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0.10±0.18 have been suggested for dense coniferous forest
4,5
; this
would increase the EESF by approximately 25 t C ha
-1
in the most
snow-covered regions.
Here I have considered forestation under present-day conditions,
but the effects of future CO
2
rise and climate change are likely to
affect the magnitude of both radiative forcing terms, due to
dependencies on time-varying quantities such as the atmospheric
CO
2
concentration, snow extent and vegetation structure and
lea®ness. As the atmospheric CO
2
concentration increases, CO
2
fertilization is likely to increase carbon uptake
22
so the magnitude of
the negative sequestration forcing should therefore increase,
although associated climate changes may exert additional positive
or negative effects on sequestration. Warmer temperatures may
reduce the extent of snow cover
23
, but the leaf area index (LAI) of
potential vegetation may increase
24,25
, so the albedo forcing could
either increase or decrease. The effect of vegetation on surface
albedo is not necessarily proportional to biomass, so the net
contribution to radiative forcing may not evolve linearly through-
out a forest's development; albedo depends on canopy density and
architecture, and can become low rapidly, whereas carbon seques-
tration depends largely on woody biomass which is more gradually
accumulated. Other contributions to forcing may also require
consideration; for example, the longwave radiation budget could
be affected by modi®ed surface emissivity
25
, although the sign of
such changes is uncertain
25,26
.
The work I report here has focused on perturbations to the Earth's
radiation budget, which is the fundamental driver of the climate
system. Forestation may also in¯uence the climate by modifying the
¯uxes of heat, moisture and momentum between the land surface
and atmosphere. Whereas boreal forests warm their local climate
through reduced albedo, tropical forests tend to cool and moisten
their local climates by greatly enhancing evaporation. Both may also
in¯uence distant regional climates via the atmospheric circu-
lation
9,27
. Assessment of the effect of forestation on climate at a
given time in the future will require simulations with a climate
model that incorporates vegetation dynamics
25,28
and other atmos-
pheric, terrestrial and oceanic components of the carbon cycle
28
,in
which forest growth occurs at appropriate rates in relation to
changes in atmospheric CO
2
and snow cover. Nevertheless, my
results suggest that high-latitude forestation would exert a positive
radiative forcing through reduced albedo that in many places could
outweigh the negative forcing through carbon sequestration. If
afforestation and reforestation are required to decrease radiative
forcing rather than simply to reduce net CO
2
emissions, then
changes in surface albedo must also be considered.
M
Received 10 July; accepted 27 September 2000.
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2. UNFCCC United Nations Framework Convention on Climate Change Art. 2 (UNEP/IUC/99/2,
Information Unit for Conventions, UNEP, Geneva, 1999); <http://www.unfccc.int/resource/
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1634 (1984).
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5. Sharratt, B. S. Radiative exchange, near-surface temperature and soil water of forest and cropland in
interior Alaska. Agric. Forest Meteorol. 89, 269±280 (1998).
6. Thomas, G.& Rowntree,P. R. The boreal forests and climate.Q. J. R. Meteorol. Soc. 118, 469±497 (1992).
7. Bonan, G. B., Pollard, D. & Thompson, S. L. Effects of boreal forest vegetation on global climate.
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the climate system. Clim. Change 29, 145±167 (1995).
9. Douville, H. & Royer, J. F. In¯uence of the temperate and boreal forests on the Northern Hemisphere
climate in the Me
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forest types. Can. J. Forest Res. 25, 1157±1172 (1995).
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Clim. Change 30, 267±293 (1995).
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et al.) 65±131 (Cambridge Univ. Press, Cambridge, 1995).
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a large-scale model. Q. J. R. Meteorol. Soc. 122, 689±720 (1996).
15. Pope, V. D., Gallani, M. L., Rowntree, P. R. & Stratton, R. A. The impact of new physical
parametrizations in the Hadley Centre climate model - HadAM3. Clim. Dyn. 16, 123±146 (2000).
16. Hansen, J. E. et al. Ef®cient three dimensional global models for climate studies, Models I and II. Mon.
Weath. Rev. 111, 609±662 (1983).
17. Wilson, M. F. & Henderson-Sellers, A. A global archive of land cover and soils data for use in general
circulation climate models. J. Climatol. 5, 119±143 (1985).
18. Woodward, F. I., Smith, T. M. & Emanuel, W. R. A global land primary productivity and
phytogeography model. Glob. Biogeochem. Cycles 9, 471±490 (1995).
19. Myhre, G., Highwood, E. J., Shine, K. P. & Stordal, F. New estimates of radiative forcing due to well
mixed greenhouse gases. Geophys. Res. Lett. 25, 2715±2718 (1998).
