A M . 7.OOLOCIST, 8:19-30
(1968).
Primary Production in Terrestrial Ecosystems
GEORGE M. WOODWELL AND ROBERT H. WHITTAKER
Biology Department, Brookhaven National Laboratory, Upton, New York 11973
SYNOPSIS "Primary production" refers to energy fixed by plants. The total amount of
energy fixed is usually called "gross production." A certain fraction of gross production is used
in respiration by the plants; the remainder appears as new biomass or "net primary production."
Thus for a single plant or a community of green plants:
Net Primary Production := Gross Production — Respiration (of Autotrophs)
Similar relationships occur in ecosystems except that the organic matter and respiration of
heterotrophs must be included. The increase in total organic matter is "net ecosystem production"; respiration is the total respiration of the green plants (autotrophs) and the animal community and decay organisms (heterotrophs). Gross production is of course identical to that of
the plant community. Thus for an ecosystem:
Net Ecosystem Production = Gross Production — Respiration (of Autotrophs and Heterotrophs)
Study of these attributes of terrestrial ecosystems is difficult, both because of the complex
interrelations of the processes involved, and because of the problems of working with systems as
large as whole forests. Three approaches are in use: (1) Harvest techniques measure weight
increase (and caloric equivalent and chemical composition) of net production. A refinement ot
this approach based on "dimension analysis" has made possible important recent advances in the
study of forests. Other techniques approach gross production and respiration through measurement of exchange of gases, especially CO2. These include: (2) Enclosure studies, involving measurements of CO2 exchange in plastic enclosures of parts of ecosystems and (3) Flux techniques
based on measurement of CO2 levels in the environment. All three approaches are being applied
to a forest at Brookhaven National Laboratory to determine the production equation of this
ecosystem.
Results to date have established general ranges of such parameters of ecosystems as total
biomass, total surface area of leaves and of stems and branches, rates of decay of organic matter
in soils, rates of production of roots, and rates of photosynthesis and respiration under different
environmental conditions. In the Brookhaven forest net primary production is 1124 dry g/m2/yr
(with an energy equivalent of 492 cal/cm2/yr), and gross production is about 2550 dry g/m2/yr;
the producers or green plants thus respire 56% of their gross production. Net ecosystem production is 422 dry g/m s /yr in this young forest. The ratio of total respiration to gross production is a convenient expression of successional status; a value of 0.82 for the Brookhaven forest
indicates that this is a late successional community, but not in steady-state or climax condition
(1.0). A leaf surface area of 3.8 m2 per m2 of ground surface intercepts sunlight energy, and the
ratio of net primary production to incident visible sunlight energy gives a net efficiency of primary production of 0.0088.
These and other functional characteristics of ecosystems are currently important topics of
research—involving understanding of communities as 'biological systems, evaluation of the potential of environments to support life and man's harvest; and understanding of the fundamental meaning and consequences of man's alteration, exploitation, and pollution of ecosystems.
"Primary production" is a general term
that refers to all or any part of the energy
fixed by plants. Thus, the grain harvested
from a wheat field is primary production,
So are the chaff, the stalks, and the roots,
as well as the weeds of the same field. Less
conspicuously a part of primary production is the energy used in respiration. This
is energy that does not appear as an increase in dry weight or size of plants, but
which has nonetheless been fixed in photosynthesis and has contributed to the main-
Research carried out at Brookhaven National
Labpratory under the auspices of the U. S. Atomic
Energy Commission.
Present address of the second author: Department of Population and Environmental Biology,
University of California, Irvine, California 92664.
tenance of the plants. Clearly, total primary production or "gross production" exc e e d s that which can be measured as an in, ,
»
„
,
aease
i n size o£ t h e
Plant o r n e t Production." T h e difference is the energy that is
19
20
GEORGE M. WOODWELL AND R. H. WHITTAKER
used in maintaining the plant—energy
"burned" in respiration. The relationship
applies to any green plant or any community of green plants and can be summarized
most simply by this formula:
NPP = GP — Rs (A)
(1)
Where NPP z= net primary production, GP
= gross production, and Rs (A) , respiration
of autotrophs. Units vary, but all are convertible more or less precisely to units of
energy.
