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OCR for page 345
Ecological Effects of
Forest Clearcutting
Management of the harvesting of renewable biological resources re-
quires knowledge of the stocks of those resources and of the rates at which
they recover after harvesting. Gaining this knowledge is especially difficult
when the intervals between cropping are long and rates of recovery are
low, as they are in the case of timber harvesting. Not only are trees long-
lived, but differences between sites make it difficult to extrapolate results
from one location to another. This case study reviews the ways in which
these problems have been approached and assesses the current state of
prediction of the amounts and significance of nutrient losses from forests
after clearcutting.
345
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Case Study
CARL F. JORDAN, Institute of Ecology, University of Georgia,
Athens, Georgia
INTRODUCTION
A major goal of the forestry profession is to sustain the production of
wood in forests. In the early days of the profession, much attention cen-
tered on prevention of fire and outbreaks of insect pests as means of
sustaining high productivity. These factors still cause extensive losses of
trees, but now that many forests are managed as agricultural crops, other
problems have increased in importance. One of the common intensive
management techniques in modern forestry is clearcut logging. Clearcut
logging is often more economical than other forest management tech-
niques, but it has the potential to create serious environmental problems
out of effects that would be negligible if less intensive techniques were
used. Examples include the elimination of habitat of endangered species
and increases in soil erosion. This case study reviews research that has
evaluated the impact of forest clearcutting on nutrient stocks and site
productivity.
The first studies of nutrient loss due to clearcutting were concerned
primarily with only parts of the nutrient budget of an ecosystem, for
example, leaching losses. Improved understanding of ecosystem func-
tioning made it apparent that useful interpretation of nutrient losses due
to clearcutting required an analysis of both the nutrient stocks in the
ecosystem and the nutrient inputs and losses. To evaluate the effect of
nutrient loss during clearcutting on future productivity of a site, it is
necessary to know the quantities not only of nutrient losses due to leaching
and erosion after clearcutting, but also of losses due to tree removal,
volatilization, and fixation in the soil. Also important are the rate of
nutrient replacement through atmospheric input, rock weathering, and
fertilization and the size of nutrient stocks in the ecosystem that can buffer
short-term fluctuations in input and output.
A nutrient-budget approach to forest management is analogous to budget
management in business. Successful business management demands an
understanding of the entire operating budget, including revenues, expen-
ditures, and financial balance. This need in business management is so
obvious as to be taken for granted. Yet, in forest management, it has
become clear only recently that successful management for sustained yield
requires knowledge of nutrient inputs, nutrient losses, and nutrient stocks.
346
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ECOLOGICAL EFFECTS OF FOREST CLEARCUITING
347
These quantities are often difficult to measure, but complete budgets must
be constructed, if the impact of clearcutting on nutrient loss is to be
evaluated adequately. This case study examines how such understanding
evolved.
BACKGROUND
We do not know when the first agriculturalist observed that ash, litter,
and animal carcasses improved crop growth. Undoubtedly, the knowledge
that the addition of particular materials to the soil increases productivity
arose independently in many regions. However, only in the nineteenth
century, after chemical elements had been identified, did it become un-
derstood that nutrients constituted the common factor in soil amendments
that maintained agricultural productivity (Brady, 19741.
The first scientist to make a systematic study of nutrient circulation in
forests in relation to growth of trees was Ebermayer. The objective of his
study, "Complete Treatise of Forest Litter," was to explain the adverse
effect on forest quality of the litter removal then common in middle
European forests (Tamm, 19791. The litter was used in cow stables, and
some of the plant nutrients in it eventually reached arable fields in dung,
thus contributing to food production at a time when commercial fertilizer
was not available. However, forest growth decreased, particularly in al-
ready poor sites, and Ebermayer attributed this to the export of plant
nutrients (Tamm, 19791.
