FOREST OF DOUGLAS FIRS in western Oregon dwarfs a fiel d investigator suspended from a spar

100 feet above the ground (at

lower left of photograph, between first and second tree trunk). Use closeup studies.

of modified rock-climbing techniques allows such

74

© 1973 SCIENTIFIC AMERICAN, INC

LIFE IN TALL TREES The high forest canopy consists of more than branches and leaves. Entire communities of other plants and animals dwell

w

ith in

this

unIque ecosystem, helping to provide the trees with nitrogen

by William C. Denison

reetop in a forest, like a mountain

ground belaying the climber on the tree

In addition to these devices for climb­

peak or a deep canyon, is a re­

with a safety rope. The team members

ing and descending along the tree trunk,

mote world that is plainly visible

use small radio transceivers for commu­

we have developed a

but not easy to explore at first hand. Yet

nication between those on the ground

which we call the "spar," for working in

A

12-foot boom,

there is a strong urge to visit such private

and climbers far above in the canopy. In

areas away from the trunk (including the

domains. They challenge both our curi­

the first ascent of a tree the climber car­

ends of branches). The spar is attached

osity and our skill. With respect to tall

ries a hammer and lag screws (which

to the trunk with a special hinge and is

trees there might even be an echo of a

hold better in the bark and trunk of a

suspended at its outboard end by a pair

past when our ancestors were at home in

tree than the pitons, or spikes, used in

treetops.

rock-climbing) to assist the climb. The

of ropes [see bottom illustration on next page]. The occupant sits on a sling seat suspended from the spar and can move

It is perhaps a little surprising, there­

climber drives a lag screw into the tree

fore, that although investigators have

trunk, fastening a hanger as far above

along the spar by alternately standing in

been drawn to the direct exploration of

his head as he can reach, attaches a pair

stirrups while sliding the seat along and

mountain heights and sea floors, relative­

of stirrup-loop ladders to the hanger,

then moving the stirrups while sitting in

ly little study has been given to the tree­

climbs the ladder stirrups to install the

the seat. By pulling one or the other of

top world. It offers intriguing questions

next hanger and proceeds in this way

the two ropes supporting the outboard

to a biologist. The forest canopy is a dis­

to the top of the tree [see top illustration on next page]. The initial climb takes

end of the spar the occupant can move

tinctive habitat, providing its own spe­ cial conditions of moisture, light, tem­

several hours and is hard work, even for

180 degrees. Thus we can work effec­

perature and other qualities. What kinds

a person in excellent physical condition.

tively anywhere in the canopy within

of plant and animal life does it support?

At the top of the ascent the climber

Does it harbor an integrated community?

attaches to the trunk a rope that will be

What roles does it play in maintaining

used for subsequent ascents and a large

the forest ecosystem as a whole?

pulley for the belaying rope that the

horizontally around through an arc of

12 feet of the trunk.

}\fter establishing access to the treetop,

we carried out a systematic survey

My colleague Lawrence H. Pike and I

climbers use for additional support. To

not only of .the wood and foliage of the

at Oregon State University undertook an

ascend the climbing rope the climber

tree but also of the population of epi­

explora tion of treetop life as part of a

holds in each hand a Swiss-made Jumar

phytes: the many plants that grow on

U.S. study of ecosystems in western co­

Ascender, which is a form of clamp that

other plants but are not parasitic. We

niferous forests under the aegis of the

grips the rope tightly when weight is put

recorded the results of the survey on

International Biological Program.

