Access to the Upper Forest Canopy
with a Large Tower Crane
Sampling the treetops in three dimensions
Geoffrey G. Parker, Alan P. Smith, and Kevin P. Hogan
Figure 1. Sketch of the canopy crane operating in a forest showing the tower (a), jib
(b), counterjib and counterweight (c), the operator's cab (d), and the gondola (e)
attached to the hook. The radius and height of the jib are 82 and 52 m, respectively.
T he uppermost forest canopy isa frontier ofscientific research(Erwin 1983). It is the pri?
mary interface between the atmo?
sphere and the forest and is a reservoir
of biological diversity. But understand?
ing of this important portion of the
forest is far from adequate because of
the difficulties in gaining access to the
tops of trees. Most techniques cur?
rently available to study canopies pro?
vide limited flexibility and maneuver?
ability, little safety, and almost no
access to the important outermost
canopy zone.
The inability to study the function?
ing of the upper forest canopy in situ
has stalled progress on a variety of
critical research pursuits. For example,
the examination of forest/atmosphere
interactions (crucial to understanding
global climate) and rigorous analyses
of canopy biodiversity (an essential
basis for conservation decisions) are
both severely limited by the lack of
controlled canopy access.
A tower crane with a long horizon?
tal jib brings the previously unreach?
able portions of forests within the
range of scientific scrutiny. It allows
repeatable observations and experi?
mental manipulations on individual
Geoffrey G. Parker is forest ecologist at
the Smithsonian Environmental Research
Center, Edgewater, MD 21037-0028.
Alan P. Smith is assistant director for
terrestrial research and Kevin P. Hogan
is a postdoctoral associate at the Smith?
sonian Tropical Research Institute, Unit
0948, APO 34002-0948, Republic of
Panama. ? 1992 American Institute of
Biological Sciences.
The upper forest canopy
is viewed from an
atmospheric perspective
canopy elements, measurements of
whole-canopy organization and struc?
ture, and freedom to take spatially
detailed measurements ofenvironmen?
tal quality in forests.
With the tower crane, investigators
approach the canopy from above, from
an atmospheric perspective, rather
than from the forest floor. A cage (or
gondola) containing the investigator
and equipment is lowered into the
canopy from the jib. The jib angle as
well as the range and the height of the
gondola may be precisely controlled.
Thus, the gondola is completely ma?
neuverable in three dimensions, lim?
ited only by the presence of trunks and
branches.
There has been only one applica?
tion of a tower crane to explore the
canopy. It employed a small proto?
type crane installed in a forested park
in Panama. In this article, we discuss
the general background, rationale, and
potential of this novel access system
and describe some initial results of the
ongoing study.
The upper forest canopy
The first layer of leaves and twigs at
the top of the forest is the outermost
664 BioScience Vol. 42 No.9
canopy. The surface of this zone is at
variable heights from the ground, as it
includes canopy gaps of various ages
(depths) and extents, individual
crowns, and spaces between crowns.
This poorly explored zone may be
thought of as a subsystem of the forest
(Carroll 1980).
Environmental conditions in this
zone differ strikingly from those at
lower levels within the forest (Jones
1983, Oke 1987). The uppermost for?
est levels are more arid, windy, and
bright, with greater daily and sea?
sonal variation in temperature and
humidity. Plant tissues and canopy
fauna are exposed to more pollutants,
much more ionizing radiation, and
more desiccation. At this level, atmo?
spheric conditions are relatively unal?
tered by the forest (Parkeret al. 1989),
whereas at all other layers of the for?
est these parameters are substantially
moderated by the outermost layer (e.g.,
Lee 1983). The character and func?
tion of this outer layer is therefore of
foremost importance in understand?
ing the reciprocal effects of canopy
and atmosphere.
The forest canopy is home to much
biological diversity. The insect fauna
of tropical tree crowns is exceedingly
rich and host-tree specific (Erwin
1982). Furthermore, the diversity of
arthropods appears to be particularly
high in the upper canopy. However,
little is known about the canopy com?
munities ofany forest (e.g., Schowalter
et al. 1981). Information on these
biotic interactions is essential for wise
conservation decisions.
