume), and S is the

path length (m) of a beam through one or more

ellipsoids. The G function incorporates the effect of

the leaf inclination angle distribution function on

light interception by making interception depend-

ent on the orientation of the incoming beam. Thus,

both models are based on the radiative transfer

scheme described by Norman and Welles (1983)

and are similar in this respect to other three-di-

mensional radiative transfer models for plant can-

opies [for example, MAESTRO11 (Wang and Jarvis

1990a; Brunner 1998)].

Direct-beam radiation and diffuse radiation are

separately computed in the OLTREE model. The

diffuse transmittance is obtained by integrating

Eq. 1 for all zenith and azimuth angles, assuming

that the diffuse solar radiation is isotropically dis-

tributed. Incoming diffuse and direct-flux density

are calculated in PAR and NIR wavebands accord-

ing to work by Spitters and colleagues (1986), as a

function of the solar zenith and the daily incident

radiation. The circumsolar portion of the incoming

radiation is added to the direct beam. The Norman

and Jarvis (1975) theory for scattering processes in

horizontal canopies was applied following the as-

sumptions and procedures proposed by Norman

and Welles (1983).

The SolTran model is based on the ray-casting

model described by Martens and colleagues (2000).

Transmission of direct beam and diffuse PAR are

computed separately. Sky diffuse PAR radiance

distribution was calculated from equations in work

by Grant and colleagues (1996). Diffuse transmit-

tance to a point is calculated by integrating Eq. 1 at

1� (in this case) azimuth and zenith angles, ac-

counting for the tree objects in the scene. Scatter-

ing is neglected. Sun and sky conditions (that is,

solar geometry, top of canopy PAR, and direct/dif-

fuse fraction) can be prescribed or simulated for

any desired temporal or spatial integration.

A spatially explicit description of the plant can-

opy is needed for both models, including details

about the location, orientation, and size of each

tree ellipsoid (crown), the leaf-angle distribution,

and the leaf-area density. For these simulations, we

used the field measurements for the 1761 trees in

the 4-ha area mapped around the crane (UW Plot

in Figure 1). These data were abstracted to specify

the location (x, y, z coordinates) and size (radii in x,

y, z dimensions) of an ellipsoid to represent each

crown in the 4-ha area. Ellipsoids were oriented

vertically (z direction) directly over the mapped

location (x, y coordinates) of the base, thus not

accounting for leaning of any bole. The radii of the

ellipsoids were based on tree diameter as derived

from the empirical model of Song (1998) that was

developed for this stand. Each ellipsoid is assigned a

leaf-area density, here 0.38 m2 m)3, regardless of

species, based on total ellipsoid volume (959,105

m3) in the 4-ha area and a presumed LAI of 9.12

(Easter and Spies 1994). Recent estimates of LAI for

this stand differ by measurement method and

cluster near 9 (Parker and others 2004, Thomas and

Winner 2004b) and 8.2–9.2 (Roberts and others

2004). Both canopy models require specification of

a G function. For OLTREE, a G function was built as

proposed by Ross (1981) based on a leaf-inclination

function. The leaf-inclination function was derived

from averaged measurements made at the site for

all species (Thomas and Winner 2000a). SolTran

assumed a spherical leaf-inclination distribution

function. For the OLTREE model, the average PAR

leaf reflectance and transmittance was 0.1 and

0.06, respectively (Ross 1981). Woody surface area

was neglected [less than 0.03% (Parker and others

2004)]. Weather data were obtained from the

WRCCRF data base.

Canopy albedo (mean upwelling divided by

mean downwelling shortwave (0.3–3.0 lm) radia-

tion)12 , was estimated from measurements of in-

coming and outgoing shortwave radiation made 20

m above the canopy by a Kipp and Zonen CNR13 1

Net Radiometer mounted on the arm of the crane

(Figure 2A). Following a method used by Betts and

458 M. J. Mariscal and others

Ball (1997), the mean albedo for days without

precipitation from August 1998 through July 1999

was 7.54%, consistent with August remotely

sensed estimates by Roberts and colleagues (2004),

who report measured reflectance and transmission

for gaps at the crane site to be about 0.04–0.02. The

albedo was higher in the winter than in summer,

probably linked to increased solar zenith angles in

the winter (Figure 2B). Coniferous forest albedos

are lower than those of other plant types. The Wind

River old-growth forest’s albedo is below the lowest

published forest values for long-term continuous

measurements over forests, which were 7.6% for

two spruce/poplar forest sites and a jack pine forest

Figure 4.48 Observed and estimated photosynthetically active radiation (PAR) transmittance profiles for 16 gaps at the

Wind River Canopy Crane Research Facility (squares, field data; ·, estimates made by the OLTREE model; and no symbols,

estimates made by SolTran). Transect numbers follow Parker (1997), and those followed by (G) indicate that the location

was also defined as a gap in the simulated canopy of ellipsoids.

