Eos, Vol. 82, No. 37, September 11,2001
VOLUME 82
NUMBER 37
SEPTEMBER 11,2001 PAGES 405-416
EOS, TRANSACTIONS, AMERICAN GEOPHYSICAL UNION
Answers Sought to Big Questions About the Amazon Region PAGES 4 0 5 , 4 0 6 The Amazon region contains the largest area of tropical forest on Earth, over 5 x 10 km . While the bulk of the region remains forested, rapid development has led to the destruction of over 500,000 km of forest in Brazil alone over the past 25 years [Houghton et al., 2 0 0 0 ] . Observing this rapid change, an interdiscipli nary group of scientists began planning a continental-scale study in 1993 to learn how Amazonia functions as a regional entity in the Earth system and how changes in land use and climate will affect the biological, chemical, and physical functions of Amazonia, including the sustainability of development in the region and the influence of Amazonia on global climate. The Large Scale Biosphere-Atmosphere Experi ment (LBA) in Amazonia, led by Brazil, is the result of this planning. LBA implementation began in 1998 and has advanced rapidly since 1999.Today over 600 scientists from South and North America, Europe, and Japan are actively participating in LBA research. 6
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At the D e c e m b e r 2000 AGU Fall Meeting, LBA researchers presented 60 talks and posters in special sessions jointly sponsored by the Biogeosciences, Atmospheric Sciences, and Hydrology Sections.These concentrated on ecological work within LBA that responds to the question,"How do tropical forest conversion, regrowth, and selective logging influence car bon storage, nutrient dynamics, trace gas fluxes, and the prospect for sustainable land use in the Amazon region?" These theme areas are producing a number of new results.
What Are the Current Trends in Land Cover and Land Use? The Amazon River is the world's largest. A substantial portion of the Amazon Basin is inundated during at least part of the year, and for the first time, analyses of microwave remote sensing data are providing quantitative measures of the extent and periodicity of the
inundation. J o h n Melack reported inundation area estimates based on the 37 GHz channel of the Scanning Multichannel Microwave Radiometer (SMMR).The maximum area sub ject to inundation along the main-stem Ama zon in Brazil was about 77,000 km , while inundation in savannas within the Amazon basin ranged from 87,000 km in the Bolivian Moxos, 58,000 km in the Bananal.to 16,000 km in Roraima. Based on examination of syn thetic aperture radar data from the JERS-1 sensor, Melack reported that about 30% of the 2
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central Amazon could b e inundated at high water. Continuing analyses of the basin-wide JERS-1 data set, in conjunction with high reso lution videography flown over much of the basin, will provide a wetland classification for the whole basin. Interannual variability in rainfall affects ter restrial vegetation, as well as river basin hydrology Observations [Asner et al, 2000] and models [e.g., Tian et al, 1998] both indicate that the vegetation responds to variations in precipita tion, with greater productivity and carbon storage in wet years. One of the biggest ques tions for LBA is whether the Amazon region is a net source or sink of carbon and how much this source/sink activity varies from year to year. Jeff Richey and his colleagues are attempting to use the historical time series of Advanced Very High Resolution Radiometer (AVHRR)
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Fig. 1. Merged satellite and agricultural census data in Amazonia from the mid-1990s. The fusion technique uses regression tree analysis to represent the statistical relationship between existing global satellite land cover classifications and land use census data derived from each country of the Amazon and Tocantins basins. The resulting relationship between land use and land cover is computed at the county scale and projected into each 5-min (-10 km) cell. Preliminary analysis indicates that cropland and pasture covered 8.3 x 10 ha out of the 6.7 x 10 ha study region, with pasture accounting for 80% of the total agricultural area. (Figure courtesy of J. Cardille, University of Wisconsin, Madison.) Original color image appears at the back of this volume. 7
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Eos, Vol. 82, No. 37, September 11, 2001 images from the National Oceanic and Atmos pheric Administration's TIROS satellites to investigate this back to 1980. Analysis of this time series is complicated by problems of atmos pheric corrections, satellite orbital changes, and degradation and changes of the instruments themselves. However, Richey points out that the images give an unambiguous expression of the land surface. Seasonality is apparent, and major changes in surface features—for example, the flooding of hydro-electric reservoirs—are clearly visible. Richey expects significant infor mation will be retrieved from these images to help constrain the interannual variability of carbon cycling over the past 2 decades. Both wetland and upland areas of the Ama zon region have b e e n under intense develop ment pressures for at least 3 decades.The