Interest in the role of nutrients in forest growth continued throughout
the early part of the twentieth century, but only within the last several
decades have scientists begun to quantify total forest-ecosystem nutrient
budgets and changes in budgets due to various logging practices. One
reason for interest in nutrient loss resulting from the clearcut method of
forest harvesting is the obvious increase in soil erosion, nutrient leaching,
and consequent stream eutrophication after such operations (Tamm et al.,
19741. Another, which is examined here, is the effect of nutrient loss on
the ability of the soil to supply enough nutrients to produce another stand
of trees.
Observation of soil erosion and increased eutrophication of drainage
streams after clearcutting left little doubt that nutrient loss was occurring,
but did not indicate whether it was great enough to affect productivity of
the site. Because tree growth responds to fertilization in at least some
cases, nutrient loss is potentially harmful. But nutrients lost through leach-
ing and erosion might be quickly replaced by weathering and other pro-
cesses. No long-term studies have been designed specifically to test the
effects of nutrient leaching on site productivity. Studies at Hubbard Brook,
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SELECTED CASE STUDIES
New Hampshire (Bormann and Likens, 1979), showed that a northern
hardwood forest that had been clearcut and then treated with herbicide
experienced substantial amounts of nutrient leaching, but recovery of the
forest through natural succession did not appear to be noticeably inhibited
by lack of nutrients.
Clearcuaing has rarely resulted in nutrient loss so serious that trees
could not grow on the denuded site. Occasionally, on very steep slopes
in regions of heavy rainfall, such as some areas in western British Co-
lumbia, clearcutting has been followed by soil erosion severe enough to
expose bedrock, and sites have consequently been permanently deforested
(J. P. Kimmins, personal communication). In most cases, however, at
least some tree growth has occurred after clearcutting, so the environmental
question is not whether trees can grow after clearcutting, but how fast
they can grow. The rate is important to industries or agencies concerned
with timber as a crop, because trees that grow rapidly yield a greater profit
than trees that grow slowly. Rate of forest recovery also is important for
nonmarket values of forests, such as the reduction of soil erosion on
watersheds, scrubbing of polluted air, support of fish and game, and
provision of habitat for some rare species (Farnworth et al., 19811.
APPROACHES TO EVALUATING LOSSES
Leaching arid Erosion
Nutrient leaching losses often increase as a result of clearcutting. After
removal of trees, evapotranspiration on the site decreases, so the amount
of water percolating through the soil increases; the result is an increase
in leaching potential. A decrease in nutrient recycling is also important
in increasing leaching. In undisturbed forests, root uptake of nutrients and
their incorporation in biomass reduces nutrient loss. Nutrients from de-
composing litter and soil organic matter on a clearcut are not taken up by
trees, but are exchanged on the clay surfaces of mineral soil, where they
are susceptible to loss through leaching and erosion. These ecological
ideas are commonly accepted and are discussed in many texts on ecology,
agronomy, soil conservation, and forestry. However, emphasis often is
on the effect of the nutrients on lake and stream eutrophication and fish
productivity, rather than on future productive capacity of the site losing
the nutrients.
Recent studies have revealed some of the mechanisms by which nutrients
are leached. After clearcutting, the activity of nitrifying bacteria in the
soil increases (Likens et al., 1969; Vitousek et al., 1979), owing to
increased temperatures, decreased competition for ammonium from tree
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ECOLOGICAL EFFECTS OF FOREST CLEARCUlTING
349
roots, decreased allelopathic inhibition of nitrifying bacteria, and other
factors (Reiners, 1981; Swift et al., 19791. As a result, ammonium from
the mineralization of organic matter is oxidized to nitrate, and nitrate
anions and nutrient cations that have been exchanged for hydrogen ions
on soil surfaces are rapidly leached.
An important effect of forest disturbance, especially in mountainous
terrain, is soil erosion. Soil erosion results from exposure of mineral soil
to the direct impact of raindrops. The impact breaks soil aggregates,
causing pores and channels in the soil to be filled, the soil surface to
become less permeable, and the surficial runoff to be greater than when
soil surfaces are covered with litter. Although soil erosion often accom-
panies clearcut logging, recent studies have shown that it is construction
of access roads, not the clearcutting itself, that causes most of the erosion
(Douglass and Swift, 19771; removal of logs by cable appears to lead to
less erosion.