The tradition of canopy access
Ascending into the forest canopy is an
old and lofty human enterprise; ex?
ploration of the treetops for scientific
reasons is relatively recent (Bates 1960,
Perry 1986). Mitchell (1982) described
its history, particularly the methods
used in tropical forests. The various
techniques range from simple to com?
plex, with a wide variation in safety
and flexibili~y.
Although tree trunks are often
climbed directly (e.g., Denison et al.
1972), climbing ropes attached to
limbs high in the tree is the more
common access method (e.g.,
Nadkarni 1988, Perry 1977). This
approach uses the vertical rope tech?
nique developed for mountain climb-
October 1992
Figure 2. View of the gondola during
microclimate measurements 25 m above
the ground in a dry forest near Panama
City, Panama. The gondola is a welded
steel cage measuring approximately 1 X 1
X 2 m.
ing, caving, and rescue work (Padgett
and Smith 1987). It is easily learned,
and the danger and exertion can be
reduced with practice. However, the
availability of secure anchor points
limits movement to the vicinity of
supporting stems or limbs. Rope sys?
tems can be designed to provide some
lateral freedom. For example, the rope
web of Perry and Williams (1981)
permitted a climber access to a large
volume of canopy space, but it was
elaborate to install and arduous to
use. Climbing is generally not suitable
for the upper quarter of the height of
the forest, where limbs are too small
to safely support a person's weight.
Only a small portion of the outer
canopy may be reached with fixed
towers, masts, or scaffolding. Aerial
walkways (Mull and Liat 1970), cable
tramways (Perry 1986), and horizon?
tally supported spars (Denison et al.
1972, Mitchell 1982) provide addi?
tional lateral movement, although
usually at levels below the outer
canopy. Ultimately, fixed structures
are limited in spatial flexibility. Also,
their installation may cause some
modification of forest structure: for
example, the position of a tower will
always remain a gap in the canopy.
A large, semirigid platform con?
structed of inflated tubes in a hexago?
nal array supporting a strong mesh
netting was placed on the top of over?
story trees in French Guiana (Halle
1990). With this canopy raft, research?
ers moved and worked directly on 600
square meters of upper canopy sur?
face, where animal and plant collec?
tions were readily made. The raft was
deployed with a large hot-air diri?
gible, and it was easily moved from
one location to another.!
The raft must be installed in closed
canopies of uniform height. Because
the presence of the raft alters the
exchange between atmosphere and
canopy, it is not suitable for measure?
ments of environment conditions or
of plant physiological responses. Fur?
thermore, the raft tends to depress the
crowns of the support trees, settling
slowly downward. Finally, several
pilots and a large ground crew are
necessary to support the raft opera?
tion while in position.
Cranes and rafts have unique and
complementary potentials. Whereas
the crane is employed for long-term
measurements of many parameters at
a given location, the raft is useful for
some short-term sampling at numer?
ous sites.
Canopy access lags behind
ocean access
Methods for studying canopies can be
compared with those used in another
10. Pascal, 1992, personal communication.
665
-4~4~0.....L.--..L.----I.----l_2-0-L---&.;;;::::::t.-.Il.OIllllllOl:l=:'L.-..L.-2~0----l--JL.....-~40
meters
In the water, means for remotely
controlled sensing and sampling were
developed, including drones and ro?
bot submersibles. Novel ecosystems
(e.g., the sea floor vents) and archaeo?
logical sites (e.g., the wreckage of the
Titanic) were first discovered using
such tools. However, analogous de?
vices for research in forest canopies,
such as cameras and sensors mounted
on balloons, are only now in develop?
ment. Manned submersibles were long
ago employed to allow people access
to the deep-sea environment. Sub?
mersibles were first lowered into the
depths from a surface vessel; indepen?
dent and maneuverable submarines
were devised subsequently. Compa?
rable instruments for human access of
the forest canopy (i. e., manned ascen?
dibles) have not, until just recently,
been available.