Primary production is of special significance in ecology because it is the energy
fixed by plants that supports all life. This
fact makes primary production, both net
and gross, a basic parameter of plant communities in the same sense that net production or rates of growth and rates of metabolism (including photosynthesis and respiration) are basic parameters of organisms.
Production can also be considered on the
level of the ecosystem, if we consider the
ecosystem as a community of plants, animals, and saprobes, together with the physical environment. Community and environment are of course linked by energy flow
and by the cyclic turnover of materials between them. Considering the ecosystem as
a whole, energy of sunlight is captured and
bound into organic compounds by green
plants (autotrophs), and this energy is dispersed back to the environment by respiration of both the autotrophs and the heterotrophs (consumers or animals, and reducers
or saprobes—bacteria and fungi). Thus we
can define a net production for the ecosystem (NEP) as the same gross production
by plants minus the total community respiration by autotrophs and heterotrophs:
NEP = GP - [Rs(A) + Rs(H)]
increase in both the plant and animal communities. The curve expressing the weight
of organic matter against time may well be
the common S-shaped curve of growth; slow
in the first years, logarithmic in the middle
years of 10-75, and flattening as the system
stabilizes in maturity approaching climax
(Fig. 1). We use the term "climax" in the
restricted sense of a community approaching a steady state in relation to its own environment (cf. Whittaker, 1953). In the
climax, net production of the ecosystem
(the slope of the curve of Fig. 1) should
approach zero, implying little or no further increase in the mass of living and dead
organic matter in the ecosystem.
THE STUDY OF FOREST PRODUCTIVITY
Three generalized techniques (cf. Odum,
1959) are in use for measurement of characteristics of ecosystems directly related to
primary production. "Harvest techniques"
involve harvesting plants in plots or collecting selected plants for detailed measurements. The harvest techniques include refinements applicable to forests which we
call "dimension analysis" (Whittaker, 1962;
Whittaker, et ah, 1963; Whittaker and
Woodwell, 1967a). Two additional techniques are based on measurement of rates
of CO, exchange: (1) Enclosure studies
based on measurement of CO2 uptake and
release in chambers enclosing parts of the
community, and (2) CO2 llux techniques
involving measurement of the daily increase
and decrease of the level of CO2 in the environment of the ecosystem. Our work at
Brookhaven National Laboratory involves
the use of all three of these approaches in
a study of the function of an oak-pine
forest.
(2)
Honest techniques
hence,
One of the most effective approaches to
primary productivity on land is by systemNet ecosystem production varies system- atic harvest of the standing crop. The techatically along a succession. For instance, nique is especially well adapted to comalong the succession from field to forest in munities that are small in stature such as
the Eastern Deciduous Forest Zone, one ex- field, grasslands, and tundra. In these compects an increase in the standing crop of munities it is usually necessary to harvest
organic matter, living and dead, due to an the standing crop on small, replicated plots
NEP = NPP — Rs m )
(3)
21
PRIMARY PRODUCTION IN TERRESTRIAL COMMUNITIES
0
20
40
60
80
100
120
TIME (YEARS)
140
160
180
FIGURE 1
FIG. 1. Field-to-forest succession in the eastern United States (YVoodwell 1967). Some oscillations in the climax are assumed.
at several times during the season, plotting
the changes and inferring from this curve
the rates of production. An adjustment
must, of course, be made for losses to dead
organic matter and for consumption by
animals, and these corrections cannot always be easily measured. It is also necessary to separate organic matter produced
in previous years from the current production, sometimes not a simple matter. On
the whole, however, this direct measurement of production is the simplest and
most satisfactory technique where it can be
applied.
In forests the problems in using the harvest techniques are much more complex,
and it is impractical to use plots replicated
at different times during the seasons. Instead, procedures have been developed
which are based on detailed measurements
of a number of harvested trees. In plantations in Britain, Ovington and his colleagues have made measurements on "average trees" (Ovington, 1956, 1965; Baskerville, 1965). Average dry-weights and
other dimensions of these trees have then
been multiplied by the numbers of trees
per unit of ground surface area in the forest to obtain dry-weight biomass and other
dimensions of the forest. When forests of
different ages are compared in this way,
the increase in biomass with age can be
determined and net production estimated.
