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ND-10 Abstract

An Integrated Use of Experimental Modeling and Remote Sensing Techniques to Study Carbon and Nutrient Dynamics During Tropical Land Use Change

Gregory Paul Asner — Carnegie Institution (US-PI)
Mercedes Bustamante — UnB - Universidade de Brasília (SA-PI)
Alan R. Townsend — University of Colorado (US-PI)

This project began prior to the LBA call for proposals, and is funded by NASA's New

Investigator Program. Thus, this abstract will be slightly different than most of the

others in that we will briefly outline relevant work already accomplished as well as list

our future plans. The majority of our work is in the Santarem/Tapajos region (lat/long:

3S, 56W).





 Project Goals





 The overall objectives of this project can be divided into three main areas:





 1) Process-level measurements of biogeochemical patterns following conversion of

moist tropical forest to pasture, with an emphasis on changes in P, Ca, Mg, K and their

effects on C and N cycles. This work is being done along two pasture-age gradients south

of Santarem; one on high clay soils (avg. 54% clay), and one on extremely sandy soils

(avg. 92% sand).





 2) Remote sensing- Biogeochemistry links, including (a) regionalization of

pasture biogeochemical information using high-spatial resolution Landsat TM data and a

radiative transfer inverse modeling technique, (b) analysis of seasonal trends in pasture

vs. forest canopy greenness and energy absorption using an AVHRR spectral unmixing method,

and (c) analysis of the biotic response of the uncleared areas of the Amazon Basin to

inter-annual climate variability.





 3) Estimates of how tropical land conversion affects the use of atmospheric 13CO2

data as a tracer of regional fluxes in the global carbon cycle.





 Brief Synopsis of Progress to Date







As mentioned above, we are focusing on the Santarem region, where we have established

several sites that comprise pasture age gradients (2-15 years old) on two soil types (sand

and clay). Data from these sites show a suite of biogeochemical changes as pastures age,

including substantial losses of soil organic matter, N, P, Ca, Mg and K. In addition,

while both soils demonstrate these declines, there are significant differences at every

pasture age between the soil types. Preliminary foliar data from pasture grasses suggest

that the strongest increases in nutrient limitation are in N, P and Ca.





Early results from radiative transfer (RT) model inversions using both field-collected

spectral data and satellite optical data suggest that changes in productivity (NPP) and

biomass which accompany changes in biogeochemistry can be resolved at regional scales

using satellite data, thereby allowing much more meaningful spatial and temporal analyses

of land use effects on carbon and nutrient cycles than can be approached using standard

remote sensing classification or ground techniques alone. We are also finishing a

basin-level analysis of inter-annual variability in NPP, linked to precipitation anomalies

associated with the El Nino Southern Oscillation (ENSO). These results show significant

inter-annual variability of Amazon moist forest physiology and carbon uptake which, when

combined with carbon efflux data from land cover / land use change analyses by other

groups, may change our perspective on spatial and temporal variation in sources and sinks

of CO2.





Finally, we have used data and modeled simulations of changes in 13C of

respired CO2 following deforestation, along with continental-scale databases of

land cover change for the last 30 years (eg. Houghton's data), to estimate the current

imbalance in atmosphere-biosphere exchange of 13C that is due to tropical land

conversion. We have then made preliminary calculations of how this imbalance, or

"dis-equilibrium", affects atmospheric 13C-based estimates of global

terrestrial vs. oceanic net fluxes of CO2 (cf. Ciais et al 1995, Francey et al.

1995). Our preliminary results suggest that this land-use dis-equilibrium must be taken

into account, as incorporation of our estimate into the Francey et al (1995) analysis

changed their partitioning of terrestrial vs. oceanic exchange by an average of 0.65

GtC/yr. This change manifests itself primarily as an increase in CO2 uptake by

terrestrial ecosystems in the tropics.





 Current Activities (spring 98)







 1) Continued work on the biogeochemistry of our pasture sites, including

Hedley-style fractionations of soil P, potential N mineralization, and foliar lignin

analyses.





2) RT inversions of TM data to see if the limited spectral information in the latter

can still provide information on changes in biomass and NPP with pasture age





3) Regional estimates of pasture LAI and NPP at TM scale-resolution for the

Santarem/Tapajos region







Future Plans (summer 98 - spring 00)







 Pasture Biogeochemistry:





We are planning at least 3 more trips to the Tapajos sites; these trips will be timed

so that we can collect data in both dry and wet seasons. We plan to supplement the data we

have on soil and foliar C, N, P and base cation content with repeated measurements of

several more dynamic variables, including using ion exchange resin bags to measure

availability of N, P and base cations in both wet and dry seasons in all pasture sites. We

will also measure dissolved organic nitrogen (DON) and phosphorus (DOP) in soil solution

in both wet and dry seasons, and plan to install several Prenart tension lysimeters in a

subset of the pastures as a pilot experiment to look at leachate losses of all the above

elements. In addition, we are planning a series of small-scale fertilization experiments

in forest and pasture sites to investigate microbial nutrient limitation. Finally, we are

planning a number of isotopic measurements aimed at understanding rates of organic matter

turnover and loss in these pastures. In collaboration with Jason Neff (Stanford Univ.), we

will measure 13C and 14C of bulk soil carbon, fractionated SOM, soil

CO2, and dissolved organic carbon (DOC) in forests and pastures on both soil

types. We will also repeat the foliar analyses we've already done in the wet season, and

will measure aboveground biomass of pasture grasses in both wet and dry seasons.





 Remote Sensing:





In addition to continued work on the projects outlined above, we are actively planning

to use MODIS, MISR and ASTER data when it becomes available in 1999. We intend to

spectrally unmix the MODIS data to recover pasture and forest reflectance signatures. Due

to the daily 1km-resolution coverage of MODIS, we think that the analyses of land-cover

phenology and NPP will be greatly improved using this instrument. MISR data will aid in

determining the degree of aerosol and water vapor contamination which has been a constant

problem with the currently available AVHRR data. Using both MODIS and MISR, we will

improve estimates of greenness, canopy energy absorption, and NPP. Concurrent AVHRR data

may also allow for retrocalibration of at least the more recent AVHRR data.





ASTER data will be used in conjunction with Landsat 7 data to continue the inverse

modeling--biogeochemistry effort. Both of these datasets will be critical for spectral

unmixing of the MODIS data. Unmixed MODIS data also has the potential to be used for RT

inverse modeling, particularly due to its SWIR (1300-2500 nm) spectral coverage (Asner et

al. in review). The SWIR range contains significant information on bare soil extent and

canopy litter fraction (Asner 1998), and we are working as part of the Schimel EOS IDS

group to develop the algorithms needed to retrieve critical pasture canopy characteristics

using unmixed MODIS data.

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