change in land cover, particularly clear-cutting and conversion of forest to pasture and agri cultural uses, is readily visible from satellite sensors such as Landsat [Skole and Tucker, 1993]. However, the fate and use of deforested lands are difficult to determine from satellite imagery Jeff Cardille and colleagues used regression tree analysis to c o m b i n e existing agricultural census data from all Amazon region countries with a land cover map [Hansen et al, 2000] from the mid-1990s to produce a map of agricultural land use (Figure 1). Results of this type give scientists greater insight into potential b i o g e o c h e m i c a l changes than sim ple land cover maps. Accounting for potential ly large errors in the census data will b e a challenge for the next phase of this research. The combination of census data and satel lite imagery is a powerful tool, but it still fails to identify one growing land use in the Amazon region: selective logging. In selective logging, about o n e to six trees of marketable species are harvested per hectare. Recent results based on surveys of saw mills suggest that logging annually affects an area nearly as large as the area clear-cut [Nepstad et al., 1999]. Unfortu nately, logging is difficult to observe from satellite imagery Greg Asner showed that the signal of log storage decks that has been used to identify logging on Landsat/SPOT images was lost after 2 years due to vegetation regrowth. Asner conjectured that either greater spectral resolution or multiple view angles would b e needed to identify logging sites from satellite sensors over longer periods following selec tive logging events.
What Happens When the Land Use Changes? Land-use changes c a n cause a variety of biogeochemical c o n s e q u e n c e s . Alex Krusche and Trent Biggs both demonstrated that the conversion of forest to pasture and urbaniza tion in the Brazilian state of Rondonia led to significant changes in river chemistry In par ticular, deforestation increases levels of dissolved inorganic and organic nitrogen in surface waters. Land-use changes also modified soil chemistry Karen Holmes used abundant data from the Rondonia State PLANAFLORA survey to demonstrate that geologic parent material
and land use interact to control soil chemistry in developing landscapes. In Rondonia, soils derived from more basic rocks are more likely to b e cleared and burned.The burning signal is apparent in the elevated pH of surface soils on the basic rock types. However, at depth, those soils are acid, much like neighboring soils derived from acid rocks. Land use trumps parent material effects in this expres sion of soil chemistry An important trend in land management in the Amazon is the increasing extent of fire. Fires cause large changes in the atmospheric concentration and composition of trace gases and aerosols. Liane Guild and Luciana Gatti, respectively, modeled and measured the effects of fire. Those effects include major increases in carbon monoxide, nitrogen oxides, ozone, aerosols, and particulates during the dry season. According to Guild's model, inten tional fires set following forest felling to clear land dominate the burning emissions. O n c e deforested, these areas continue to contribute to emissions as they are re-burned to maintain pasture. Marcia Yamasoe used retrievals from AERONET Sun-photometers spread across the Amazon region of Brazil to show that photosynthetically active radiation can b e reduced by 25% or more at times of heavy smoke. Aerosol mass increased by a factor of greater than 15, and diurnally aver aged surface ozone concentration increased by a factor of two in the dry season compared to the wet season according to results presented by Paulo Artaxo.
Where Has All the Carbon Gone? Through fire and other processes, land clearing and land-use changes modify ecosystem car bon budgets. Chris Potter and his colleagues used a combination of satellite observations of fire o c c u r r e n c e from the IGBP DIS global fire product [Stroppiana et al, 2000] and biomass and productivity estimates from the CASA ecosystem model to evaluate the carbon budget for the Amazon region. According to their calculations, the region acted as a net source for carbon in a range of 0.2 to 1.2 Pg yi from 1992 to 1993.Net emissions were strongly dependent on annual rainfall patterns.The CASA model predicts that Amazon primary forests can b e net sinks of about 1-2 Mg-C ha" y" during relatively wet years, but are C sources of about the s a m e magnitude during severely dry El Nino years. Net emission calculations are strongly dependent on the a c c u r a c y of the fire maps and the underlying land cover maps that differentiate forest and pasture areas. LBA researchers are working to improve the accuracy of this critical data. 1
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Despite the rapid changes, the Amazon is still mostly forested. How does the world's largest tropical forest contribute to the global budget of carbon? Tans et al. [1990] found that global-scale observations pointed to a Northern Hemisphere terrestrial sink for the so-called "missing" carbon. Most subsequent analyses also place this sink in the northern temperate and boreal forest ecosystems.