A top-down approach to
canopy access
Designed for use under the congested
conditions of narrow European streets,
tower cranes are intended to be "quiet,
relatively unobtrusive, and safe"
(Shapiro and Shapiro 1988). In ham?
merhead-style cranes, the horizontal
jib and counterweights, motors, and
the operator's cab are supported atop
the tower, which minimizes the crane's
footprint on the ground. The compo?
nents of such a canopy access system
are illustrated in Figure 1. Powered by
separate electrical motors for the jib
azimuth (slewing motion), the range
(trolley), and the height (hook) of the
load, such cranes can rapidly hoist
and transfer as much as 20 metric tons
at a time. A crane operator controls
the motion from a vantage above the
load. The tower position may be fixed
on a concrete foundation or on a
moveable base mounted on a wide?
gauge railroad track. Variously shaped
areas may therefore be covered.
When the crane is employed for
canopy access, the scientist and ex?
perimental apparatus are lowered into
the forest within a frame cage (the
gondola) attached to the crane hook.
For measurements of environmental
quality, a smaller instrument package
(pod) may be deployed. The position
of the gondola or pod is controlled by
the crane operator; remote radio con?
trol is also possible. A positioning
system can be provided to allow accu-
4020
and climbing spikes and grippers. To
extend the duration of the stay, artifi?
cial habitats were constructed, for
example, the underwater Sealab and
platforms and walkways in the canopy
environment.
-20
-20
-25
-30
-35+---~---r----~--.----~---,;---~----1
-40
o
-15
inaccessible environment, the deep
ocean. Researchers in both situations
encounter depth/height and time lim?
its. Equipment was devised in each
case to increase range and maneuver?
ability, for example, scuba equipment
20
o
Distance, m
Figure 3. (a) Contour map of the upper canopy surface of a tropical dry forest in the
Parque Metropolitana made in September 1990. North is to the right, and the crane
tower sits at the center of the diagram. The contour interval is 5 m, referenced to the
highest leaf. The lowest point on the ground is 35.9 m below. Deep regions (gaps) are
shaded dark, whereas high levels (tall overstory crowns) are lightly shaded. (b) A
north-south section through the center of the surface in Figure 3a. Note the
exaggeration of the vertical axis.
20
a
40 r--..,..-...,---r----r--,-~ _ _T_-
b 0.------------------------,
-s
-10
666 BioScience Vol. 42 No.9
Figure 4. Diurnal changes in light intensity above (a) forest, in f..lmol m-2 S-1, leaf water
potential in MPa (b), leaf fluorescence ratio (c), and initial fluorescence F in relative
units (d) in several species of canopy trees, from measurements taken ~n 13 April
1991. In all panels, the squares are Anacardium excelsum, circles are Didymopanax
morototoni, and triangles are Luehea seemannii. Plotted points and bars are means
and standard errors of 4-6 samples. Light was averaged over 5-minute intervals with
a LI 190 quantum sensor and recorded with a LI-1000 data logger (LiCor, Lincoln,
NE). Water potential was measured in the forest canopy by using a pressure bomb
immediately after cutting twigs from the branch. Additional leaves were harvested
and lowered to a mobile laboratory on the forest floor and maintained in the dark.
Fluorescence was measured within 10-40 minutes with a chlorophyll fluorometer
(PAM 101; H. Walz, Effletrich, Germany). When a dark-adapted leaf is stimulated
by a low level of light (insufficient to induce photosynthesis), it fluoresces at a level
designated F
o
? In the presence of a high-intensity lamp flash (sufficient to saturate the
light-harvesting system), it fluoresces at a maximal rate, Fm. The difference between
these levels (Fm - F) is called variable fluorescence, F
v
? The fluorescence ratio (F/Fm)
is a useful indicator of photoinhibitory damage (Demmig and Bjorkman 1987,
Schreiber et al. 1986). The increases in F
o
in Didymopanax and Luehea suggest
possible photodamage, but the decrease in Anacardium suggests that pigment systems
have been produced that protect against photodamage.
a
b
2400
1800
C
U. 1200
"-
"- 600
0
0.0
-0.5
~
-1.0~
-1.5
-2.0
0.84
Life history of canopy biota. The biol?
ogy of canopy organisms, except for
wide-ranging vertebrates and inverte?
brates, can be studied directly with
the canopy access system. Repeated
surveys of numerous individual leaves
would yield rates of appearance, her?
bivory, and mortality. The identity,
visitation rates, and success of over?
story pollinators could be studied at
close hand, as could the breeding sys?
tems of trees, the production of fruits,
and frugivory. Flying insects could be
sampled over a wide area with traps
deployed from the gondola or jib.
