note that the magnitude of this estimated temperate zone sink
was to a large degree determined by the value estimated for the
tropical source term. Recent research to estimate the CO,
partitioning as a function of latitude and time from 1990 to
1993 indicated that the northern temperate zone was a large
terrestrial sink in 1992 and 1993, whereas the biosphere in
the northern tropics (from equator to 30°N) was a large source
of carbon (Ciais et al. 1995). The authors attributed this
difference to either a problem with their intra-hemispheric
transport between the tropical and temperate latitudes or that
previous deforestation estimates were lower than the actual
deforestation. It is also interesting to note that they estimated
that the southern tropics was a small terrestrial sink in 1992
and 1993. This is surprising because this latitudinal band
contains Brazil and Indonesia. Estimates of CO, emission
from tropical land use rank these two countries as the top two
in terms of total emissions due to deforestation. The weak
terrestrial sink in the southern tropical zone suggests that
either the forests have a positive net ecosystem production
(due to fertilization of undisturbed forest and/or secondary
growth formation) or previous estimates of deforestation have
been overestimated (Ciais et al 1995).
The current net flux of carbon between the biota and the
atmosphere due to land use change is uncertain. Three factors
contribute to this range of uncertainty: (1) rates of
deforestation, (2) the fate of deforested land (i.e. the amount of
secondary forest regrowth and re- clearing), and (3) the amount
of biomass and soil organic matter and their response to
disturbance, including anthropogenic reductions of carbon
stocks due to forest thinning or degradation. Models
developed with improved geographic and temporal data on
deforestation rates, better parameterization of the dynamic
nature of deforestation and reforestation, and improved data on
above- and below- ground carbon response characteristics are
needed to more accurately estimate net biotic carbon flux.
1.2 Rate of Tropical Deforestation.
The current rate of deforestation is unknown. There are only a
few estimates of tropical deforestation available and they may
be in error by as much as 50 percent. Moreover, without
exception all published sources of data on tropical
deforestation have come in non-spatial, tabular form.
Therefore, both the rate and geographic distribution of this
critical forcing parameter needs to be developed in a way
which is objective, quantitatively reproducible, and useful as
an input dataset for numerical carbon models.
The FAO Forest Resource Assessments for 1980 and 1990
estimated that Southeast Asia comprised 22% and 21% of the
global deforestation in closed tropical forests from 1971-
1980 and 1981-1990, respectively (FAO 1993). Estimates by
the World Resources Institute suggest the rate of deforestation
in some Southeast Asian countries has more than doubled
since the late 1970s (e.g. Indonesia). But there is considerable
uncertainty in these numbers due to the lack of a precise and
consistent methodology. Two additional reports, one by an
478
independent consultant to Friends of the Earth and another by
the World Resources Institute suggest the rate of deforestation
in closed forests has increased by 100-140% since the late
1970s. However it is evident from inspection of these data
that there are large differences between these two sources, and
at the level of individual countries they are frequently greater
than the large difference for their global totals.
1.3 Modeling carbon flux.
The simplest way to calculate the net flux of carbon from
deforestation is to multiply the area deforested by the average
difference in carbon stocks between forests and cleared land.
Such a calculation, however, does not take into account the
time lags associated with the release of carbon from the long-
term decay of soil organic matter and dead wood and plant
material. Nor does it take into account the accumulation of
aboveground biomass in secondary vegetation or the
accumulation of soil carbon as a long- term response to
pasture formation. A more accurate approach is to develop a
dynamic model of changes in carbon after disturbance, which
takes advantage of large spatial data bases on deforestation
and ecosystem distributions. This approach would utilize
remote sensing as the source of geographically-referenced
deforestation data, along with geographically-referenced data
on vegetation and soils which could be parameterized through
detailed in- situ measurements in different environments. Until
recently, it has been impossible to know the precise location
of deforestation (since the primary source of data has been
tabular statistics), thus prohibiting the accurate assignment of
deforestation to vegetation and soil types, and, hence,
biomass and soil carbon stocks. An aggregate approach also
lacks the ability to define temporal shifts in the types of
ecosystems disturbed. The Landsat Pathfinder HTF project
focused on creating spatially explicit and temporally
consistent data base on extent and rates of tropical
deforestation. This database is ideally suited for use in models
to estimate average annual flux of carbon due to tropical
deforestation over the past two decades.
2. PROJECT OBJECTIVES
In more specific terms, NASA LANDSAT Pathfinder Humid
Tropical Inventory Project has three major objectives:
e Utilize LANDSAT data to map deforestation in closed
tropical forests at sub-kilometer resolution for a base
period (e.g. 1986);
e Utilize LANDSAT data to quantify and map the rate of
deforestation by mapping the change from other time
periods (eg.1973, 1992); and
* Create a Landsat data set and science products for
distribution in a digital geographic information system
(GIS) format, and develop an information management
system (IMS) to manage data orders, archiving and
processing and distribution of products.
International Archives of Photogrammetry and Remote Sensing. Vol. XXXII, Part 7, Budapest, 1998
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