Light Profiles in an Old-growth Forest Canopy 459

site in Canada [see (Betts and Ball 1997); see also

Mukammal (1971), Stewart (1971), Tajchman

(1972), Jarvis and others (1976), Betts and Ball

(1997), and Ni and Woodcock (2000)]. The

WRCCRF albedo is consistent with the relationship

Stanhill (1970) described for mean albedo and

canopy height.

To simulate the 16 PAR transmission profiles

measured by Parker (1997), we derived gap loca-

tions (x, y coordinates) from his data. We specified

vertical transects at each location consisting of 30

points at 2-m height intervals from 0 to 58 m. We

assumed clear sky conditions for 27 July 1995 [date

of measurements by Parker (1997)] at noon Pacific

Standard Time (sun elevation angle about 63�) and

a direct fraction of incoming PAR to be 0.75. Re-

sults for instantaneous PAR (lmol m)2 s)1) trans-

mission under these conditions were converted to

relative transmittance (below-canopy PAR/above-

canopy PAR) for comparison to Parker’s results.

Simulations of daily canopy PAR transmission for

the summer and winter solstices and fall equinox

used SolTran, with calculations made at a grid of

vertical transects. The grid consisted of 21 · 21 lo-

cations with 5-m horizontal spacing with the lower

left coordinate at 250 m easting, 275 m northing

(Figure 1). This allowed an ample buffer on the

south side to avoid edge effects at low sun eleva-

tions when the direct-beam radiation penetrates

through the side of the canopy to the ground. At

each of the 441 locations, we simulated a vertical

transect of 31 points (from 0- to 60-m height at 2-m

intervals). At each point, daily PAR (mol m)2

day)1) was integrated from computations of in-

stantaneous PAR (lmol m)2 s)1) made at 15-min

intervals throughout the daylight period.

Direct radiation and diffuse radiation were esti-

mated from light-transmission profiles measured in

nine canopy gaps at 5-m intervals (Weiss 2000).

These data, along with the NREL data base and

local solarimeters at the WRCCRF, provided inde-

pendent estimates of canopy light penetration and

the proportion of direct and diffuse radiation.

To study the spatial radiative environment, si-

mulations were made for the central part of the

forest, west of the crane (Figure 1B), for an area of

200 · 100 m. Over this area, 2000 cells of 5 · 3 m

were defined, and diffuse and seasonal transmit-

tance at 5-m canopy height intervals (derived from

the hemispheric photos) in the forest were calcu-

lated. Also PAR and shortwave reflectance was14 es-

timated for these cells.

RESULTS15

The simulated 4-ha canopy area consisted of

1761 ellipsoids with a total volume of 959,105 m3.

The vertical distribution of leaf area of the simu-

lated canopy of ellipsoids (Figure 3) was derived at

1-m height intervals. Maximum leaf area of the

simulated canopy occurred at about 24-m height

(Figure 3A), which was also the median height.

This simulation approximates the results reported

by Chen [Figure 2 in Parker and others (2004)].

Approximately 80% of the leaf area occurred be-

tween 10- and 39-m height (Figure 3B), also con-

sistent with results of Parker, Chen, and Van Pelt

[Figure 2 in16 Parker and others (2004)].

We compared light profiles measured by Parker

(1997) with model simulations by OLTREE and

SolTran in these 16 gaps. To test whether gaps in

the simulated canopy occurred where Parker’s

measurements were made, we used SolTran to

calculate transmittance of a vertical ray at each

location (a narrow definition of a gap). We found

that seven locations (5, 9, 12, 13, 15, 16, and 19,

using Parker’s original location numbers) met this

criterion for a gap. The other nine locations were

found to have one or more ellipsoidal crowns di-

rectly within the gap, such that a vertical ray would

be intersected at heights ranging from 52 to 2 m.