Foliage and stem arthropods could be
collected from canopy elements di-
epiphytes, lianas, and vines) can be
conducted within the upper canopy
environment with the canopy crane.
For example, leaf physiological re?
sponses to experimental manipula?
tions of atmospheric environment
(e.g., temperature, ultraviolet light,
carbon dioxide and ozone, and rain?
water quality) can be studied where
such conditions might occur. Mea?
surements of gas exchange (e.g., ni?
trogen fixation, photosynthesis, and
transpiration) may be conducted on
leaves or whole branch systems with
the canopy crane.
E
U. 0.80
"-
> 0.76
U.
0.72
14~d13
0 12
U. 11
10
9
5 7 9 11 13 15 17 19
Time of day
Direct observation ofcanopy elements.
Direct measurements and experimen?
tal manipulations within actual cano?
pies are critical. Observations on plants
or tissues in noncanopy environments
or on young specimens of overstory
species are not suitable proxies for
direct measurements within the canopy
(Boardman 1977, Cregg et al. 1989,
Kramer and Kozlowski 1979, Leverenz
and Jarvis 1979). The natural canopy
environment is not comparable to that
of large gaps or forest edges (Pearcy
1987), and it is not easily reproduced
in the laboratory. The gondola of the
canopy access system can help install
and service experimental chambers
and instruments to make the neces?
sary direct measurements.
Studies of individual canopy ele?
ments (e.g., branches, twigs, leaves,
canopy can be achieved with little
damage to the forest. Where roads are
available, a second, mobile crane is
used to raise the tower crane. Other?
wise, installation by helicopter is an
option. The footprint of the tower
base can be relatively small: the foun?
dation required for the largest avail?
able crane is less than 10 meters wide.
Novel research opportunities
The canopy access system opens a
variety of new research opportunities.
Initial efforts are intended to describe
the details of the canopy components,
the canopy environment, and varied
interactions. Additional possibilities
are expected to become evident once
the system is in use.
rate determination of the gondola's
three-dimensional coordinates, so that
a position may be revisited and indi?
vidual canopy elements be resampled
at will.
The gondola must be small for flex?
ible access but large enough to accom?
modate the researchers and their equip?
ment. An open gondola design allows
the investigator to grasp a variety of
upper canopy elements. The gondola
body is shaped for easy penetration of
and extraction from the canopy. A
roof provides some protection from
rains. Lights in the gondola would
allow nocturnal operations.
Tower cranes have a good safety
record in the construction industry
(Allen 1985, Shapiro et al. 1980).
Strict standards are in force for crane
operation (ANSI 1984) and their use
as personnel hoists (CIA 1989). Be?
cause the load requirements ofcanopy
access are far less than those of con?
struction applications, a canopy ac?
cess system has a far greater margin of
safety. A harness and lifeline secures
the investigator within the gondola. A
safety line and descending system pro?
vide a means for leaving a stranded
gondola.
The canopy crane would yield ac?
cess to an unprecedented region of
canopy space. The longest reach avail?
able is approximately 80 meters (more
than 2.0 hectares of coverage). In a
50-meter-tall forest, the accessible
volume of a fixed crane of this size
would exceed a million cubic meters.
Cranes on tracked bases could poten?
tially access much more area.
Installation of the crane in the
October 1992 667
0.42
approximately 0.5 ha of forest. This
reach provides access to approximately
150,000 cubic meters ofcanopy space
in the 30-meter-tall stand. To trans?
port personnel and equipment, a small
enclosed gondola was built and at?
tached to the crane hook (Figure 2).
The gondola has windows on all sides
that open easily, with mesh screening
to protect against stinging insects. The
gondola position is controlled by a
crane operator, who follows direc?
tions given from the gondola by hand,
voice, or walkie-talkie.
The gondola provided a steady
working platform when aloft; anchor?
ing to adjacent branches was rarely
necessary. Movement between sam?
pling points was generally quite rapid
and smooth. Although abrupt stops
could cause the gondola to rock, the
overall sensation was secure. More?
over, the presence of the crane ap?
peared not to disturb animals: iguanas
sunning in the upper canopy were not
troubled when the gondola ap?
proached within 2 m (Anderson 1990).