The approach is suited to plantations of
even-aged trees. It cannot be applied to
most natural forests, which include trees
of different ages and of a wide range of
sizes.
To study natural forests with populations of uneven-aged individuals, techniques of dimension-analysis have been developed which use logarithmic regressions
of critical dimensions on easily-measured
characteristics, such as diameter at breast
height for trees (Ogawa, et al., 1965; Whittaker and Woodwell, 1967a). The regressions are calculated from data drawn from
plants (harvested and measured in detail)
which span the variation in size that occurs
in the stands. The regressions are applied
to diameters at breast height of trees in
sample quadrats to compute biomass, production, surface, and volume dimensions
for the forest. The field technique is tedi-
22
GEORGE M. WOODWELL AND R. H. WHITTAKER
ous, involving the harvest of 15 or more
individuals of each species, with extraction of roots; sampling for number and dimensions of branches, leaves, and fruits;
and analysis of annual increments of stem
wood and bark. The calculations are numerous and are facilitated by access to a
computer and to assistance in programming. The results, however, provide a wide
range of information on forest structure as
well as yielding an estimate of net primary
production. They also provide a basis for
converting measurements of gaseous exchange in chambers to a gross production
value for the forest. The system of dimension analysis has been applied to only a
few forests as yet, but the Brookhaven computer program has been generalized to
accept data from any forest, and we hope
that additional forest communities will be
studied in this way through the International Biological Program.
Dimension analyses, building on the
techniques of earlier workers, have established not only improved methods but also
have yielded a body of information on the
ranges of important structural parameters
of forests (Whittaker, 1966). This research
has been simplified by recognizing that the
forms of woody plants, trees and shrubs
of a very wide range of sizes, follow the
same general design and can be related
allometrically, as indicated in Figure 2
(Whittaker and Woodwell, 1967a). It is
thus possible to relate complex characteristics of plants to basal diameters or other
easily measured features. From such studies
we have learned to expect the total standing crop (dry weight biomass) of aboveground plant parts in many mature temperate-zone forests of favorable environments to range from 30-50 kg/m2, that of
open forests and woodlands of less favorable environments to range from 5-20
kg/m 2 . In the Great Smoky Mountains,
which support some of the most highly developed temperate-zone forests in the
world, above-ground standing crops of undisturbed cove forests have been estimated
at 60 kg/m 2 (Whittaker, 1966), a value exceeding those reported even for the tropical
rainforests for which data are available
(Ovington, 1962; Miiller and Nielsen, 1965;
Ogawa, et al., 1965; Yoda, 1967). Biomass
of the coastal redwood forests, however,
should much exceed that of the cove forests.
Roots are a less accessible part of the
standing crop for which data are limited.
It has been assumed that the dry weight of
roots is about 20-35% of the above-ground
biomass, but root weight may be as low as
10% of above-ground weight in some mature forests (data of Ovington, 1962). The
Brookhaven forest, in contrast, is a small
forest adapted to fire, in which young
above-ground shoots grow from older roots
which have survived the fire. Here root
weight is 56% of above-ground biomass.
Net primary production is believed to be
in the range of 1200-1500 g/m2/yr for most
mature temperate-zone forests of favorable
environments, and in the range of 400-1000
g/m2/yr for many woodlands and smaller
forests (Whittaker, 1966). Production of the
Brookhaven forest—probably the most intensively studied of all—is slightly less than
that of forests of favorable environments—
1124 g/m2/yr. In large forests {e.g., the
cove forests) net primary production is 1/40
to 1/50 of biomass. The distinctive characteristics of the Brookhaven forest (as a
fire-adapted, small-tree community) appear
to be not so much its net production value
as its low biomass (9.9 kg/m2), its high
ratio of production to biomass (1/9), and
its high ratio of root weight to aboveground weight.