A general approach to determining nutrient loss during and after clear-
cutting of forests and during site preparation for a new stand has been to
measure leaching and erosional losses from clearcut areas directly and to
compare losses with those in undisturbed areas that serve as a control.
Measurements have usually been made where drainage from a watershed
flows over a weir resting on bedrock, so that subsurface drainage is
insignificant. Continuous monitoring of water flow and nutrient content
permits measurement of total nutrient loss. Clearcut watersheds are com-
pared with control watersheds or watersheds subjected to other treatments,
such as conversion to grassland. In some cases where discrete watersheds
were not available, losses have been studied by means of lysimeters (soil
water collectors). Losses have also been measured by comparing soil and
ecosystem nutrient stocks before and after cutting or between cut and
control plots.
Many studies have shown increases in rate of nutrient loss during forest
disturbance. For example, the studies of the watersheds at Coweeta Hydro-
logic Laboratory in North Carolina showed higher rates of nitrate and
sediment loss in recently cut catchments than in control catchments (Monk
1975; Swank and Douglass, 1975, 1977; Webster and Fatten, 1979~.
Studies of nutrient dynamics in conifer forests of the Pacific Northwest
also showed an increase in loss rates after clearcut harvesting and slash
burning (Feller and Kimmins, 1984; Gessel and Cole, 1965; Miller and
Newton, 19834.
An important study of nutrient loss after clearcutting was carried out
in northern hardwoods at the Hubbard Brook watershed site in New Hamp-
shire (Bormann and Likens, 1970; Borrnann et al., 1968; Likens et al.,
1970~. Bormann et al. (1968) concluded that
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350
clear-cutting tends to deplete the nutrients of a forest ecosystem by (i) reducing
transpiration and so increasing the amount of water passing through the system; (ii)
simultaneously reducing root surfaces able to remove nutrients from the leaching
waters; (iii) removal of nutrients in forest products; (iv) adding to the organic substrate
available for immediate mineralization; and (v) in some instances, producing a
microclimate more favorable to rapid mineralization.
SELECTED CASE STUDIES
This conclusion was controversial, because clearcutting at the Hubbard
Brook site had been followed by herbicide treatment to prevent regrowth
of vegetation. Aubertin and Patric (1974) wrote that "there is a substantial
difference between the Hubbard Brook treatment and conventional clear-
cutting. Conventional clearcutting also features complete forest cutting;
but all saleable wood is harvested and rapid forest regeneration is en-
couraged." In an experimental hardwood watershed in West Virginia,
Aubertin and Patric measured negligible nutrient losses after clearcutting
that was carried out to resemble conventional clearcut logging techniques.
Other clearcutting studies in the Hubbard Brook region showed that nu-
trient losses are less if herbicides are not used after clearcutting, but can
still be important because the soils in the region are shallow (Pierce et
al., 19721; harvesting removes a larger proportion of the total nutrient
pool there than in areas with deeper soils.
Because of the importance of nutrient uptake by vegetation, alternating
contour strip cuts with undisturbed forest on a mountainside should de-
crease nutrient leaching. Experiments with strip cuts showed that this
technique can reduce nutrient leaching in northern forests (Hornbeck et
al., 19751. The effect should be even greater in the humid tropics, where
the potential for leaching is extremely high (Jordan, 19821. Once vege-
tation becomes re-established in the clearcut strips, the uncut strips can
be harvested.
~.