However, during the 1990s, data from several sources indicated that Amazon forests may also b e a substantial sink for c a r b o n [Grace et al., 1995; Phillips et al., 1998]. Scott Denning demonstrated that current estimates from global inversions are uncertain for the Amazon region b e c a u s e of the lack of regional data on C 0 concentrations. According to this analysis, weekly aircraft-based air sampling currently being tested by LBA investigators could reduce the current uncertainties from atmospheric inversion models by 75% to within a few tenths of a Pg-C per year. Tower-based estimates of c a r b o n uptake by forests were the subject of several posters and oral presentations at the AGU Fall Meeting. Pavel Kabat, Antonio Nobre, Bart Kruijt,and their colleagues presented data from eddy correlation measurements of C 0 flux by the Brazilian and European EUSTACH/CARBONSINK consortium from three moist forest sites with about 2 years of nearly continuous meas urements. Michael Goulden, Humberto R o c h a , Scott Miller, and their colleagues presented data from a fourth moist forest site collected over about 7 months. When analyzed using standard approaches, all these data suggest that the forest sites are absorbing 5 - 1 0 Mg-C ha" y .At these sites, calm conditions dominate at night; therefore, the flux estimates that depend upon turbulent conditions may b e inaccurate. When the data are s c r e e n e d to eliminate periods of low night-time turbulence, the high uptake rates are often—but not always— much smaller. In s o m e cases, small net emis sion rates may b e calculated. All researchers pointed out that the interpretation of noctur nal fluxes from eddy correlation data over for est at their study sites in the Brazilian Amazon region remains very difficult.The search for a potentially large error term in tower fluxes, noc turnal or otherwise, is not over yet. 2
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Vourlitis and Priante reported eddy correla tion data from sites in seasonally dry transitional forests in the data of Mato Grosso, Brazil.There, the forest appears to b e absorb ing carbon in the wet season and emitting carbon during the dry season. How do ground-based ecological studies help us constrain the capacity of the Amazon forest ecosystems to absorb carbon? Plinio Camargo demonstrated that C data may b e used to estimate diameter increases for trees during the period from 1965 to the present. For older trees, average-diameter growth may b e estimated through their entire life spans. R e c e n t growth rates for 4 individual trees studied ranged from 2 mm to 12 mm y . Over their full life spans, these trees averaged from < 1 to about 3 mm y" in diameter growth. If sufficient data c a n b e gathered, this approach should b e useful in reconstructing stand histories. 14
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Are trees growing more rapidly now than in the past? Based on detailed stand data, Jeff Chambers and his colleagues developed a model for woody biomass.Their results show that an increase in stand productivity by 25% ultimately results in an increase of 20% in woody biomass.This increase takes from 100 to 200 years to b e realized. Given the
Eos, Vol. 82, No. 37, September 11, 2001 above-ground woody biomass estimated by Michael Palace and his colleagues of about 140 Mg-C ha" , this would translate into an annual increment in the range of 0.3 to 0.6 Mg-C ha^y" . Should we b e looking below ground for car b o n sinks? To date, studies of soils in Amazon forests suggest that they have a limited capaci ty to absorb carbon [Trumbore,2000]. But there are many uncertainties regarding biolo gy underground. Using sequential coring techniques,Whendee Silver and her colleagues showed that live fine roots appear to turn over in about 1 year. However, Susan Trumbore and her colleagues found that fine root structural material was 6 - 1 9 years old. Can these results b e reconciled? 1