Given favorable conditions of temperature, moisture, and time on land, a forest
usually develops. Within the wide range of
net primary production values exhibited
by terrestrial communities (Odum, 1959;
Lieth, 1962; Newbould, 1963; Westlake,
1963), the forest production range of 12001500 g/m 2 /yr may be considered a norm
for climax communities of favorable environments. Certain communities exceed
this range, among them some marshes and
tropical grasslands (with production of
1500-3000 g/m 2 /yr and above), forests of
redwood flats, and probably some other
floodplain forests. The environments of
these communities are especially favorable
23
PRIMARY PRODUCTION IN TERRESTRIAL COMMUNITIES
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GROUND-LEVEL DIAMETER
BASAL DIAMETER IN CENTIMETERS
FIGURE 2
FIG. 2. Interspecies regressions for production of leaves and stem wood in shrubs and trees,
plotted against stem basal diameters. Both scales are logarithmic. Points represent individuals
of ten species ranging from low shrubs (Vacciniaceae at Brookhaven National Laboratory) to
medium-sized trees (Oak Ridge data of Whittaker, et al., 1963).
in some combination of high moisture
availability, warm temperature, and continuing replenishment of nutrients. Unstable communities of favorable environments—intensively cultivated sugar cane,
rice, and maize, and young successional
forests, particularly of warm-temperate and
tropical climates—may also exceed this
range. Net primary productions of shrublands and grasslands mostly range downward from values near 1000 g/m 2 /yr for
heath balds (Whittaker, 1963) and highgrass prairie (Kucera, et ah, 1967), to values
of a few hundred grams—probably 200-300
g/m 2 /yr in desert grassland [130 g/m2/yr
above-ground only in the Larrea community studied by Chew and Chew (1965)]
and sedge-tundra (Bliss, 1962). Productivities of shrublands and grasslands thus
overlap broadly with one another, and with
those of woodlands and small forests of
severe environments. Production of deserts
ranges down from 200-300 g/mB/yr as far
as one wishes to pursue the gradients of
diminishing productivity in response to decreasing moisture and/or temperature. De-
24
GEORGE M. WOODWELL AND R. H. WHITTAKER
spite the overlap of different community
types, 1000 and 200 g/m2/yr may serve as
convenient reference points separating the
high productivities of most forests; the intermediate productivities of most woodlands, shrublands, and grasslands; and the
low production of most deserts.
The distribution of net production
among the various parts of terrestrial communities is also becoming better known.
Production of leaves ranges upward to
about 700 g/m2/yr in the tropical rainforests for which data are available (Bray
and Gorham, 1964). In temperate zones it
is 300-400 g/m 2 /yr in many forests; e.g., in
the Brookhaven forest, it is 367 g/m2/yr
(with a chlorophyll content of 1.9 g/m2 of
ground surface). In the larger forests of
the Great Smoky Mountains leaf production is 360-420 g/m2/yr (Whittaker, 1966)
or about 30% of above-ground net primary
production and about 25% of total net
production. The largest fraction of net
production in forests of favorable environments is the woody growth of stems— 3540% in some of the forests of the Great
Smoky Mountains. Man is consequently
able to harvest as stem wood about onethird of the total net primary production
of stable forests of favorable environments,
a somewhat larger fraction in some unstable forests, and a smaller fraction (down
(o around 15%) in forests of unfavorable
environments. Man harvests as seeds 2535% of above-ground net primary production in some temperate-zone cereal crops
(Filzer, 1951).
There are few estimates of the fraction
of net primary production that reaches the
organic horizon of soils. Leaves, fruits,
flowers, dead wood, and bark fall to the
ground and accumulate with organic matter from roots to form an organic horizon
in the soil which may contain several thousand g/m2 (dry weight). In the Brookhaven forest the total contribution to this
horizon has been estimated as 342 g of
litter and 311 g of roots, making a total of
653 g/m 2 /yr or about 58% of the net primary production (Woodwell and Marples,
1968). Another 8% of the production enters the animal community dh-ectly through
insects feeding on leaves (Whittaker and
Woodwell, 1967£>). The remaining net production, neither harvested by the animals
nor contributed to the soil as dead organic
matter, is the annual increase in biomass,
or dry weight standing crop, of the plant
community in this young forest.