Biomass Removal
Nutrient losses due to leaching are brief, and often small, compared
with those due to tree removal (Cole and Bigger, undated; Hornbeck and
Kropelin, 1982; Kimmins, 1977; Sollins and McCorison, 1981; Swank
and Waide, 19801. As evidence accumulated that nutrient losses due to
biomass removal during clearcut operations were often much greater than
losses due to leaching and erosion, scientists shifted their attention to
studies of nutrient stocks in forests and soils and to losses from ecosystems
due to biomass harvest. Results from various sites and management strat-
egies were presented in several major symposia (Ballard and Gessel, 1983;
Leaf, 19791. Generalizing about the studies is difficult, because each site
had its own combination of soil type, soil nutrient stocks, biomass nutrient
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ECOLOGICAL EFFECTS OF FOREST CLEARCU7TING
351
stocks, management technique, rates of nutrient input via rainfall, nitrogen
fixation, fertilization, and nutrient losses through leaching and denitrifi-
cation. Generalizations valid for sites of a specific type in a given region
do not apply to sites of other types. Each nutrient also appears to behave
differently. Only by measuring the stocks and dynamics of all the critical
nutrients at each site can one estimate accurately the impact of nutrient
removal on productivity of the site. Even then, estimation might be in-
sufficient, because trees can exhibit uptake beyond immediate needs, and
this can be misleading in the prediction of requirements.
One general finding is that the extent of nutrient loss depends on which
parts of the trees are removed from the site. Leaves have higher concen-
trations of nutrients than do stems, so stripping logs of leaves before the
logs are removed from the site decreases nutrient losses. For evergreens,
branches with leaves are cut off before the tree bole is removed. In the
case of deciduous species, an alternative is to harvest during the winter.
However, because some nutrients are withdrawn from leaves into stems
before the leaves are shed, nutrient losses would be smaller if trees were
cut while in full leaf and mechanically defoliated. Harvesting of whole
trees, including roots, removes larger proportions of nutrients.
Nutrients Remaining in Soil
Simply measuring the nutrients lost because of clearcutting and site
preparation does not indicate the effect of the nutrient loss in site pro-
ductivity. Estimates of the quantities of nutrients remaining in the site and
of the rates at which they are replenished are also needed. Scientists
attempting to predict the effect of nutrient loss on site productivity must
determine nutrient stocks remaining in the soil and the fractions of those
stocks available to growing trees (Johnson et al., 19821.
Nutrients are held in the soil in various ways (Brady, 19741. Some
cations, such as potassium, can be part of the lattice structure of minerals,
where they are relatively unavailable to roots; or they can be exchanged
on clay surfaces, where they are readily available. Phosphorus can exist
in relatively soluble forms, but, in the presence of low pH and high
aluminum concentrations, it becomes bound in compounds that are not
readily taken up by plants. Soil can contain large amounts of nitrogen; if
the nitrogen is bound in organic matter that is resistant to decomposition,
however, nitrogen shortages might be critical. Thus, knowing the total
stocks of nutrients in the soil is not sufficient.
For many agricultural crops, reagent-soluble nutrients in soil have been
correlated with crop growth, and solubility of nutrients has been used as
an index of the availability of nutrients to crop plants. However, such an
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352
SELECTED CASE STUDIES
index usually is not applicable to tree growth. Many tree species are
adapted for extracting nutrients from soil when "availability," as mea-
sured by solubility, is low. Some of these adaptations are large root
biomass, mycorrhizal symbiosis, slow root uptake, and long life, which
enable trees to survive during periods of nutrient shortage (Chapin, 19801.
The effects of these variables are not easy to measure.
Nutrient Replenishment
Site productivity depends on relative rates of nutrient replenishment
and nutrient loss. The atmosphere contains a stock of nutrients adsorbed
on the surface of aerosols, such as dust and pollen, that either settle out
of the atmosphere gradually as "dry fall" or are washed out by precipi-
tation. Nutrient input from the atmosphere can be substantial. Nitrogen
can be contributed to a site by nitrogen-fixing species, which can be present
because they occur naturally or through planting of species symbiotic with
nitrogen-fixing bacteria. Nutrients can also enter an ecosystem through
the weathering of subsoil or parent rock.
Nutrients lost through clearcutting can be replaced by fertilizers, but
fertilization of forests is often economically infeasible. For most agricul-
tural crops, fertilization is profitable, because annual sales are high and
the time between investment in fertilizers and return of investment in crop
sales is short. The economic benefits of fertilizing forests are more difficult
to calculate, because the value of harvestable products that accumulate
each year is low, because the period between investment and harvest is
long, and because forests have other values besides their use for wood.