We

on it and loosens when weight is re­

punch cards, a procedure that enabled

enlisted a group of physically fit univer­

moved. Slings of nylon webbing that

us to store the data in a computer and

sity and high school students in our

serve as stirrups for the climber's feet are

to undertake computer analyses. The

climbing party and set out for the top of

suspended

and the

data were first used in preparing a "map"

a 450-year-old stand of Douglas fir in the

climber goes up the rope by taking the

of the tree, showing the location and

H. J. Andrews Experimental Forest in

weight off one foot and stirrup, sliding

amount of living and dead matter on the

western Oregon, where the trees soar to

the jumar up the rope with one hand, re­

tree trunk and on each branch system.

from the jumars

a height of 200 feet or more and the

storing the weight on the stirrup to tight­

The survey enabled us to estimate the

lowest branch is usually at least 60 feet

en the jumar and then repeating the

biomass, or weight of living matter, of

above the ground. Since the project pre­

process with the other hand and foot.

each branch system and of the tree as

sented a problem roughly corresponding

With practice one can climb the rope in

a whole. These estimates were compiled

to scaling the rock face of a mountain­

this way with little more effort than it

by considering separately the amount of

side, we borrowed ideas from rock­

takes to climb an ordinary ladder. One

foliage (obtained by observing what per­

climbing

can also rest one's feet from standing in

cent of the horizontal area within a

the stirrups or free both hands for work

branch system was covered by foliage),

at any point by sitting back on a seatlike

the amount of wood (in the branch and

sling that is attached to one jumar.

twigs) and the amount of epiphytic plant

to

develop

techniques

for

climbing the tall trees. As in mountaineering, our climbers work in teams, with a teammate on the

75

© 1973 SCIENTIFIC AMERICAN, INC

life inhabiting each branch. We checked our estimates of the biomass by making detailed measurements of typical sys­ tems. We found, for example, that in a Douglas fir 185 feet tall the foliage had an estimated total weight of 187 pounds and the epiphytic lichens and mosses growing on the tree weighed 38 pounds. It was already well known, of course, that trees are inhabited by a great va­ riety of plant and animal life. The plants that grow on tree trunks, branches and foliage include bacteria, algae, fungi, mosses, lichens and ferns. (In warm cli­ mates even advanced species of plants such as orchids are epiphytes.) The ani­ mals that live out their lives in trees show a similar range in size and diversity: pro­ tozoans, nematodes, higher invertebrates such as arthropods and mollusks, aud various vertebrates (including primates in warm climates). As botanists, we fo­ cused our attention on the plant life in our exploration of treetops.

CLIMBING STIRRUPS

he forest canopy proved to be a sub-

T system

of considerable complexity.

The variety of its plant life reflects a variety in the habitats within the cano­ py. These habitats differ widely in the ASCENT OF TREE is accomplished by means of the mountaineering methods designed for

amount of available moisture and light,

vertical rock faces, except that lag screws are used instead of pitons to secure hangers. The

in temperature and in the age and sur­

first climber up the tree rigs the climbing and belay ropes used by subsequent climbers.

face texture of the supporting structure (twig or branch). As a twig grows and ages into a branch, its surface harbors a succession of different organisms, begin­ ning with pioneering lichens and pro­

SUSPENSION ROPES

gressing through a series of complex communities. Diane Nielsen and Diane M. Tracy, two students who started with ns as undergraduates and, pioneering our climbing techniques, were the first to ascend to the canopy, found that the diversity of habitats and the- number of species of epiphytic plants increased the higher they went up the tree. Each habi­ tat has a characteristic flora, with cer­ tain species predominating. For exam­ ple, most of the large fir trees in the forest where we have been working are not strictly vertical but lean a little to one side. As a result their trunks have an up­ per side and a lower side, and the upper side is moister than the lower because rainwater streams down that side. The upper and lower sides therefore differ in the prevailing lichen species growing on them. On the branches aloft in the can­ opy certain large foliose (flat and leaf­ like) lichens predominate on the upper side of the branch, and the lower side

BELAY ROPE

tends to favor the lower plants known as liverworts.

HORIZONTAL SPAR provides a movable base among the tree branches. The investigator is supported by a sling seat hung from the spar and can study a 12·£00t length of branch.

76

© 1973 SCIENTIFIC AMERICAN, INC

Habitat by habitat, we are cataloguing the characteristic epiphytic communities

and obtaining a census of the many plant varieties. Pike has already noted 121 dif­

FEET

ferent species of lichens. The number

180

will undoubtedly increase as we explOre

I

\ I \

-

-'

I

"

I I

more trees.