A further contribution from dimension
analysis has been a series of measurements
of the surface area of plant communities.
Surface area is important because of its
relationship to exchanges of gases, water,
and heat with the atmosphere. The surface
area of leaves is conveniently expressed as
a "leaf-area index"—the ratio to a unit
ground surface area of the surface areas of
the leaves above it. Surface areas are customarily calculated for one side only of
broad leaves, but for the whole surface of
the needles of conifers. It is usual for surface areas of leaves, thus calculated, to exceed ground surface areas by 3-6 times in
broadleaf forests and by 12-15 times in
dense conifer forests (Whittaker, 1966;
Whittaker and Woodwell, 1967c). This extensive leaf area is stratified, intercepting
light at various levels of forest foliage from
tree crowns to herbs. The extents of leaf
surface are presumably optimal for community photosynthesis, lying between low
areas which would fail to intercept light
potentially useful for production, and
higher surface areas which would waste increasing amounts of net production on
leaves which received inadequate light to
photosynthesize efficiently (Duncan, el al.,
1967).
The non-photosynthetic bark surface is
less. It usually includes 0.3-0.6 m2 of stem
surface plus 1.2-2.2 m2 of branch surface,
making a total of 1.5-2.8 m2 of bark
surface per m2 land surface (Whittaker
and Woodwell, 1967c). Branch surface increases more rapidly with increasing size of
plant than does either stem surface or leaf
area, a relationship that appears to set an
upper limit on the size of woody plants
based on the ratio of photosynthetic surface to respiring surface. Bark and leaf
surfaces together absorb and scatter sunlight, so that in dense forests less than 1%
PRIMARY PRODUCTION IN TERRESTRIAL COMMUNITIES
25
taken up, and released, makes for difficulty
in this approach to measurement of production.
Enclosures for measurement of metabolism
If one is dealing with plankton, or a
Quite a few attempts have been made to grassland, or an agricultural field, it is posmeasure community metabolism by enclos- sible to enclose a piece of the whole ecoing portions of ecosystems and measuring system, including all plant parts or all
CO2 exchange rates. The use of CO2 as above-ground plant parts, in the measurean expression of over-all metabolism is at- ment chamber. One does not so easily put
tractive, because it offers a rapid, short- forests into chambers, but H. T. Odum
term, direct measure of function. A net (1965) enclosed a section of a tropical rainreduction in the concentration of CO2 in forest in a giant plastic cylinder open at
the air implies net photosynthesis; an in- the top. Air was drawn downward from
crease implies an excess of respiration over the top of the cylinder and exhausted at
photosynthesis. However, the measure- the bottom. Turbulence at the top limited
ments need to be taken around the clock the usefulness of this system in measuring
and throughout the seasons to obtain val- respiration.
ues for annual production. As in dimenAn approach which seems more promission analysis the data to be summarized ing, but not free of difficulties, involves
and integrated are extensive. Current tech- measurement of rates of CO2 exchange in
niques at the Brookhaven forest include small chambers enclosing fractions of
automatic monitoring, recording, and sum- plants. Through such measurements the
marization of CO2 levels in air flowing into metabolism of plant parts can be related
and out of measurement chambers.
to environmental conditions, including
The technological problems associated temperature, light, season, and availability
with such measurements are formidable, of water. Then, knowing environmental
even with the blessings of automation. variation and the structure of the forest
Some of the most intractable problems from dimension analysis, it is possible to
arise from the effects of enclosures, which convert these measurements into values of
often involve an increase in temperature, production and respiration for the forest
a reduction in the amount and/or the ecosystem.