Parker (1997) described the relative size of these 16

gaps as large (locations 1, 6, and 15), medium (lo-

cations 2, 5, 12, 16, and 19), small–medium (loca-

tions 9 and 13), small (locations 3, 4, 7, and 18),

and very small (location 10). We found that gaps in

our ellipsoidal canopy corresponded with Parker’s

at one of the three large gaps, four of the five

medium gaps, and both small–medium gaps, but

Figure 5.49 Simulated versus observed instantaneous

photosynthetically active radiation (PAR) transmittance

for 16 gaps at the Wind River Canopy Crane Research

Facility (squares, field data · estimates made by the

OLTREE model and diamonds, field data · estimates

made by the SolTran model.

460 M. J. Mariscal and others

none of the small or very small gaps were open to

the ground surface. Thus, of the 16 gap locations

measured by Parker (1997), fewer than half can be

considered deep gaps (continuously open from

ground surface to top of canopy) in the simulated

canopy of ellipsoids. This discrepancy illustrates the

complexity of the actual foliage and branch distri-

bution compared to the simulated ellipsoid volume.

The PAR transmittance profiles measured by

Parker (1997) were individually compared to pro-

files simulated by the two models for each of those

locations (Figure 4). There is much variation

among the 16 profiles as well as among the three

methods used to estimate the profiles. In some

cases, there was good agreement among the two

modeled profiles and the measured profiles (for

example, locations 10, 13, 16, and17 18, all small or

very small gaps). In other cases, the measured

transmission profiles differed greatly from both of

the modeled profiles, although the modeled profiles

were similar (for example, locations 1 and18 12, large

and medium gaps). Furthermore, all three profiles

differed for other locations (for example, locations

2, 3, 5, and19 19, medium and small gaps). The

agreement is best for small gaps, suggesting that as

foliage fills the space it approximates a random

distribution that the models more closely fit. Gen-

erally, differences among the methods in predicting

the height at which transmission decreased sharply

appear to account for most of the differences

among the profiles. Also, the simulated profiles

were usually smoother than the measured profiles.

For example, this is especially apparent for location

2, where measured transmittance fluctuates widely

along the profile. Overall, the common feature of

most profiles is high transmittance near the top of

the canopy and very low transmittance near the

bottom of the canopy, with an abrupt decline in

transmittance [the lumicline described by Parker

(1997)] in the midcanopy region. This pattern re-

sults in a bimodal distribution of transmittance

values (both simulated and observed).

Comparison of PAR transmittance simulated at

individual points (in both space and time) with

measured values is the most difficult test for the

models. The point differences between simulated

and measured PAR estimates are seen by plotting

the 480 points (Figure 5). The bimodal distribution

of transmittance values is clearly apparent—most

points have either high or low PAR transmittance

values, as was seen in the individual profiles (Fig-

ure 4). Intermediate values of PAR transmittance

(0.3–0.8) are less frequent in the measured data set

(2.6% of the points) than in either of the modeled

data sets (OLTREE, 9.2%; SolTran, 13%). Correla-

tions between estimated versus measured PAR

values for each of the models yield statistically

significant (P < 0.01) relationships (OLTREE,

r2 = 0.49; SolTran, r2 = 0.42). However, given that

the estimated values range from 0.0 to 1.0 across

the full range of observed values, the predictions of

instantaneous PAR transmission by either model

are unsatisfactory.

Better agreement between simulated and meas-

ured PAR values is likely to be obtained by com-

paring temporally integrated values (for example,

daily PAR), spatially averaged values, or both. Be-

cause Parker’s (1997) data are instantaneous

measurements, we could not test the models

against temporally integrated values, but we did

test them against spatial averages of the 16 gaps at

each of the 30 heights simulated (Figure 6). There

were strong correlations (r2 = 0.94 and P < 0.01)

between spatial averages of measured transmit-

tance and those simulated by the models. Linear

regression equations for these relationships in-

dicated that intercepts were near zero (OLTREE,

0.055; SolTran, 0.089) and slopes were slightly less

than 1.0 (OLTREE, 0.90; SolTran, 0.87). Thus, the

models provide good predictions of the spatial

average of PAR transmittance in gaps.