Budgets arid Sensit~vi~ Analysis
Nutrient budgets of ecosystems have proved extremely difficult to mea-
sure precisely, because of high variability in both space and time. In
addition, such important quantities as the proportions of nutrients available
to plants and the rate of mineral weathering are often hard to measure;
errors in their measurement could result in large errors in estimates of
total ecosystem nutrient budgets. Therefore, these estimates are often
viewed skeptically by both scientists and managers. The key question,
however, is whether the error is important in relation to the environmental
or ecological problem being addressed.
As an example of a simple sensitivity analysis to assess the effect of
biomass removal on remaining nutrient stocks and the effect of error on
the predictions, consider the following hypothetical case. Assume that the
standard deviation around the amount of calcium in the soil is relatively
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ECOLOGICAL EFFECTS OF FOREST CLEARCU7TING
353
large and that the average value of a set of samples could be different
from the true average value by 30%. Is this error important? If calcium
were present in the aboveground biomass at 500 kg/ha, all of which would
be removed by clearcutting, and present in the soil at 5,000 kg/ha, an
error of 30% in the estimate of calcium in the soil would not change the
conclusion that clearcutting will not have an important effect on total
calcium stock. If calcium were present at 500 kg/ha in the biomass and
100 kg/ha in the soil, an error of 30% would not change the conclusion
that clearcutting would have a very important effect on total calcium stock.
However, if calcium were present at 500 kg/ha in both the biomass and
the soil, it would be important to be able to estimate more accurately the
true value of calcium in the soil, because a 30% error could lead to very
different management conclusions.
This relatively simple type of analysis has yielded predictions of nutrient
depletion due to biomass removal in several regions. For example, calcium
depletion due to clearcutting of ridge forests in the southern Appalachians
might result in reduced growth of the next crop (West and Mann, 1983),
and nitrogen is often a limiting nutrient in the conifer forests of the Pacific
Northwest (Peterson and Gessel, 1983~.
Simulation models of forest growth that incorporate the effects of nu-
trients can assist in answering questions about the required degree of
accuracy of field measurements and experiments. Only recently have mod-
els with increased sophistication been developed. A model like the nitrogen
model of Swank and Waide (1980), which predicts yield under various
management strategies on the basis of nitrogen dynamics in the treated
ecosystems, can be used to assess the effect of an error of a given mag-
nitude on the overall conclusion and can aid in a decision as to whether
more accurate measurements are necessary. The latter model permits eval-
uation of management alternatives and their consequences, such as the
effect on forest yield of a change in the length of rotation.
The importance of sensitivity analyses is often not appreciated by eco-
logical and environmental scientists. Perhaps the reason is that sensitivity
analyses often contradict what many scientists have been taught, which
is that the greatest possible accuracy should always be sought. Sensitivity
analyses show us where a large amount of less accurate data might be
more valuable than a smaller amount of more accurate data.
CONCLUSION
Studies to evaluate the effect of clearcutting on nutrient depletion and
site productivity have progressed a long way. Early studies were concerned
with nutrients removed by leaching and erosion after clearcutting. Then
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354
SELECTED CASE STUDIES
it was recognized that nutrient stocks remaining in the ecosystem after
logging were important, and studies of those stocks were carried out.
Nutrient dynamics and their effects on stocks and productivity were in-
cluded in the analyses. Recent work has used systems analysis to keep a
better account of the multitude of continuously changing factors in the
ecosystem.
The proof of the effectiveness of the approach will lie in tests of the
predictions generated by the models. From the perspective of applications,
~ ,
the most important prediction is that of wood yield. Because of the time
required for such validation, it Is too early to appraise the approach.
Regardless of how accurate the predictions prove to be, they will be useful
to the scientists who formulated the models on which they were based.
Discrepancies between predictions and results will point up weaknesses
in the models and indicate what studies are necessary to improve the
accuracy of the predictions.