quality of light, and a variation of rates
Equipment used to measure rates of CO2
of air movement affecting rates of gas ex- exchange in the comparatively simple oakchange of leaves. Further difficulties arise pine forest of central Long Island is shown
in converting rates of photosynthesis and in Figure 3. It includes photosynthesis
respiration in a small chamber to those chambers, chambers for measurement of
in a whole ecosystem, and in determining CO2 exchange of stems of trees, and refrigthe relationship of photosynthesis and res- erated chambers for measurement of rates
piration. Respiration as related to tem- of CO2 exchange for the surface of the
perature in the dark is used to estimate, soil, ground cover, and shrubs, plus autofrom temperature, the rate of respiration matic recording and data-handling equipin the light, which is hidden by the more ment. From this system it is possible, as a
rapid photosynthetic binding of CO2. It first stage, to relate the metabolism of
is apparently true that the rate of respira- plant parts and ecosystem components to
tion differs at a given temperature in the environmental variables. We can, for inlight and in the dark, but there is presently stance, within certain limits, predict rates
no sound basis for making appropriate of respiration of tree stems from temperacompensations. Uptake of radioactively ture and season as indicated in Figure 4.
labeled CO2 by plants or plant parts in More elaborate considerations make possmall chambers can be used as a more di- sible estimations of community photosynrect approach to CO2 assimilation, but the thesis and respiration and some partitionfact that this CO2 is being turned over, ing of the latter into such components as
of incident sunlight reaches the surface of
the ground.
26
GEORGE M. WOODWEIX AND R. H.
MEASUREMENT
OF C 0 2
EXCHANGE
WHITTAKER
OF FOREST
LEAF
OTHER
INPUTS
CHAMBER
AIR
SAMPLING
SYSTEM
-^-
*) ' r
RECORDER
H2O
ANALYZER
TIMED
SWITCHES
EXHAUST
CO 2
GROUND-COVER
CHAMBER
COMPRESSED
AIR
SUPPLY
REFRIGERATION
COMPRESSOR
ANALYZER
EXHAUST
FIGURE 3
FIG. 3. Automatic sampling and data-recording system for monitoring metabolism of plants in
the field. Air is supplied at a constant rate to each of the three types of assimilation chambers
shown. The exhaust from these chambers is sampled, and its COa content is measured and compared with the COa content of the air supplied. Measurements from each chamber, together
with pertinent environmental data, are recorded on punched tape and compiled by computer.
(BNL Biology Department Annual Report, July 1, 1964, BNL 867 (AS-18), p. 134).
respiration by foliage, woody tissues, and
soil organisms.
CO2 flux
A simpler, and perhaps more promising,
way of measuring production is to measure
effects of the ecosystem on the concentration of CO2 in the ecosystem's environment,
whether water or air. This technique yields
data applicable to the ecosystem only, and
not always specifically referable to the plant
community. Several approaches have been
used in agricultural ecosystems with varying degrees of success; all depend on accurate measurement of atmospheric conditions as a basis for measurement of the flux
of CO2. Diurnal curves of the rise and fall
of CO2 in water are used as an approach
to production by plankton.
Few data are available from forests, principally because of the difficulty of obtaining reliable estimates of air movement
coupled with CO2 concentrations in communities as large and complex as forests.
The difficulties can be greatly simplified
when meteorological conditions are simplified, as they are during temperature inversions. Under these conditions there is
little or no vertical movement of air, and
the rate of increase in the concentration of
CO2 can be taken as a measure of the rate
of respiration of the forest (Fig. 5). Temperature inversions, however, occur commonly at night, restricting the measurement to respiration only.
We have recently exploited this possibility at Brookhaven by monitoring the CO2
content of air at various levels in the forest
PRIMARY PRODUCTION IN TERRESTRIAL COMMUNITIES
SEASONAL COURSE OF RESPIRATION
OF SCARLET OAK STEMS
268
272
276
2 BO
284
288
292
ABSOLUTE TEMPERATURE
FIGURE 4
FIG. 4. Regressions expressing rate of CO2 production of stems of scarlet oak trees as a function of
temperature. The first number for a curve-segment
is the month and the second the year of measurement. The differences in rates at a given temperature between winter and summer follow the pattern of progressively higher rates as spring progresses, lower rates through fall and winter.
from ground surface to well above the
canopy during 40 temperature inversions
throughout one year. Rates of CO2 production vary with temperature and with
season. Spring and summer rates were 2-3
times higher than winter rates at the same
temperature, reflecting the lower rates of
respiration characteristic of dormancy.