/""---'\ \ ,

{

\

,

'

'

I

\ e find, then, that the forest canopy V is an active, well-populated system (comparable in many respects to the for­

160

---- � ,-, ,

est floor), and we must suppose that the epiphytcs living in the canopy contribute substantially to the nourishmcnt and vi­

I

ability of the forest as a whole. They un­

"

doubtedly take up water, minerals and other substances from the atmosphere.

f I I

/

/

{

"

\

\ \ I \ \

140

Through photosynthesis and other proc­ esses they produce nutrients that are re­ leased to thc forest's animal dwellers and the trees. \,y'e have been particularly in­ terested in investigating the epiphytic plants' role in capturing nitrogen for the

120

forest's needs. In an old-growth Douglas-fir forest the supply of available nitrogen is not abundant. Relatively little of this essen­ tial element is brought in directly from the atmosphere by rainfall, and the for­ est floor is largely barren of nitrogen-fix­

100

ing plant life. It seemed likely, therefore, that lichens growing in the canopy, some of which were known to fix nitrogen, might be important contributors to the forest's nitrogen economy. Our notice was attracted particularly

80

to one lichen, Lobaria oregana, that is by far the most abundant species in the treetops. The forest floor is littered with fallen pieces of its green, lettucelike thal­ lus (plant body). It has been established that Lobaria fixes nitrogen from the at­

60

mosphere, presumably through the agen­ cy of a blue-green alga that is embodied in granular packets within the thallus. In order to evaluate the importance of this lichen in the forest's overall econ­ omy, we worked out estimates of its

40

probable annual contribution to the ni­ trogen supply.

EAST >

Sterling A. Russell of Oregon State University,

who has investigated Lo­

baria's nitrogen-fixing productivity, esti­ mated that the lichen fixes nitrogen at a

20

maximum rate of about 50 nanomoles (billionths of a unit of molecular weight) per hour per gram of the lichen's fresh weight, which is equivalent to 200 nano­ moles per gram (dry weight). We esti­ mate that in our Douglas-fir forest the

o

amount of Lobaria growing on the trees is between 350 and 450 pounds per acre. If we took the 450-pound figure and as­ sumed that Lobaria fixes nitrogen at the

SCHEMATIC MAP of a Douglas fir more than

180 feet high is based on climbers' studies.

The fir had a forked top; the shorter fork, on the east side of the tree, is offset for clarity. The map shows only branches that project east or west and that are more than

1.6 inches in

maximum rate throughout the year, we

diameter. Solid lines indicate living branches, broken lines dead ones. The enclosed area

would arrive at a figure of slightly more

accompanying a branch is proportional to the weight of its foliage. Where branch is less

than 10 pounds per acre as this lichen's

than minimum diameter only the foliage area is shown

(color). Treetops were not mapped.

77

© 1973 SCIENTIFIC AMERICAN, INC

annual production of fixed nitrogen. It

SOLUBLE NITROGEN IN RAINFALL

is unreasonable to suppose, however, that the lichen sustains the maximum rate the year round.

ATMOSPHERIC NITROGEN FIXED BY BACTERIA AND LICHENS

We decided to calculate a lower limit: the minimum amount of nitrogen Lo­

baria must fix to support its own growth. Its thallus, we estimate, adds about a fourth (in dry weight) in new growth each year. Taking the lower estimate of the amount of Lobaria in the forest (that is, 350 pounds per acre), the annual new

NITROGEN TAKEN UP BY FOLIAGE AND EPIPHYTES AFTER RELEASE BY FIXERS

growth would amount to 90 pounds per acre. Since the nitrogen content of the thallus is 2. 1 percent of its total dry weight, the 90 pounds of yearly new growth per acre would contain roughly 1. 8 pounds of nitrogen per acre. Thus we conclude that Lobaria ore­ gana contributes from 1. 8 to 10 pounds of nitrogen per acre per year to the for­ est-certainly less than 10 pounds but probably substantially more than

NITROGEN ABSORBED BY EATERS OF EPIPHYTES

1. 8

pounds. The nitrogen trapped by the lichen is released in several ways for the eventual nourishment of the trees. The chief route of the contribution is through Lobaria's fall and decay. The annual fall· of dislodged Lobaria from the canopy to the forest Boor amounts to roughly 80 pounds per acre; most of this fall is peeled off by rain, snow and ice during the winter. Decomposing on the ground, the fallen thalli release about 1.8 pounds of nitrogen per acre per year to the roots of the trees and other plants.