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Bormann, F. H., G. E. Likens, D. W. Fisher, and R. S. Pierce. 1968. Nutrient loss
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Brady, N. C. 1974. The Nature and Properties of Soils. Macmillan, New York.
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Cole, D. W., and C. M. Bigger. Undated. Effect of Harvesting and Residue Removal on
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Douglass, J. E., and L. W. Swift. 1977. Forest service studies of soil and nutrient losses
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_ _ _ , ,
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Jordan, C. F. 1982. Amazon rain forests. Am. Sci. 70:394-401.
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A. L. Leaf, program chairman. Impact of Intensive Harvesting on Forest Nutrient Cy-
cling. Proceedings of a Symposium at Syracuse, New York, August 13-15, 1979. North-
east Forest Experiment Station, Broomall, Pa.
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from soil as a consequence of forestry operations. Ambio 3:211-221.
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1979. Nitrate losses from undisturbed ecosystems. Science 204:469-474.
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Committee Comment
As illustrated by this case study, methods of analyzing nutrient budgets
of ecosystems have developed slowly as scientists gradually have rec-
ognized the need to perform a complete accounting of nutrient stocks and
fluxes if the significance of changes in flux rates induced by clearcutting
is to be understood. This represents discovery of methods that were already
well known in the business world, and it might be asked whether ecologists
would have progressed more rapidly if they had been better versed in
budget analyses as practiced in various disciplines. The answer is probably
yes, but it is also clear that ecosystems are so different from businesses
that those techniques have to be modified for use in studying ecosystem
nutrient dynamics. Such features of ecosystems as differences in availa-
bility of nutrients due to variation in soil types and in the types of plants
growing on them have no close parallels in the business world.
Whereas much progress has been made in measuring total stocks and
fluxes of nutrients, site variation is great enough that very few general
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predictions are possible. There is no reason to believe, in principle, that
developing a broad predictive ability is impossible, but it seems evident
that the necessary data base is large and that its components might not
yet all be identified. That is true even if we simply wish to predict growth
rates of trees. If we also wish to predict the dynamics of other constituents
of an ecosystem, such as herbivores, carnivores, and detritivores as well
as interactions among plant species even more extensive data are re-
quired. Results of such analyses could be surprising. For example, lower
availability of nutrients, in combination with physical stress, such as
drought, might render trees much more susceptible to attacks by defoliating
insects or fungi (Fearnside and Rankin, 19851. These attacks might de-
crease yields and change growth forms of the trees, leaving yields of
usable wood products much lower than would be predicted simply on the
basis of the availability of nutrients to support tree growth.
An important conceptual advance illustrated by this case study is sen-
sitivity analysis. Sensitivity analysis is especially important when systems
consist of many interacting factors that are of uncertain influence or that
are difficult to measure accurately. Time and budgetary constraints prevent
accurate measurement of all factors of interest, and choices need to be
made as to which factors should receive the most attention. Predictions
generated by models are unequally sensitive to variation in different pa-
rameters. Sensitivity analysis is a powerful method for deciding which
factors need to be measured most accurately and which ones require only
crude estimation. Sensitivity analysis helps to avoid consequential errors
errors that could lead to inappropriate management decisions. The natural
inclination of most scientists to measure all quantities as accurately as
possible might actually lead to poorer predictive abilities, given the in-
vestment of comparable resources, than an approach designed to obtain
accurate measurements only for the quantities identified as critical.
The study of ecosystem nutrient dynamics is made difficult not only
by the number of interacting factors, but also by the long duration of the
most important processes. Development and testing of models might re-
quire decades of work that taxes the patience of investigators, funding
agencies, and managers who must make decisions, whether or not suf-
ficient data are available for them to predict the consequences of their
decisions. This emphasizes the need for cooperation among all who are
involved in developing plans for managing and using forest ecosystems.
Reference
Fearnside, P. M., and J. R. Rankin. 1985. Jari revisited: Changes and the outlook for
sustainability in Amazonia's largest silvicultural estate. Interciencia 10:121-129.
Representative terms from entire chapter:
nutrient losses