20. Keeling, C. D. & Whorf, T. P. Atmospheric CO
2
Concentrations - Mauna Loa Observatory, Hawaii, 1958-
1997 (NDP-001, Carbon Dioxide Information Analysis Centre, Oak Ridge, Tennessee, 1998).
21. Willmott, C. J., Rowe, C. M. & Mintz, Y. Climatology of the terrestrial seasonal water cycle. J. Climatol.
5, 589±606 (1985).
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climate change. Nature 393, 249±252 (1998).
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366 (1997).
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vegetation feedbacks in climate change simulations. Nature 387, 796±799 (1997).
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-induced
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carbon-cycle feedbacks in a coupled climate model. Nature 408, 184±187 (2000).
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(1964).
Acknowledgements
I thank S.E. Lee and F.I. Woodward for providing data from the Shef®eld University
vegetation model, and P.M. Cox, J.M. Edwards, R.L.H. Essery, W.J. Ingram, G.J. Jenkins,
J.E. Lovelock, S. Nilsson, I.C. Prentice, P.R. Rowntree, K.P. Shine, P.J. Valdes and D.A.
Warrilow for advice, comments and discussion. This work forms part of the Climate
Prediction Programme of the UK Department of the Environment, Transport and the
Regions.
Correspondence should be addressed to the author (e-mail: rabetts@meto.gov.uk).
.................................................................
An arti®cial landscape-scale ®shery
in the Bolivian Amazon
Clark L. Erickson
Department of Anthropology, University of Pennsylvania, 33rd and Spruce Streets,
Philadelphia, Pennsylvania 19104-6398, USA
..............................................................................................................................................
Historical ecologists working in the Neotropics argue that the
present natural environment is an historical product of human
intentionality and ingenuity, a creation that is imposed, built,
managed and maintained by the collective multigenerational
knowledge and experience of Native Americans
1,2
. In the past
12,000 years, indigenous peoples transformed the environment,
creating what we now recognize as the rich ecological mosaic of
the Neotropics
3±6
. The prehispanic savanna peoples of the Boliv-
ian Amazon built an anthropogenic landscape through the con-
struction of raised ®elds, large settlement mounds, and earthen
causeways
7,8
. I have studied a complex arti®cial network of
hydraulic earthworks covering 525 km
2
in the Baures region of
Bolivia. Here I identify a particular form of earthwork, the zigzag
structure, as a ®sh weir, on the basis of form, orientation, location,
association with other hydraulic works and ethnographic analogy.
© 2000 Macmillan Magazines Ltd
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The native peoples used this technology to harvest suf®cient
animal protein to sustain large and dense populations in a
savanna environment.
The zigzag structure is a particular form of arti®cial earthwork,
found in the seasonally inundated savanna of Baures, Bolivia
(Province of Ite
Â
nez, Department of the Beni) (Figs 1 and 2).
Zigzag structures are linear segments of raised earth (1±2 m wide
and 20±50 cm tall) that change direction every 10±30 m (Fig. 3).
Shrubs, palms, and termite mounds cover the structures. Many
zigzag structures cross the savanna from one forest island to
another, distances of up to 3.5 km; others terminate 5,000±
1,000 m from the forest edge (Fig. 4). Funnel-like openings, 1±
3 m long and 1±2 m wide, are present where the structures form a
sharp angle (Fig. 5). The structures are associated with small circular
ponds. Dense networks of interconnected zigzag structures form
enclosures of 10±80 ha. A total of 48.431 linear kilometres of weirs
were measured in a sample area of 16.755 km
2
of savanna, a density
of 2.891 linear km per km
2
.
The zigzag structures are arti®cial constructions created by
raising earth removed from the adjacent savanna. Canals or
barrow pits ¯ank some of the zigzag structures. Although over-
lapping in distribution, the zigzag structures are distinct from the
long, wide and straight causeways and canals that cross the savanna
between forest islands (Fig. 4). The narrow and irregular zigzag
structures would be inef®cient for transportation. The zigzag
structures do not appear to have functioned as check dams or
berms for ¯ood-recessional farming. There is no evidence of crop
furrows or ®eld platforms between the structures.
On the basis of location, form, patterning, associations and
ethnographic analogy, I identify the zigzag structures as ®sh weirs.
Fish migrate to and spawn in the seasonally inundated savannas of
Baures during the wet season
9
. Many ®sh are trapped in water bodies
as the ¯oodwaters recede. The zigzag structures provided a means to
manage and harvest these ®sh. The zigzag structures are similar to
®sh weirs built by native peoples in Bolivia
10±14
and throughout the
Americas
15±17
. Two characteristics shared by ®sh weirs include
construction of barriers across shallow bodies of water and V-
shaped openings where ®sh are trapped. Weirs, ranging in length
from several to hundreds of metres, are constructed of soil, rock,
reed, branches, logs, aquatic vegetation and/or basketry. Super-
structures of perishable materials or a dense wall of vegetation
probably covered the earthen ®sh weirs of Baures.
