B41C-01 08:15h
Slow Growth Rates of Amazonian Trees: Consequences for Carbon Sequestration and Forest Management.
Growth rates for tropical forest trees estimated from radiocarbon ages and dendrometer measurements illustrate differences in forest age and structure among three sites located in the eastern, central and western Amazon basin. Although growth rates vary dramatically among individual trees\, overall the slowest growing trees (averaging \sim0.1mm yr$^{-1}$ as opposed to 0.3mm yr$^{-1}$ diameter increment) are found in the central Amazon. Small individuals (DBH \<30cm) have slower growth rates than larger diameter trees, and trees in this size class with radiocarbon ages $<$500 yr are encountered at all sites. Only \sim2MgC ha$^{-1}$ year$^{-1}$, or \SIM7% of annual photosynthesis, is allocated to growth of living wood at the eastern and central Amazon sites. Rates of C allocation to stem growth are similar across the three sites we studied because slowest growth occurs at the central Amazon site that has highest stem density and greatest biomass. Extrapolating our growth increment data to forest stand, we estimate the mean age of individual trees is \SIM350 years in the central Amazon but \SIM200\-250 years in the other two areas. The mean age of C making up the trees has a smaller range of \SIM250\-310 years, because of the greater fraction of biomass in larger individuals in the eastern and western Amazon sites. These residence times for C are longer than those of 100\-180 years obtained by simply dividing the total biomass C by the rate of C allocation to new wood for the same reason. We estimate that $<$20% of trees at all sites should have ages $<$300 years, and that maximum tree ages of $<$1000 years, though not common, are in accord with the growth rates we find. The fact that many Amazon trees attain ages greater than several centuries should be accounted for in management practices in these forests.
B41C-02 08:30h
The residence time of carbon in Amazonian primary forests
The residence time of carbon is a major determiner of the capacity of an ecosystem to function as a source or sink of carbon. The overall residence time of carbon in primary forests is determined by (1) what fraction of photosynthetic products get respired quickly and (2) the residence time of C allocated to living plant tissues, and (3) the time each of these components takes to decay, including what fraction is oxidized to CO2 versus what becomes stabilized in soil organic matter. Using radiocarbon to determine the age of carbon in various pools in forests, we conclude that: (1) carbon use efficiency of these forests is low, with ~70% of photosynthetic products respired within a year, and only 30% allocated to growth of wood, root and leaf tissues; (2) carbon resides on average for 2-3 years in leaves and 3-10 years in fine roots; very rapid or ephemeral root turnover is assigned in our budgets to autotrophic respiration (3) the mean age of carbon in living trees is longer (200-260 years) than the mean residence time of carbon derived from the biomass stock divided by annual wood growth increment (40-100 years) because most of the biomass is in the largest, fastest growing, trees, while most of the individuals are smaller, slower growing, shaded trees; (4) decomposition rates are rapid, but potentially recycling of carbon in the microbial community leads to a significant decadally cycling pool in near-surface organic matter. We will summarize these findings and use them with models of carbon dynamics to estimate carbon storage and loss potential on interannual to decadal timescales. The overall age of heterotrophically respired carbon (carbon derived from microbial decomposition) is 6-10 years, with much of the time lag due to the time spent by C in living leaf and root tissues. Even when combined with 70% autotrophically respired C with residence times of <1 year, this significant time lag can lead to large interannual variation in net ecosystem exchange given relatively small, regionally coherent, shifts in gross photosynthesis. On decadal and longer timescales, the dynamics of wood and fast-cycling soil organic dominate the capacity for C storage under scenarios such as CO2 fertilization, or recovery from periodic disturbance.
B41C-03 08:45h
Forest Gap Formation Dynamics and Carbon Balance at the Landscape Scale
The carbon balance of landscapes is critically dependent on the frequency and spatial distribution of large canopy gaps ($<$ 400 m$^{2}$) which are difficult to quantify using traditional forest inventory methods. For this project, both modeling and remote sensing investigations were employed to better understand how carbon cycling in Amazon forests varies with large gap dynamics. A site specific model was parameterized based on recent studies from across the entire Amazon basin, and modified to include spatial and temporal variability in disturbance rates, including data from a logging experiment in the Central Amazon. Remote sensing investigations were employed to quantify spatial variability in large gap dynamics. Due to the combination of large quantities of downed wood and upturned soil, large gaps have unique spectral features than can be identified using hyperspectral imaging methods. Large gaps (blowdowns) were clearly identified on both IKONOS and HYPERION images for the Central Amazon near Manaus. HYPERION data were minimum noise fraction (MNF) transformed to reduce spectral dimensions for analyses, and pixels representing pure blowdown endmembers were extracted. Spectral unmixing was carried out using a robust method, and the fractional abundance of large gaps in each pixel was mapped using a conservative threshold of 30%. Additional HYPERION images were obtained for other Amazon sites, and a similar spectral unmixing of large gap fraction was carried out. Results demonstrate that carbon balance dynamics at the landscape-scale can be quite different than those measured on relatively small forest inventory plots, and underscore the important of improving our understanding of gap formation dynamics.