Profiles of mean transmittance were created by

averaging values for the 16 gaps at 2-m height in-

tervals for each of the models. Results for mean

transmittance show close agreement among the

estimated and measured values (Figure 7A) as

would be expected from the high correlations pre-

sented in Figure 6. The profiles of mean transmit-

tance exhibit a more gradual decline in

transmittance with height, unlike the profiles for

Figure 6.50 Simulated versus observed instantaneous

photosynthetically active radiation (PAR) transmittance

for 480 points averaged by height (30 height levels si-

mulated for the 16 vertical gap transects). Simulations

were made with OLTREE and SolTran.51

Light Profiles in an Old-growth Forest Canopy 461

the individual points that exhibit a sharp decline in

transmittance with height (the lumicline). Profiles

of standard deviation of PAR transmittance (Fig-

ure 7B) show more variation among the three es-

timates. Overall, there is a pattern of relatively

lower variance at the top and bottom of the

canopy, with higher values in midcanopy (about

22–32 m) for all three estimated profiles. As ex-

pected, the modeled estimates show less variation

in standard deviation with height than do the

measured values.

These results (Figures 6 and 7) demonstrate that

both models, OLTREE and SolTran, provide good

estimates of spatially averaged PAR transmittance

profiles in canopy gaps. Because the models are not

intrinsically limited in spatial or temporal scope as

are the measurements, they can be applied to PAR

estimates over longer periods and extended to sites

of similar canopy structure.

We used SolTran to simulate daily transmitted

PAR flux (mol m)2 day)1) profiles for summer

(solstice), fall (equinox), and winter (solstice) days

under clear-sky conditions (Figure 8). The profiles

were derived from spatial averages of a grid of 441

points at each 2-m height interval. The profiles

of mean flux showed strong seasonal variation

(Figure 8A). Incoming PAR flux was 33.4 mol m)2

day)1 at winter solstice, 65.0 mol m)2 day)1 at fall

equinox, and 91.4 mol m)2 day)1 at summer sol-

stice (Figure 8A). Photosynthesis is saturated at

90% maximum rate at 50% maximum (50 mol

m)2 day)1) PAR (Winner and others 2004), indi-

cating that for most of the year the midcanopy to

the upper canopy (more than 35 m) is light satu-

rated. At ground level, transmitted PAR flux was

0.81 mol m)2 day)1 at winter solstice, 2.6 mol m)2

day)1 at fall equinox, and 6.7 mol m)2 day)1 at

summer solstice (Figure 8A). The height at which

PAR flux was decreased to half of the aforemen-

tioned20 canopy value was 40.5 m at winter solstice,

37.0 m at fall equinox, and 35.1 m at summer

solstice. The standard deviation of transmitted daily

PAR flux also showed strong seasonal differences.

There is a trend for maximum standard deviation

to increase from winter through summer, thus

paralleling the trend in incoming PAR flux. The

height at which the maximum standard deviation

occurred also showed a seasonal trend with the

highest variance at winter solstice (44 m) and

decreasing to about 38 m at summer solstice.

This parallels the trend for the height at which

relative transmittance decreases to 0.5 as described

previously.

Average diffuse PAR values for nine gaps and

corresponding standard error are shown in Figure 9

for both measured (hemispheric photo sites) at 5-m

height intervals and estimated profiles (using the

OLTREE model). The overall agreement between

modeled values and both hemispheric photography

and light measurements supports the use of these

models for predicting the light environment in this

old-growth forest.

Monthly mean profiles of direct-beam penetra-

tion at the nine sites measured by hemispheric

Figure 7. Vertical profiles of instantaneous photosynthetically active radiation (PAR) transmittance in gaps derived from

model simulations and observations. A: Mean PAR transmittance, averaged for 16 canopy gaps. B: Standard deviation of

PAR transmittance in gaps. Simulations were made with OLTREE and SolTran.

462 M. J. Mariscal and others

photographs were calculated for January, March,

and June (Figure 10). Attenuation of direct-beam

radiation is less gradual than is diffuse attenuation.

Clearly, most of the radiation is intercepted be-

tween 20- and 40-m height, and less direct-beam

penetrates deeply into the canopy than diffuse ra-

diation. As the solar declination is small (that is,

from January to June), direct beam penetrates

deeper, and thus the illumination at every height is

more uniform. Overall, predictions markedly agree

with observed values, although the model overes-

timates attenuation higher in the canopy in June.

Spatially explicit diffuse transmittance was esti-

mated from OLTREE for an area of 2000 5 · 2-m

cells (Figure 11). The three-dimensional distribu-

tion of predicted transmittance over this block is

shown for the forest floor (0-m height) and at

canopy heights of 10, 20, 30, 40, 50, and 60 m.