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How Else Do Amazon Forests Affect the Atmosphere? It is well known that the tropical forests of the Amazon region play a key role in regulat ing the water and energy budgets of the atmos phere. Evapo-transpiration a c c o u n t s for over 50% of the rainfall in the Amazon Basin and is particularly important for recycling water dur ing the drier times of the year.Tropical forests also have important c h e m i c a l effects. Alex Guenther pointed out that globally, vegetation produces about 1 Pg-C or volatile organic car b o n c o m p o u n d s (VOC). Plants release a daz zling variety of volatiles to the atmosphere. Jurgen Kesselmeir,Uwe Kuhn,and Jim Greenberg all measured strong light-dependent emissions of isoprene from vegetation. According to Greenberg, 2 5 - 4 0 % of all the tree species sur veyed emitted isoprene. All three researchers found evidence of light-dependent terpene emissions from Amazon forest trees. Surveys in many tropical regions show that the forest soils are strong sources of nitrogen oxides. Work presented by Franz Meixner, Christof Ammann, Andreas Gut, and their col leagues showed that although the forest soils produced considerable amounts of nitric oxide (NO), very little of this NO escaped the forest canopy layer. Especially during daytime, reac tion of NO with ozone ( 0 ) yields the reactive gas N 0 that deposits to vegetation surfaces. In contrast, b e c a u s e 0 concentrations were very low at night, NO could escape the c a n o p y layer. Measurements in the canopy layer of longlived gases by Patrick Crill and colleagues show that nitrous oxide ( N 0 ) and m e t h a n e (CH ) concentrations consistently correlated with C 0 concentrations at night.These results agreed with c h a m b e r measurements of a soil 3
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source for N 0 . However, these measurements present a surprise regarding methane. Local soils consumed methane, suggesting that there is another dispersed methane source in the forest. Important LBA research was presented out side of the special LBA sessions. In particular, results from the NASA Tropical Rainfall Mea surement Mission (TRMM) that focused on the Rondonia LBA-TRMM campaign were fea tured in sessions on hydrology Earle Williams, Danny Rosenfeld, M.O. (Andi) Andreae, and Paulo Artaxo contributed papers on the rela tionship between cloud condensation nuclei concentrations (CCN) and cloud physical properties. In contrast to other tropical conti nental regions, these researchers observed that the cloud structure during the wet season in Rondonia greatly resembles the o c e a n i c cloud regime. S o m e LBA-TRMM researchers began to call Amazonia the "Green Ocean." During the wet season campaign in January and February 1999, clouds were generally very shallow, had no ice phase, and very little, if any lightning. In contrast, during the dry season, clouds in Rondonia had a more continental appearance, probably b e c a u s e of the high lev els of CCN derived from biomass burning.The high CCN content of clouds may have impor tant c o n s e q u e n c e s for the ecosystem. Dry sea son clouds were very inefficient in producing precipitation.The high aerosol concentration continued to suppress precipitation in Amazo nia during the transition from the dry season to the wet season. 2
More Information Available The mainly e c o l o g i c a l themes presented here represent only a part of the LBA scientific activity. More information on the LBA project is available at http://www.lba.cptec.inpe.br. LBA maintains an o p e n data policy and a search and retrieval facility for meta-data (http://beija-flor.ornl.gov/lba). LBA w e l c o m e s new participants and contributions.Those interested in future participation in LBA should contact the LBA Scientific Steering Commit tee through the Web site noted above.
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Acknowledgments The authors thank B.Kruijt,C. Potter,!Cardille, PArtaxo,L.Guild, J. Melack,and PM.Crill for their helpful c o m m e n t s . T h e LBA Program is guided by the Brazilian Ministry of S c i e n c e and Technology (MCT).The LBA Ecology
Questions Raised about Benefits of Artificial Reefs PAGES 4 0 5 , 4 0 7 Twenty-seven subway cars lined on a barge awaited their fate off the coast of Delaware, about 26 kilometers east of the Indian River Inlet, on August 22. A priest blessed the dark-red cars donated by the New York City Transit Authority, and
prayed for the safety of all c r e a t u r e s using them.The song,"Sidewalks of New York" filled the festivities:"East side, west side, all around the town..." A woman tossed tokens from those subway lines into the sea. Then, down went the cars, having first been scrubbed clean, with windows removed for better circulation. Shoved by a bulldozer, the cars were deployed in 35
Project is sponsored by NASAs Terrestrial Ecology and Land Cover and Land Use Change Programs.