There are important differences between the zigzag structures of
Baures and contemporary ®sh weirs. Most ethnographic ®sh weirs
are ephemeral and rebuilt each season. Traditional weirs are con-
structed in rivers, streams or permanent bodies of water. In contrast,
the zigzag structures are permanent earthworks built across a
seasonally ¯ooded savanna. They are also more numerous, longer
and more densely placed than ethnographic ®sh weirs. In addition
to controlling and harvesting ®sh within the savanna, the weirs and
large causeways may have been used for water management. The
earthworks could have extended the period of inundation by
capturing the ®rst rains and holding ¯oodwaters into the dry
season
18
.
The savanna ®sheries of the Bolivian Amazon are productive.
Estimates of 100,000 to 400,000 ®sh have been calculated for a single
hectare of abandoned river channel in the savannas
9
. Yields of
1,000 kg per hectare per year have been recorded for shallow
ponds in tropical savannas
19
. Large numbers of Pomacea gigas
snails are found beside the zigzag structures. In addition to ®sh,
these edible snails may have been managed and raised in the weir
structures and ponds. In the past, these snails were eaten in Baures
and the gastropods are found in precolumbian sites in Bolivia and
Brazil
20,21
. The nutritional status of Pomacea is probably similar to
other tropical snails, low in calories and protein
22
. Pomacea gigas
reproduce and grow at an impressive rate and an average of 23.8
snails per m
3
is recorded in Bolivian wetlands
23
. The arti®cial ®sh-
eries of Baures potentially produced hundreds of tonnes of edible
snails as a secondary food source.
The most common vegetation associated with the ®sh weirs and
ponds is the palm Mauritia ¯exuosa
24±26
(Fig. 3). A single tree can
produce up to 5,000 edible fruits each year and a single hectare
yields 10±60 t of fruit. The fruits are high in vitamins C and A, oil
(12%) and protein (4±5% dry weight). The ground tissue produces
large amounts of edible starch. Edible larvae of the palm beetle
thrive in the decomposing trunks. In addition, the palm is a
favoured food of game animals and ®sh. The ®bres of the fronds
and trunks are used for basketry, mats, hammocks, bowstrings,
thatch and roof beams. The palm may have been encouraged or even
cultivated on the earthworks.
Arti®cial ponds overlap with the distribution of zigzag structures
(Fig. 3). These measure 0.5±2 m deep and 10±30 m in diameter; the
largest hold water year round. The ponds teem with ®sh such as
buchere (Hoplosternum sp.), yallu, cunare
Â
(Cichla monoculos),
palometa (Serrasalmus sp.), sa
Â
balo (Prochilodus nigricans) and
bento
Â
n(Erythrina sp), snails, birds, reptiles and amphibians. Con-
temporary hunters stalk the game animals and birds that congregate
at the ponds. Arti®cial ponds provided a way to store live ®sh and
snails until needed.
The complex of ®sh weirs and ponds of Baures is a form of
intensive aquaculture. The earthworks did not necessarily involve
the mobilization of large amounts of labour. I estimate a total of
1,515 linear kilometres of ®sh weirs in Baures based on a sample
of aerial photographs. Using labour estimates for experimental
Guapore River
San Martin River
San Joaquin R.
Negro River
Blanco River
Itonamas River
or Itenez River
Bella Vista
Baures
Bolivia
0 200
kilometres
Santa Cruz
La Paz
Sucre
Trinidad
Prehispanic Hydraulic
Complex of Baures
Detailed Map
kilometres
080
Figure 1 Map of the Baures prehispanic hydraulic complex. It is located between the
San Joaquõ
Â
n river to the west and the San Martõ
Â
n river to the east and between 13
o
30' and
14
o
20' latitude South.
South Block
North Block
Savanna
Forest island
kilometres
020
N
S
EW
Figure 2 The ®sh weirs (zigzag structures and ponds). They are concentrated in the North
Block (447 km
2
of savanna excluding forest islands) and the South Block (77 km
2
). The
structures are restricted to savannas near forest islands that are ¯ooded by shallow water.
© 2000 Macmillan Magazines Ltd
letters to nature
192 NATURE
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construction of raised ®elds (5 m
3
of earth per person per 5-hr day
and weirs measuring 2 m wide and 0.5 m tall), the weirs required
300,000 person-days of labour or the equivalent of 1,000 people
working 30 days a year over a period of 10 years. Small groups of kin
or communities constructed and managed the weirs recorded in
ethnographic accounts
10±14
. Similar social groups may have been
responsible for the weirs and ponds of Baures.