Diffuse transmittance at the soil surface for these

gaps was about 10% of incoming radiation; virtu-

ally all incoming irradiance is transmitted at 50-

and 60-m height in the canopy. Heterogeneity was

highest in the middle of the canopy.

DISCUSSION

Our results demonstrate the utility of these spa-

tially explicit radiative transfer models for predict-

ing PAR flux in a complex, old-growth forest

canopy. Both models provided excellent predic-

tions of spatially averaged vertical profiles of PAR

transmission in the old-growth forest gaps (Fig-

ures 6 and 7). However, predictions for individual

points in the canopy showed poor agreement with

instantaneous measurements (Figure 5). Simulated

vertical profiles of PAR transmission for individual

gaps (Figure 4) displayed lumiclines similar to

those of the measured profiles made by Parker

(1997). The models predicted the mean and vari-

ance for the light field (composed of many points)

in the canopy and thus are suitable for further

investigations of the light field at other space

and time scales (Figure 8). The models can pre-

dict stand-level estimates of radiative transfer

Figure 8. Vertical profiles of daily photosynthetically

active radiation (PAR) transmitted through the canopy

(not only gaps) at the Wind River Canopy Crane Re-

search Facility simulated by SolTran. A: Mean. B:5252

Standard deviation for first day of summer, fall, and

winter.

Figure 9. Average direct-beam photosynthetically active

radiation (PAR) transmittance profiles for nine gaps

estimated from hemispheric photos (Weiss 2000) and

OLTREE-predicted PAR.

Light Profiles in an Old-growth Forest Canopy 463

(Figure 8) anywhere within the canopy and are not

restricted to gaps as are direct measurement tech-

niques [for example, see Parker (1997) and Weiss

(2000)].

Both models appeared to perform equally well

(Figure 6), even though they differed somewhat in

formulation. The simplifying assumptions made by

SolTran (for example, spherical leaf-angle distri-

bution, and21 neglecting within-canopy scattering)

did not adversely affect predictions. Indeed, Mari-

scal and colleagues (2000) concluded that PAR

scattering within an olive tree canopy could be

neglected, a conclusion also reached for other

canopies (for example, see Ross 1981). However, if

the models were extended to simulations of near-

infrared radiation in the canopy, neglecting scat-

tering would likely increase error, due to the large

near-infrared reflectance of leaves.

The accuracy of both models is largely dependent

on the specification of canopy structure in the

model. This is apparent from the poor correlation of

the point-by-point comparison of PAR transmit-

tance (Figure 5). A small error in relative spatial

arrangement can yield a large difference in PAR

flux, especially if it results in a change from pri-

marily diffuse radiation to direct-beam radiation,

which can be orders of magnitude greater. The

inaccuracy of the canopy ellipsoids to match actual

foliage distribution is also directly apparent in our

result that, of the 16 gaps we simulated, only seven

were also gaps in the canopy of ellipsoids (Fig-

ure 4). Inaccuracies in the canopy description may

result from errors in describing the location, size,

orientation, and shape (for example, cylinder or

cone) of the ellipsoids or the foliage within them.

Location errors are likely to be greatest for the z

coordinate because it was derived from a statistical

relationship with tree diameter (Song 1998) and is

not a specific measurement of an individual crown.

The ellipsoidal crown size was also derived from

statistical relationships (Song 1998) and therefore

does not perfectly describe a particular crown.

Orientation of each ellipsoid was assumed to be

vertical above the mapped location (x, y coordi-

nates) of the base, but many trees at this site are

known to lean (D. Shaw personal communication).

Finally, the ellipsoidal shape may not be an ap-

propriate assumption for all crowns. The slight

underprediction of transmittance by the models

(Figure 7A) just below the top of the canopy (38–

45 m), and the overprediction by the models of

standard deviation of transmittance at the same

levels (Figure 7B), may indicate that the ellipsoidal

shape defines too much crown volume at these

heights. Some crowns at this site appear to be better

approximated by a conical shape (Ishii and others

2000a), or even a more complex, dissected shape

(Van Pelt and North 1999). Future improvements

in simulating canopy light environments are de-

pendent upon better spatial descriptions of the

canopy, perhaps by accounting for the leaning of

boles or through more detailed crown descriptions

[for example, see Martens and others (1991)].