B41C-04 09:00h
Forest Canopy-Atmosphere Gas Exchange Rates in the Tapajos National Forest, Para, Brazil, Determined by Radon-222 Measurements
Continuous canopy air and soil-air flux measurements of radon-222 have been combined to quantify canopy-atmosphere gas exchange rates and canopy air residence times in Amazonian old growth and selectively logged forests in the Tapajos National Forest near Santarem, Para, Brazil, as part of the LBA project led by Brazil. Radon canopy air and soil flux measurements, when fully integrated with LBA studies led by other investigators including tower eddy covariance fluxes, forest canopy gas inventories and soil gas fluxes can provide independent quantification of gas production, consumption and within the canopy plus net canopy-atmosphere fluxes. Canopy and above-canopy air radon activities at up to ten tower elevations at forest sites decrease systematically with height above the soil surface. Diel radon activity variations in the Tapajos forest canopy at both sites are characterized by dual maxima peaking near approximately 0900 and 1730 local time that occur respectively as a result of nocturnal stratification and late afternoon stratification during the early evening transition. Radon inventories within the lower 10m of the forest canopy typically range by over 200 percent over a diel cycle. Soil-air radon fluxes were determined using portable radon fluxometers capable of repeated thirty-minute flux measurements on soil collars installed around the tower sites. Radon flux divergence within the forest canopy can be utilized to quantitatively determine the net rates and canopy vertical distribution of CO2 or methane and other trace gas production and consumption processes when combined with their soil flux and canopy profile measurements. The combined canopy and soil flux radon data also yields canopy air residence times throughout the diel cycle.
B41C-05 09:15h
Boat-Based Eddy Covariance Measurements of CO2 and H2O Exchange Over Amazon and Tapajos Rivers and Lakes
Recent reports suggest that gas evasion of carbon dioxide from the Amazon river and its tributaries to the atmosphere may play an important role in the regional carbon budget. These gas transfer rates were estimated using air-water concentration gradients and gas transfer coefficients (piston velocities) derived from floating chamber measurements. Chamber techniques have inherent uncertainties due to their effect on the near-surface air turbulence. The micrometeorological technique of eddy covariance is attractive since it is a direct measurement of gas exchange and samples over a much larger area. In August 2004, we mounted equipment on a small riverboat to measure CO2 and H2O fluxes from the rivers and lakes near Santarem, Para. The motion of the boat was recorded using an inertial measurement package combined with a GPS receiver, and subtracted from the measured winds. We experimented with both thin-wall Teflon tubing and headspace equilibrators to measure the concentration of CO2 in the water continuously. Our sampling strategy included both "under-way" measurements and stationary (moored) 24-hour measurements on the Amazon and Tapajos rivers, and lakes connected to these rivers. CO2 concentration in the Amazon river and a connected lake was 3000-5000 ppm, much higher than the Tapajos river and a connected lake (range 400-1200 ppm). The "signal-to-noise" ratio was therefore greater for fluxes measured above the Amazon. Preliminary calculations indicate fluxes of order 1-2 micromoles/m2/s over the Amazon and 0.6-1 micromoles/m2/s over the Tapajos, and the calculated gas transfer velocity agrees with existing ocean-based parameterizations.
B41C-06 09:30h
Mesoscale Circulations and Atmospheric CO2 Variations in the Tapajos Region, Para, Brazil
We have investigated mesoscale variations of atmospheric CO2 over a heterogeneous landscape of forests, pastures, and large rivers during the Santarem Mesoscale Campaign (SMC) of August 2001. The variations of atmospheric CO2 concentration were simulated using the Colorado State University (CSU) Regional Atmospheric Modeling System (RAMS) with 4-level nested grids that included a 1-km finest grid centered on the Flona Tapaj"rs. Surface fluxes of CO2 were prescribed in the model using idealized diurnal cycles over forest and pasture vegetation derived from flux tower observations, and over surface water using a value suggested by in situ measurements in the Amazon River. Heterogeneous vegetation types were derived from the 1-km International Geosphere-Biosphere Programme (IGBP) land-cover dataset version 2.0. Our simulation ran from the 1st through the 15th of August 2001, which was concurrent with the SMC. Evaluation against flux tower observations and the SMC field measurements shows that, in many respects, the model captures observed meteorological variables and CO2 concentrations reasonably well. The results also demonstrate that the local topography, differences in roughness length between water and land, the "T" shape juxtaposition of Amazon and Tapaj"rs Rivers, and the resulting horizontal and vertical wind shears, all facilitated the generation of local mesoscale circulations. Possible mechanisms producing a lower level convergence line near the east bank of the Tapaj"rs River during strong trade-wind conditions are also explored. Our modeling study is helping us to understand observed patterns of CO2 fluxes and concentration distribution obtained from flux towers and light aircraft.
B41C-07 09:45h
Modeling Studies of Carbon Cycling at the Tapajos National Forest using the NASA-CASA Ecosystem Model
The NASA-CASA model is being compared to measurements of energy, water, and carbon exchange at Tapajos National Forest (TNF) tower sites. Daily and monthly model estimates of plant water flux and soil water content, ecosystem productivity, biogeochemical processes, trace gas emissions, and net carbon sequestration are evaluated here for prediction errors and seasonal trends. High resolution (less than 1-km) land cover images for 'footprint' areas of LBA tower sites are being used to define ecosystem model estimates and validation of predictions against measured tower fluxes of carbon and water exchange. Initial model results replicate the seasonal patterns in measured NEE fluxes at km 67, and are consistent with the findings that trees in the eastern Amazon are deeply rooted and their carbon fluxes are not highly stressed during relatively dry seasonal periods. Increased woody debris from past disturbance events could increase the CASA model's predicted loss of carbon to the atmosphere (increase NEE flux). Hence, LBA ecosystem models must better account for recent changes in the forest floor and soil carbon pools at tropical forest sites, specifically those related to disturbance.
http://geo.arc.nasa.gov/sge/casa/