A

nimals feeding on the lichens in the tree provide a second means of con­

veyance of the nitrogen. We have ob­ served great numbers of invertebrates,

NITROGEN RELEASED BY

including nematodes, mites and insects,

LITTER FALL AND DECAY

eating away at Lobaria thalli. Certain vertebrates, such as the rodent called the red tree vole, supplement their diet with this lichen, among other plants. The animals consuming the lichen are

NITROGEN PATHWAYS in the forest can­ opy are indicated schematically. First (top) some nitrogen (less than one pound per acre annually) is washed into the canopy by rain. Far more nitrogen enters the system through the action of nitrogen.fixing bacteria on the fir needles and of blue·green algae in some lichens. The nitrogen.fixers pass the vital element along three pathways. Rainwater leaches some nitrogen from the living and dead tissue of the fixers; both the tree and its epiphytes absorh the rainwater. Fixed nitrogen follows a second path when herbiv­ orous animals feed on epiphytes and then excrete. Finally, litter from the death and decay of both the epiphytes and the herbi­ vores adds further nitrogen to the ecosystem.

78

© 1973 SCIENTIFIC AMERICAN, INC

BLUE-GREEN ALGAE

GREEN ALGAE

Lobaria oregana, is shown in cross section. Like all lichens, Lobaria is a symbiotic association of a fun. gus (gray areas) and two algae (colored areas). One of the Chlo·

Lobaria thallus (light color). Populations of a blue· green alga, Nostoc, are also present (solid color) in bulges called cephalodi. ums. Nosloc, one of the Cyanophyta, fixes atmospheric nitrogen at a significant rate. As a result Lobaria contributes from 1.8 to 10

rophyta, or green algae, is the principal symbiont; it lives in the

pounds of nitrogen per acre to the fir·forest ecosystem annually.

PRINCIPAL NITROGEN·FIXER among the epiphytic lichens in the forest.canopy community,

fed on in turn by predators, and the ni­

the most prolific). Probably the total is

trogen is transferred to the forest soil

less than the 44 pounds per acre that is

The question of the differences in

through the predators' excreta.

sometimes applied in the form of fer·

microclimates is particularly interesting. The climate at the top of the canopy is

are related to various tree environments.

Lobaria undoubtedly provides some

tilizer to promote growth in Douglas-fir

transportable nitrogen even from its po·

forests. Our survey indicates, however,

obviously very different from the climate

sition high in the treetops. At its maxi­

that in an old-growth stand of Douglas

low in the tree, so that an epiphyte high in the canopy is subjected to greater in·

mum rate of nitrogen. fixation it prob­

fir the nitrogen-fixing plant life in the

ably traps more than it uses for new

canopy can serve as the main pathway

tensities of light and sharper fluctuations

growth, and this soluble excess of nitro­

for the introduction of new nitrogen, an

of temperature and humidity than one

gen may be leached from the thallus by

element required by all forest life-forms,

on a low branch. In the top of the tree

rain and washed down the tree. The

small or large, plant or animaL Epiphytic lichens such as

lichens dry out within a few minutes

Lobaria

after a rain, whereas those on the lowest

thalli decaying in the canopy. Rainwater

seem to be particularly susceptible to

branches may stay damp for months in

flowing down over the branches and

poisoning by pollutants in the atmo·

the season of intermittent rains. As we

leaves loses nitrogen to mosses and other

sphere. In western Oregon these lichens

have seen, there are marked climatic dif­

epiphytes that do not fix nitrogen, and

are disappearing from forests as urbani­

ferences even between the upper and the

perhaps some nitrogen is fed to the tree

zation and industrialization of the land

lower sides of a branch, with resulting

itself through its foliage. In any event,

advance toward the woodlands. Here is

differences in the epiphytic communities

the epiphytes that take up nitrogen from

further evidence that the atmospheric

of the two sides. Climatic differences

the rainwater eventually release it to the

pollution that too often accompanies in­

within the tree also affect the growth of

ecosystem through their decay. We esti­

creasing density of population is inimical

the branches themselves; for example,

mate that in our old· growth Douglas for·

to nearby forested lands.