Numerous observations and experi?
ments have been initiated during this
ongoing prototype study, including
studies of leaf physiology (photosyn?
thesis, transpiration, water relations
and wa ter use efficiency, and
photoinhibition), the physical organi?
zation of the upper canopy, competi?
tive interactions between lianas and
their hosts, and the structure of com?
munities of stingless bees and ants.
Here we describe measurements on
the structure of the upper canopy and
observations on the physiology of
overstory leaves.
Structure of the dry forest
upper canopy
Knowledge offorest structure has com?
monly been obtained from observa?
tions taken at the ground. However,
many of the details of the treetops are
hidden from view. The bathymetry of
the outer canopy was assessed by
measuring the depth from a fixed
height to the upper canopy surface.
We lowered a weighted measuring
tape from the gondola at 672 loca?
tions more or less evenly distributed
throughout the area covered by the jib
(mean interpoint distance 3.1 m). The
species of tree leaf and the coverage by
lianas or vines was noted at each
contact point.
ral distribution of important environ?
mental variables within a volume of
forest. The lateral and vertical change
of atmospheric parameters can be
finely resolved at any desired loca?
tion. The light environment of the
upper part of canopy gaps may be
assessed directly.
Long-term measurements of envi?
ronmental characteristics or material
fluxes (e.g., Baldocchi et al. 1988), in
other studies usually made with in?
struments fixed on towers or masts,
may be made wherever desired. Re?
peated sensor observations at prede?
termined positions within the canopy
could be scheduled to run automati?
cally: one could program the access
system for unattended sampling.
Prototype trial
In September 1990, the Smithsonian
Tropical Research Institute installed a
small rented crane in a forested subur?
ban park (the Parque Metropolitana)
on the outskirts of Panama City. The
stand is a diverse, dry Pacific forest,
approximately 75 years old, that sits
on the shoulder of a small hill. This
venture is the first to study the upper
forest canopy using a tower crane.
The prototype crane has an effec?
tive reach of almost 40 m, covering
A cooperative research facility
The access system provided by a tower
crane could benefit scientists interested
in all aspects of canopy science. Many
investigators could cooperate in the
use of the facility and the study of the
focal canopy. Sharing of major scien?
tific instruments is common, such as
telescopes in astronomy, particle ac?
celerators in high-energy physics, and
deep-water vessels in oceanography.
The initial cost of the tower crane
can range from $250,000 to $750,000,
depending on the configuration. After
installation, the operating expenses
are relatively low: electrical power,
the salary of a crane operator, and
some routine maintenance. The initial
cost of the canopy raft and dirigible is
roughly comparable (approximately
$550,000). However, the expense of
operating the raft is higher because it
requires not only fuel for the dirigible
but also the salaries of a large support
crew (as many as ten people during
intensive field studies).
I
8 10 12 14 16 18 20
0.22 L-....L----L-...>.......-l.--'--....J.-.....L....-J..-~.l.__L.___l..__'_____'
6
Environmental measurements in three
dimensions. The capacity to mount
short-term sampling at any point in
three-dimensional space allows accu?
rate assessment of the spatial-tempo-
0.32
Time of day
rectly or sampled by using insecti?
cides. Other canopy animals, such as
arboreal birds, herpetofauna, and
mammals, could be studied above the
forest from the vantage afforded by
the crane.
0.52
Figure 5. Leaf reflectance (fraction of
incident light that is reflected) at both
531 nm (closed symbols) and 505 nm
(open symbols) for leaves of Anacardium
(a), Didymopanax (b), and Luehea (c).
Reflectance was measured with a
spectroradiometer with integrating sphere
(Lieor, Lincoln, NE). The symbols for
the species are as in Figure 4. Patterns of
leaf reflectance for Anacardium and
Luehea are consistent with possible pro?
duction ~fprotective pigments (c.f. Bilger
et al. 1989, Gamon et al. 1990).