Authors Michael Keller, Complex Systems Research Center,University of New Hampshire, Durham, USA; Reynaldo Victoria, CENA-USr?Piracicaba, SP13416-000 Brazil; and Carlos Nobre, Centro de Previsao do Tempo e Estudos Climaicos, Cachoeira Paulista,SP 12630-000, Brazil. For more information, contact Michael Keller at
[email protected] or USDA Forest Service, International Institute of Tropical Forestry, Rio Piedras, Puerto Rico, USA.
References Asner, G. P, A. R.Townsend, and B. H. Braswell, Satellite observation of El Nino effects on Amazon forest p h e n o l o g y a n d productivity Geophys. Res. Lett., 27, 981-984,2000. Grace, J. et al., Carbon dioxide uptake by an undis turbed tropical rain forest in southwest Amazonia, 1992 to \993,Science, 270,778-780,1995. Hansen, M. C , R. S. Defries, J. R. G.Townshend, and R. Sohlberg, Global land cover classification at 1 km spatial resolution using a classification tree approach, Int. J. Remote Sens., 21,1331-1364,2000. Houghton, R. A., D. L. Skole, C. A. Nobre, J. L. Hackler, K.T. Lawrence, and W. H. Chomentowski, Annual fluxes of c a r b o n from the deforestation and r e g r o w t h in the Brazilian Amazon,Nature, 403, 301-304,2000. Nepstad, D. C , A.Verissimo, A. Alencar, C. Nobre, E. Lima, PLefebvre, PSchlesinger, C. Potter, PMoutinho, E. Mendoza, M. Cochrane, and V Brooks, Larges c a l e impoverishment of Amazonian forests by logging and t\re,Nature, 398,505-508,1999. Phillips, O. L., Changes in the carbon b a l a n c e of tropical forests: E v i d e n c e from long-term plots, Science, 282,439-442,1998 Skole, D.,and C.Tucker,Tropical deforestation and habitat fragmentation in the Amazon: Satellite data from 1978 to 1988, Science, 260,1905-1910,1993. Stroppiana, D.,S. Pinnock, and J.-M. Gregoire,The Global Fire Product: Daily fire o c c u r r e n c e from April 1 9 9 2 - D e c e m b e r 1993 derived from NOAAAVHRR d a t a , M J Remote Sens.,21,1279-1288,2000. Tans, PP, I.Y Fung, a n d T.Takahashi, Observational constraints on the global atmospheric C 0 budget, Science, 247,1431-1438,1990. Tian, H.,J.M. Melillo, D. W Kicklighter, A. D. McGuire, J. V K. Helfrich III, B. Moore III, and C. J.Vorosmarty, Effect of interannual climate variability on c a r b o n storage in A m a z o n i a n ecosystems, Nature, 396, 664-667,1998. Trumbore, S., Age of soil organic matter and soil respiration: Radiocarbon constrains on belowground C dynamics, Ecol.Applic, 70,399-411,2000. 2
minutes to join a heap of military vehicles, old tires, and other used materials already accu mulated at Reef Site 11, a 1.3-square-nauticalmile artificial reef. The ceremony "was a little bit surreal," said Jeffrey Tinsman, Delaware reef program coordinator with the state's Division of Fish and Wildlife. Over the next year, the division lans to deploy a total of 400 "retired" subway cars, out of 1300 the transit authority is unloading to interested states. South
Eos, Vol. 82, No. 37, September 11, 2001
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Fig. 1. Merged satellite and agricultural census data in Amazonia from the mid-1990s. The fusion technique uses regression tree analysis to represent the statistical relationship between existing global satellite land cover classifications and land use census data derived from each country of the Amazon and Tocantins basins. The resulting relationship between land use and land cover is computed at the county scale and projected into each 5-min (-10 km) cell. Preliminary analysis indicates that cropland and pasture covered 8.3 x 10 ha out of the 6.7 x 10 ha study region, with pasture accounting for 80% of the total agricultural area. (Figure courtesy of J. Cardille, University of Wisconsin, Madison.) 7
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