The weirs also show some evidence of integration at a higher
scale. Individual zigzag structures often cross the savanna from one
forest island to another (Fig. 4). Assuming each forest island was an
autonomous settlement, weir construction may have involved inter-
community cooperation. Although individual weirs could operate
Figure 3 Oblique photograph of a ®sh weir and arti®cial ponds between forest islands in
the savannas of Baures. Fish weirs are the zigzag structures, lower left to upper right;
arti®cial ponds are the circular features surrounded by palms (approximately 20 m in
diameter). The diagonal feature (upper left to lower right) is a contemporary path.
Forest islands
Major causeways
Minor causeways
Fish weirs
Palomarial
Naranjal
kilometres
01
N
Figure 4 Map of ®sh weirs (irregular lines) and causeways (straight lines) in Baures. Based on aerial photographs.
© 2000 Macmillan Magazines Ltd
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independently of other weirs, the presence of an integrated network
of causeways, canals, weirs and ponds suggest a higher order of
water management
8,18
. As a permanent food-producing infrastruc-
ture, the weirs must have been valuable real estate. The networks of
causeways and canals may have promoted communication and
alliances between individual communities exploiting the ®sh weirs.
Groups in the Colombian Amazon jealously protect and guard
riverine ®sheries, valuable resources that are owned and inherited by
clans and chie¯y lineages
16
. The presence of moated, and presum-
ably palisaded, settlements on many of the forest islands suggests
potential tension over the ®sheries and other resources
27
.
Colonial accounts describe the use of causeways in Baures for
communication and transportation between settlements
7,10,12,27
.
Weirs in lakes and streams are described
10,12
, but there is no mention
of the zigzag structures in the savanna. To date the ®sh weirs, I
excavated a large causeway directly associated with zigzag
structures
28
. Burned wood from the base of the causeway ®ll was
radiocarbon dated to 335 years
BP (before present) 6 20 (OS-17293)
or an uncalibrated calendar date of AD 1615 (AD 1595±1635). The
corrected date at 68.2% con®dence is AD 1490 (0.26) AD 1530; AD
1560 (0.74) AD 1630. Depending on the context, the sample may
date or predate the original construction. The Spanish did not
control the Baures region until 1708; thus, the earthwork probably
predates European occupation.
The earthworks of Baures are an example of creation and active
management of an anthropogenic landscape by native peoples. The
linear causeways and canals were a sophisticated means of regulat-
ing water levels within the savannas to enhance and manage
seasonal aquatic resources. The network of ®sh weirs provided a
means of controlling and harvesting ®sh, in addition to enhancing
the habitat and availability of aquatic and terrestrial fauna. The
arti®cial ponds were a means of concentrating and storing live ®sh,
providing drinking water and improving game habitats. Palms
growing on earthworks provided additional foodstuffs and materi-
als. Using this simple, but elegant, technology, the people of Baure
converted much of the landscape into an aquatic farm covering 500
km
2
. Rather than domesticate the species that they exploited, the
people of Baure domesticated the landscape. The ®sh weirs and
ponds produced abundant, storable, and possibly sustainable
yields of animal protein. Thus, they were able to sustain large
dense populations in what many would consider a marginal
environment.
M
Received 2 July; accepted 1 September 2000.
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Acknowledgements
Fieldwork and analysis were supported by the NSF, the Heinz Charitable Trust Founda-
tion, the American Philosophical Society, the Research Funds of the University of
Pennsylvania Museum, and Corporacio
Â
n del Beni. K. Lee originally reported the earth-
works of Baures in the late 1950s. A.Vranich, O. Saavedra and F. Bruckner did the ®rst
study of the Baures earthworks in 1995. I thank the Bolivian Direccio
Â
n Nacional de
Arqueologõ
Â
a y Antropologõ
Â
a, the Prefectura and Alcaldia of the Department of the Beni,
authorities of Baures and Bella Vista, W. Winkler (project co-investigator), K. Lee,
H. Schlink, R. Bottega, R. Pinto Parada, E. Bruckner, A. Bruckner, C. Bruckner, O. Rivera,
R. Langstroth, W. Denevan, A. Vranich, P. Stahl, R. Dunn and D. Brinkmeier.
Correspondence and requests for materials should be addressed to C.E.
(e-mail: cerickso@sas.upenn.edu).
0
50
metres
A
B
C
Figure 5 Plans of ®sh weirs (zigzag structures). Small parallel openings in the weirs are
present every 50±200 m.
© 2000 Macmillan Magazines Ltd