668 BioScience Vol. 42 No.9
Of the canopy trees, the leaves of
Enterolobium cyclocarpum (Corotu)
cover 29.8% of the projected surface
of the upper canopy in this stand;
Anacardium excelsum (Espave) cov?
ers nearly as much (28.5%). At least
25 other species constitute the outer
canopy, including species of Luehea
(Guacimo; 7.44%), Cecropia obtusi?
folia (Guarumo; 2.7%), and Didymop?
anax morototoni (Mangabe; 1.6%).
Additionally, leaves of lianas or vines
covered 35.6% of the upper canopy
surface and were occasionally so dense
it was difficult to distinguish the spe?
cies of the host tree.
The uneven surface of this canopy
had a mean depth of 13.9 m (standard
deviation 7.95 m ) relative to a plane
defined by the highest leaf. The outer
canopy has a very convoluted shape:
we estimate the total area of this
rumpled upper surface to be more
than 2.5 times the projected area. A
contour map of the upper surface
(Figure 3a) details that structure. Three
large crowns are apparent at the right
of the figure; another is to the left.
Gaps of various sizes are evident, in?
cluding several that do not penetrate
to ground level and may not be visible
to a ground-based observer.
Figure 3b gives a central section
through the canopy, illustrating some
of the hills and valleys in this complex
surface. The forest floor was rarely
glimpsed (only 3.7% of the points)
from the vantage of the outer canopy.
This initial 0 bservation is being con?
tinued with studies of the dynamics of
canopy structure at several spatial
scales and the interactions between
leaves of lianas and their host trees.
Physiological observations on
canopy leaves
Most research on forest plant eco?
physiology has been concentrated in
the understory and has emphasized
responses of plants to low and fluctu?
ating light. However, uppermost?
canopy leaves are subject to prolonged
exposure to full sun, which may dam?
age photosynthetic systems. This dam?
age, referred to as photoinhibition,
causes reduction in efficiency of pho?
tosynthetic utilization of light (Powles
1984). Water stress and high tempera?
ture can increase this sort of damage
(Powles 1984).
Photoinhibition can be analyzed
October 1992
through measurements of leaf fluores?
cence (Figure 4). Some plants may
avoid damage from intense solar ra?
diation by producing carotenoid pig?
ments that dissipate excess solar en?
ergy as heat, protecting photosynthetic
systems (Anderson and Osmond 1987,
Demmig and Bjorkman 1987).
Changes in reflection of light from
leaves can indicate altered levels of
these pigments. Changes in leaf fluo?
rescence can also suggest production
of protective pigments (Kitajima and
Butler 1975).
Photoinhibition in leaves of Ana?
cardium excelsum, Didymopanax mo?
rototoni, and Luehea seemannii was
measured directly in the upper canopy
(20-25 m above the ground). The
experimental canopy crane allowed
measurements of incident light in the
environment of the experimental
leaves, the direct assessment of foliar
water stress, and the sampling of se?
lected leaves for additional analyses.
As light increased in the canopy
over the day (Figure 4a), so did the
water stress (Figure 4b) and the extent
of photoinhibition (Figure 4c,d) for
all three species. There are differences
between species of canopy trees in
water stress and in the potential mecha?
nisms of protection from photoin?
hibition. Anacardium and perhaps
Luehea leaves appear to respond to
stress with production of accessory
pigments (Figure 5). Didymopanax
and Luehea appear to sustain some
damage to photosynthetic systems;
Luehea was the most water stressed
of the three. Regardless of the degree
of photoinhibition and extent of
photoprotection through pigment pro?
duction, all three species showed re?
covery from stress by early evening
(Figure 5b).
These preliminary results provided
the basis for much mor~detailed stud?
ies now in progress on the response of
canopy trees to high light stress. They
suggest that tropical forest trees shar?
ing the same environmental may use a
variety of responses to stress. Such
research is likely to reveal diverse
physiological mechanisms paralleling
the great species diversity of tropical
forests.
Conclusions
Lack of a flexible access system has
impeded fundamental research on the
upper forest canopy. A large tower
crane installed in a forest can provide
the centerpiece of such an access sys?
tem, allowing repeatable sampling in
three dimensions, unprecedented con?
trol for observations, and experimen?
tation within the canopy space of the
forest. The technology necessary to
implement this technique is readily
available and is being used success?
fully in a trial configuration in a tropi?
cal forest. Observations made from
the canopy crane suggest a wide diver?
sity of functional behaviors of over?
story leaves and a complex upper
canopy structure. The access system
reveals the canopy from an atmo?
spheric perspective, in contrast with
the common standpoint on the forest
floor.