branches in the deep shade have a tend­

O

meteorological instruments at various

rain also picks up nitrogen from dead

est the contribution of nitrogen to the

y

soil by epiph tes that do not fix the ele­ ment amounts to roughly 5.4 pounds per acre per year.

ency to prune themselves. By installing ur climbing around in the treetops so far can only be considered an early

points in the tree we expect to obtain

stage in the exploration of the microen­

more specific information relating the

We do not yet have an estimate of the

vironment of trees. We hope to learn a

growth of epiphytic communities and in­

aggregate amount of llitrogen supplied

great deal more about the communities

dividual branches to factors in their im­

to the forest by all the nitrogen-fixing

of epiphytes living in the trees of our

mediate environment.

epiphytes (of which Lobaria is certainly

Douglas-fir forest and about how they

The trees to which we have given the

79

© 1973 SCIENTIFIC AMERICAN, INC

FOUR LICHENS commonly found on tree trunks and branches in Lobaria (top left), a highly rami· fied species, Sphaerophorus globoslls (top right), a species with fir forests are the nitrogen.fixing

Hypogymnia enteromorph" I bottom left), a buttonlike .species, Ochrolechia oregona I bottom right).

cuplike fruiting bodies, and

Lichens are easily poisoned by urban and industrial air pollutants.

most study are the Douglas fir and the

most of their foliage concentrated at the

sponds to the late successional type. The

western hemlock. Viewed from a dis·

end of the branch.

old branches of a large tree show a pat­

tance, both have about the same general

The Adaptit;e Geometry of

tern of developmental response to envi­

tively short branches extended laterally

I

Henry S. Horn has observed

acteristic of its species and therefore

from a massive central trunk. Closer up,

that the form of a tree can often be re­

probably is genetically influenced. The

shape: a slender cone consisting of rela·

n his book

Trees

ronmental factors that seems to be char­

however, they are seen to have substan·

lated to the tree's ability to take hold and

further exploration of the structure of big

tially different builds.

thrive under specific conditions.

One

trees should provide useful information

An old-growth

Douglas fir has relatively few branches

shape, for example, is characteristic of

about the adaptive capacities of individ­

and they are widely spaced, sometimes

trees that spring up as the early settlers

ual species.

with gaps of up to 70 feet between

in place of a forest that has been felled

I hope this brief description of our

branches on the shaded side of the trunk.

by fire or clearing; other shapes allow

explorations in the treetops will en­ courage other investigators to extend the

Each branch is a complex system; often

the trees to invade an established forest

it is fan-shaped with a wide spread, and

at later successional stages. Examining

exploration to other forests and other

in many cases the system is a group of

our Douglas firs and hemlocks in this

types of trees. The investigation of living

two or more young branches that have

light, we find that the widely spaced,

things in the forest canopy probably will

grown out from places where the original

fan-shaped branches of the Douglas fir

also prove to be as attractive and re­

main branch broke off. In contrast, an

(forming what Horn describes as a "mul­

warding to zoologists as it has been to us

old-growth western hemlock has more

tilayer") fit his description of trees of the

botanists. The study of small arboreal

branches per length of trunk even in

early successional type, and the western

animals at home in the treetops should

heavy shade; the branches are more

herrilock, with its evenly distributed foli­

provide information that cannot be ob­

evenly spaced up the tree, and they have

age

tained by observing them in captivity.

(forming

a

"monolayer"),

80

© 1973 SCIENTIFIC AMERICAN, INC

corre-