Our estimates of seasonal daily PAR flux (Fig-

ure 8A) are consistent with those estimated by

Weiss (2000) using hemispheric photography in

canopy gaps at this site (Figure 9). Mean PAR flux

varied with season such that relatively more PAR

was transmitted deeper into the canopy in the

summer than in the winter, with the fall being

intermediate, similar to the findings of Weiss

(2000). Our estimates of variance (Figure 8B) show

that the canopy height of maximum variance in

PAR flux increases from summer (38 m), through

fall (40 m), to winter (44 m), coincident with in-

creasing solar zenith angles. These heights are well

above the position of peak LAI (about 24 m) found

in our estimates.

Diffuse transmittance estimated by OLTREE was

spatially estimated over an area of 2000 5 · 2-m

cells, (Figure 8). The three-dimensional distribution

over this block is shown for the forest floor and at

10-, 20-, 30-, 40-, 50-, and 60-m canopy heights.

Diffuse transmittance at the soil surface was about

10% of incoming radiation; virtually all is trans-

mitted at 50- and 60-m height in the canopy. Het-

erogeneity was highest in the middle of the canopy.

It is difficult to directly measure spatially dis-

tributed transmittance within the forest. These

models can be used to predict the light environ-

ment for physiological and biometeorologic studies.

Figure 10. Monthly mean profiles of direct-beam trans-

mittance at nine sites measured by hemispheric photo-

graphs (observed), calculated for January, March, and

June, and predicted by OLTREE (simulated)53 .

464 M. J. Mariscal and others

Based on the results shown in Figure 11, light

levels in the upper canopy, at heights of 40 m

above the floor, are on an average seasonal basis,

above 800 mE22 m)2 s)1. If we consider that 90% of

the photosynthetic capacity is saturated at 1000 mE

m)2 s)1, it seems that the upper canopy assimilates

at near maximum rates. Radiative density flux at

heights of 30–20 m are below 300 mE m)2 s)1, and

it seems that the compensation point [50 mE m)2

s)1 (Paw U and others 2004)] is not reached even at

the bottom of the forest. Douglas-fir is at photo-

synthetic saturation (Winner and others 2004)

above 1000 mE m)2 s)1. Its foliage is distributed

between 20- and 50-m height (Parker and others

2004), indicating the wide range of light intensities

the foliage grows in, from the compensation to the

saturation point. Western hemlock and western red

cedar have photosynthetic rates that decline at light

levels above 500 E m)2 s)1 (Winner and others

2004). The foliage of these species is typically lo-

cated between 0- and 20-m height, thus these

species grow at light levels less than 500 mE m)2

s)1. Under climate conditions in western Wash-

ington, about 50% of seasonal incoming radiation

reaches the canopy as diffuse radiation. Because

both direct and diffuse profiles are similar, on a

seasonal basis we can consider that 50% of the light

reaching any point in the canopy is diffuse and

50% is directed beam.

Use of radiative transfer models to study canopy

PAR fluxes is promising because these models

demonstrate realistic performance in predicting the

Figure 11. Spatially explicit diffuse transmittance estimated by OLTREE for an area of 2000 cells, each 5 · 2 m.

Light Profiles in an Old-growth Forest Canopy 465

mean and variance for light fields (that is, all points

on a horizontal plane). Three-dimensional simula-

tions of light transmission in complex canopies may

provide abstractions that improve one-dimensional

conceptualizations of tree canopies used in many

ecosystem models [for example, see Anderson and

others (2000)]. Because light interception exerts a

major control over net ecosystem exchange of

carbon dioxide, three-dimensional modeling of

light interception may improve regional estimates

of carbon flux.

ACKNOWLEDGEMENTS

This research was performed at the Wind River

Canopy Crane Research Facility, a cooperative sci-

entific venture among the University of Washing-

ton, the USFS PNW Research Station, and the USFS

Gifford Pinchot National Forest. This research was

supported by the Office of Science, Biological and

Environmental Research Program (BER), US De-

partment of Energy (DOE), through the Western

Regional Center of the National Institute for Global

Environmental Change (NIGEC) under Coopera-

tive Agreement DE-FC03-90ER61010. Any opin-

ions, findings, and conclusions or recommendations

expressed herein are those of the authors and do not

necessarily reflect the view of the DOE. We wish to

thank Geoffrey G. Parker for his data and sugges-

tions on an earlier version of this report.

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Light Profiles in an Old-growth Forest Canopy 467