Acknowledgments
We thank Jose Herrera, crane opera?
tor, for his skilled collaboration on
this project. The crane was funded by
the Smithsonian Tropical Research
Institute and by the Government of
Finland, through the United Nations
Environmental Program. Additional
support for the work reported here
came from the Smithsonian Environ?
mental Research Center. We thank
Jose Luis Machado, Jorge Illueca, and
Mikko Pyhala (UNEP) for their assis?
tance. Alan Smith thanks Michael C.
Smith for first suggesting the use of
cranes for forest research in 1986.
References cited
Allen, H. 1985. There'll be some changes in the
city when the cranes come soaring in. Smith?
sonian 16: 44-53.
American National Standards Institute (ANSI).
1984. Hammerhead Tower Cranes. ANSI/
ASME B30.3-1984. American Society of
Mechanical Engineers, New York.
Anderson, A. 1990. The view from the top.
Nature 347: 5.
Anderson, J. M., and C. B. Osmond. 1987.
Shade-sun response: compromises and
photoinhibition. In D. J. Kyle and C. J.
Arntzen, eds. Photoinhibition. Elsevier, New
York.
Baldocchi, D. D., B. B. Hicks, and T. P. Meyers.
1988. Measuring biosphere-atmosphere
exchange of biologically related gases with
micrometeorological methods. Ecology 69:
1331-1340.
Bates, M. 1960. The Forest and the Sea. Ran?
dom House, New York.
Bilger, W., O. Bjorkman, and S. Thayer. 1989.
Light-induced spectral absorbance changes
in relation to photosynthesis and the
epoxidation state of xanthophyll cycle com?
ponents in cotton leaves. Plant Physiol. 91:
669
At last we can see the earth as it
really is. This small. pale ball
floating in the vastness of space.
Clearly with limits. Vulnerable,
fragile.
For almost 100 years the Sierra
Club has been fighting to protect
the earth's fragile systems. We have
successfUlly lobbied for laws to
limit air and water pollution and to
regulate poisonous toxic
chemicals. We have won protection
for swamps and meadows, rivers
and mountains, deserts and
prairies ... those natural places
which permit the earth to heal and
renew itself. We have consistently
been an effective voice for a world
healthful for all its inhabitants.
The unique power of the Sierra
Club springs from our active grass
roots membership ... volunteers
who give freely of their time and
expertise. If you want to participate
in this work, or share in the
satisfaction of it through a sup?
porting membership, contact the
Sierra Club, 730 Polk Street, San
Francisco, CA 94109, (415)
776-2211.
SIERRA
? CLUB
542-551.
Boardman, N. K. 1977. Comparative physiol?
ogy of sun and shade plants. Annu. Rev.
Plant Physiol. 28: 355-377.
Carroll, G. L. 1980. Forest canopies: complex
and independent subsystems. Pages 87-107
in R. H. Waring, ed. Forests: Fresh Perspec?
tives from Ecosystem Analysis. Oregon State
University Press, Corvallis.
Crane Institute of America (CIA). 1989. OSHA
limits use of construction cranes and der?
ricks as personnel hoists. Indicator 2: 1-2.
Cregg, B. M., J. E. Halpin, P. M. Dougherty,
and R. O. Teskey. 1989. Comparativephysi?
ology and morphology of seedling and ma?
ture trees. Pages 111-118 in R. D. Noble,J.
L. Martin, and K. F. Jensen, eds. Air Pollu?
tion Effects on Vegetation, Including Forest
Ecosystems. Proceedings of the second US?
USSR symposium.
Demmig, B.,and O. Bjorkman. 1987. Compari?
son of the effects of excessive light on chlo?
rophyll fluorescence (77 K) and photon
yield of O2 evolution in leaves of higher
plants. Planta 171: 489-504.
Denison, W. c., D. M. Tracey, F. M. Rhoades,
and M. Sherwood. 1972. Direct, non-de?
structive measurements of biomass and
structure in living, old-growth Douglas-fir.