Mean monthly temperatures, averaged over
15 years, were used with the curves of respiration on temperature to compute annual respiration of the Brookhaven forest.
The forest was estimated to release approximately 3400 grams of CO2/m2/yr,
equivalent to 2104 grams of dry matter
(C6H10O5) (Woodwell and Dykeman, 1966).
This figure is the total respiration of the
ecosystem [Rs(A, -f Rs (H) ]; it includes the
respiration of the plant community and
roots, of the soil saprobe community, and
of the animal community.
While we cannot yet partition respiration among the various segments of the
ecosystem in a really satisfactory way, we
can estimate gross production of the Brookhaven forest from the data available now.
We can also complete the production equa-
27
tions, albeit tentatively, for both the ecosystem and the plant community on the
basis of certain assumptions. The first assumption is that the organic matter in the
soil is at equilibrium, an assumption which
appears reasonable in the light of earlier
studies (Woodwell and Marples, 1967). If
we also assume that the animal community
is at equilibrium, with its metabolism supported by 8% of the leaf production or
about 29 g/m2/yr (Whittaker and Woodwell, 19676), we then have the figures
needed to approximate the net production
equation for the ecosystem. The net production of the ecosystem would be the net
primary production for the plant community (1124 grams), minus litter-fall (342
TEMPERATURE PROFILE DURING INVERSION
JUNE 11-12, 1965
' I ' I '
360 -
280 -
1300
1700
FIGURE S
FIG. 5. Concentrations of CO3 at four heights in
the Brookhaven forest (night of 11-12 June 1965)
together with a plot for the same heights of the
temperature inversion with height (at 0300 hours).
By integrating CO2 content through time for a collumn of air in the inversion the rate of CO,, release,
and hence, the rate of respiration by the forest, can
be computed.
28
GEORGE M. WOODWELL AND R. H. WHITTAKER
grams), annual loss of roots (estimated by
Woodwell and Marples as 311 grams), and
consumption by herbivores (29 grams).
The remainder is the net ecosystem production: 442 grams. Completing the equation provides an estimate of gross production, GP:
NEP = GP - [Rs(A) + Rs (H) ] (2)
442 = 2546 — 2104
From this the equation for net primary
production can be completed, providing
an estimate of the rate of respiration of the
plant community:
NPP = GP—Rs ( A )
1124 = 2546— 1422
(1)
Units are grams of dry matter per square
meter per year.
While these numbers are tentative, several interesting relationships emerge. First
is the relation between the two major
routes by which the energy of net primary
production is used and dissipated back to
environment—the herbivorous animals (and
their predators), and the detritus route of
the soil saprobes and animals. In terrestrial
communities, and especially in forests,
utilization of energy by herbivores is small
relative to that by soil organisms (apparently less than 10% in the Brookhaven
forest, although extent of consumption of
live roots by animals is unknown). Second
is the relation of autotrophic respiration,
Rs(A1, to gross production. We estimate
that in the Brookhaven forest 56% of gross
production is respired by the plants themselves. Autotrophic respiration of 50-60%
of gross productivity may be typical of
cool-temperate forests approaching maturity (Moller, et ah, 1954; Muller, 1962).
The relative amount of respiration is probably temperature-dependent, decreasing toward cooler climates and increasing to 7276% in the warm temperate and tropical
forests studied by Muller and Nielsen
(1965) and Kira, el al. (1967). A third relationship of interest is that between heterotrophic respiration, Rs, n ,, and net primary
production, NPP.
NEP — NPP — Rs (H)
442= 1124 — 682
(3)
Two limiting cases are approached by
this relation: (1) Heterotrophic harvest
and respiration are very small, and net ecosystem production is nearly equal to net
primary production, or (2) Heterotrophic
respiration equals net primary production,
and net ecosystem production is zero. The
first of these can be approached for a short
time very early in successions (e.g., during
the first growing season in an old-field herb
community). The second is approached in
climax communities, if the climax condition is defined as a steady-state in which
input of energy in photosynthesis and output of energy in respiration are equal.