Pages 147-158 in J. F. Franklin, L. J.
Dempster, and R. H. Waring, eds. Research
on Coniferous Forest Ecosystems. Pacific
Northwest and Range Experiment Station,
Portland, OR.
Erwin, T. L. 1982. Tropical forests: their rich?
ness in Coleoptera and other arthropod
species. Coleopt. Bull. 35: 53-68.
___. 1983. Tropical forest canopies: the
last biotic frontier. Bull. Ent. Soc. Am. 29:
14-19.
Gamon, J. A., C. B. Field, W. Bilger, O.
Bjorkman, A. L. Fredeen, and J. Penuelas.
1990. Remote sensing of the xanthophyll
cycle and chlorophyll fluorescence in sun?
flower leaves and canopies. Oecologia 85:
1-7.
Halle, F. 1990. A raft atop the rain forest. Natl.
Geogr. 178: 128-138.
Jones, H. G. 1983. Plants and Microclimate.
Cambridge University Press, New York.
Kitajima, M., and W. L. Butler. 1975. Quench?
ing ofchlorophyll fluorescence and primary
photochemistry in chloroplasts by
dibromothymoquinone. Biochim. Biophys.
Acta 376: 105-115.
Kramer, P. J., and T. T. Kozlowski. 1979.
Physiology of Woody Plants. Academic
Press, San Diego, CA.
Lee, R. 1983. Forest Microclimatology. Co-
lumbia University Press, New York.
Leverentz,J. W., and P. G. Jarvis. 1979. Photo?
synthesis in Sitka spruce. VIII. The effect of
light flux density and direction on the rate of
net photosynthesis and the stomatal con?
ductance of needles. J. Appl. Ecol. 16:
919-923.
Mitchell, A. W. 1982. Reaching the Rainforest
Roof: A Handbook ofTechniques ofAccess
and Study in the Canopy. Leeds Philosophi?
cal and Literary Society, Leeds, UK.
Mull, I., and L. B. Liat. 1970. Vertical zonation
in a tropical rainforest in Malaysia. Science
196: 788-789.
Nadkarni, N. M. 1988. Tropical rainforest
ecology from a canopy perspective. Pages
189-208 in F. Alameda and C. M. Pringle,
eds. Tropical Rainforest: Diversity and
Conservation. California Academy of Sci?
ences, San Francisco.
Oke, T. R. 1987. Boundary Layer Climates.
Methuen, London.
Padgett, A., and B. Smith. 1987. On Rope. Na?
tional Speleological Society, Hunstville, AL.
Parker, G. G., J. P. O'Neill, and D. Higman.
1989. Vertical profile and canopy organiza?
tion of a mixed deciduous forest. Vegetatio
89: 1-12.
Pearcy, R. W. 1987. Photosynthetic gas ex?
change responses of Australian tropical for?
est trees in canopy, gap and understory
micro-environments. Funct. Ecol. 1:
169-178.
Perry, D. R. 1977. A method of access into the
crown of emergent and canopy trees.
Biotropica 10: 155-157.
__. 1986. Life Above the Jungle Floor.
Simon and Schuster, New York.
Perry, D. R., and J. D. Williams. 1981. The
rainforest canopy: a method providing total
access. Biotropica 13: 283-285.
Powles, S. B. 1984. Photoinhibition of photo?
synthesis induced by visible light. Annu.
Rev. Plant Physiol. 35: 15-44.
Schowalter, T. D., J. W. Webb, and D. A.
Crossley Jr. 1981. Community structure
and nutrient content of canopy arthropods
in clearcut and uncut forest ecosystems.
Ecology 62: 1010-1019.
Schreiber, U., U. Schliwa, and W. Bilger. 1986.
Continuous recording of photochemical and
non-photochemical chlorophyll fluorescence
quenching with a new type of modulation
fluorometer. Photosyn. Res. 10: 51-62.
Shapiro, H. J.,J. P. Shapiro, and L. K. Shapiro.
1980. Cranes and Derricks. McGraw-Hili,
New York.
Shapiro, L. K., and H. J. Shapiro. 1988. Con?
struction cranes. Sci. Am. 258: 72-79.
BioScience Vol. 42 No.9