Thus the ratio, NEP/NPP, is an expression of successional status which decreases
from values near 1.0 to values near zero in
the course of succession. In the Brookhaven
forest the ratio is 0.39, indicating that the
forest is not stabilized but in a late stage
of succession. The ratio of total respiration of the ecosystem to gross production
may be a more convenient expression of
successional status, approaching 1.0 in the
climax. This ratio is 2104/2546 = 0.82 in
the Brookhaven forest.
In the climax condition there may be no
further increase of living and dead organic
matter in the ecosystem; community biomass and soil organic mass are in steady
state. However, perfect balancing of gross
production and total respiration of the
ecosystem is probably not common in nature. The intense activities of man convert
many communities to successional status.
It is also likely that in many climax communities, as mature and stable as they will
become in relation to their present environments, this balance is imperfect in a
small but significant way. While the living organic mass of the community may be
in a steady state, there may be a small continuing accumulation of dead organic matter reflecting a small deficiency of ecosystem respiration relative to gross productivity. A difference of 1% could not be detected with present techniques, but over the
course of a million years this difference
PRIMARY PRODUCTION IN TERRESTRIAL COMMUNITIES
would result in a large accumulation of
fixed carbon. The fossil fuels, coal and
petroleum, represent such a gradual accumulation through long periods of past geologic history.
A final relationship of interest is that of
productivity to incident solar energy. The
annual input of solar energy at Brookhaven
within the visible range of 4000-7000 A is
56,300 cal/cm2/yr. Net primary production
is 492 cal/cm2/yr; gross productivity is
1090 cal/cm2/yr. The efficiency of gross
productivity over incident sunlight is
1.93%, and the corresponding efficiency
for net production is 0.88%. The latter
value is similar to those of 0.96-1.15 for
Danish beech plantations (Moller, et ah,
1954; Hellmers, 1964) and 0.8 for a tropical forest (Miiller and Nielsen, 1965).
There are some communities of higher production and efficiency, but for most ecosystems—terrestrial and aquatic—the annual
efficiency of conversion of solar energy in
the visible spectrum into net production
potentially available for harvest by man or
other animals ranges from 1% downward.
A reasonable estimate of the efficiency of
man's harvest (in the form of wood) of the
incident solar energy of the forest combines a net primary production efficiency
of 0.9% with harvest as wood of one-third
of that net production. The resulting efficiency is 0.3%. The efficiency of harvest
from some fast-growing forests should exceed this, but that from many slow-growing forests should fall below it.
CONCLUSION
We have sought to represent one aspect
of the current interests of ecologists in forest production. It is only fair to observe
that this is only a part of their interest in
only one area of the field—that of studies
of the structure and function of ecosystems.
Thus, our research on productivity is to be
viewed in a broader context of ecosystem
problems which include the biological and
environmental controls of productivity, the
meaning of the structural design of communities, the rates and routes of movement
of nutrient elements and of organic materials through the community and environ-
29
ment, chemical influences and biochemical
antagonisms between members of communities, population processes and stabilization in communities, and the relative importances of species in relation to niches
and diversities of species in the community.
We like to think there is challenge in such
research, and a concern with its results
paralleling that of research on the individual organism and cellular biochemistry.
We have observed that man is wholly
dependent on biological productivity for
his food, and largely dependent on accumulated net production of ecosystems
for the energy of industrial societies. It is
a grimmer truth that man is pressing very
hard on his environment in ways related to
both of these facts. On the one hand man
demands the production of greater food resources for increasing populations, and on
the other he degrades and poisons the environment by his expanding technology.
We believe that if human populations cannot be controlled, mankind is headed toward massive tragedy—probably first in the
form of famine and instability in the undeveloped countries (Brown, 1967). The
concerns of ecologists are relevant to these
problems, first because of the relation of
the food problem to productivity, and second because of problems of community
degradation and the transfer and concentration of pollutants. We believe that no
reasonable, non-catastrophic solutions to
these problems exist which do not include
unwelcome restraints upon the growth of
populations and upon industry and technology. If, however, the political bases of
such restraints develop, ecologists will contribute to the solution—a more understanding management of the world's ecosystems
for human welfare.
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