# Presentation: 906033

## Understanding Interannual Net Ecosystem Carbon Exchange Variability

-  in Tropical Rain Forest: A New Research Program at La Selva, Costa Rica
- Steven F. Oberbauer1, Deborah A. Clark2, Michael G. Ryan3, and David B. Clark2
-  1Florida International University, 2University of Missouri - St. Louis , 3Rocky Mountain Research Station,  U.S. Forest Service

- Strengths of the eddy covariance technique are its focus on net ecosystem C exchange and the relatively large forest footprint measured.  It has become clear, however, that major issues need to be addressed with respect to eddy covariance measurements, particularly in tropical rain forests.  A fundamental limitation is the prevalence of still-air conditions at night (>90% of nights, preventing accurate assessment of nighttime ecosystem respiration by this technique in these forests.
- _To evaluate the validity of the eddy covariance estimates of net ecosystem C exchange (NEE) in terms of their sign, magnitude, and climatic variation, eddy flux studies need to be compared to on-site ground measurements of the key processes in forest C cycling._  _Because of the large within-landscape spatial variation in tropical forest structure and function (200+ tree species, for example), spatial stratification and replication are needed to scale up plot-based estimates to the landscape level._
- In a new research program at La Selva, Costa Rica, we will directly address these challenges to move toward assessment of tropical forest carbon flux and how it is likely to respond to on-going climate change.  We will compare independent estimates of whole-forest photosynthesis, autotrophic and total ecosystem respiration, and NEE obtained by direct and indirect approaches:
-  highly replicated direct measurement of forest structure and function across the landscape
-  process-based modeling of daytime uptake and NEE based on these ground data
-  intensive cross-comparison of these data and model results with a 3-yr eddy covariance dataset
-  over strongly contrasting climatic conditions, including the 1997/98 mega-ENSO.
-  Major products from this research will include:

*Exploration of the climatic drivers underlying our 16-yr record of interannual variation in tree growth , using the MBL-GEM ecosystem process model.  * We will parameterize MBL-GEM using our intensive direct studies of the climatic responses of canopy photosynthesis and respiration in this forest.  We will then use the model and the long-term meteorological data to evaluate three potential explanations for the more than 2-fold interyear variation in tree growth measured in this ecosystem over the period 1984-2000.

- 3

- Proyecto CARBONO

- http://www.Carbono.org

_**STUDY SITE**_
**LOWLAND TROPICAL RAIN FOREST**
**LA SELVA BIOLOGICAL STATION, **
**      ****N.E. COSTA RICA, C. AMERICA**
**10****O ****26’ N, 84****O ****00’ W****	**
**ELEVATION: 37-150 M**
**MEAN  ANNUAL TEMPERATURE: 26 C**
**MEAN  ANNUAL PRECIPITATION: 4 M**

*Quantification of  biomass-weighted photosynthetic and respiration rates in an old-growth tropical rain forest, based on highly replicated direct measurements; assessment of the impacts of within-landscape variation in canopy structure and leaf physiology on these processes.*  At each of 40 sites stratified across the landscape, we will directly assess plant biomass, rates of photosynthesis and respiration, and their climatic responses, in all strata from the forest floor to the top of the canopy.

*Scaled-up estimates of total canopy photosynthesis (GPP) and total ecosystem respiration (R**e**) and their response to short-term climate variation for a tropical rain forest.  * We will develop estimates of GPP and Re based on (1) the field data from our chamber measurements of these processes on the ground-to-canopy vertical transects; (2) MAESTRO, a photosynthesis model;  (3) unbiased estimates of leaf area and the distribution of photosynthetic and respiratory capacity derived from our sampling and from extrapolation to the whole landscape based on remote sensing; and (4) intensive meteorological measurements on site

*Assessment of the adequacy of modeled responses of  forest C uptake to climate variability by comparing MAESTRO estimates to GPP estimates derived from eddy covariance measurements and estimates of soil and wood respiration*. Data manipulations and extrapolations are required to estimate GPP from the daytime and nighttime eddy covariance NEE estimates.  Similarly, process models of forest C uptake involve numerous assumptions and need to be tested for biological realism.   We will compare the GPP responses to climatic variation that are derived from the 3-yr eddy covariance dataset to those projected for the same time period by the multi-layer MAESTRO model.

*Inter-comparison of three independent approaches to estimating GPP, R**e**, and NEE for tropical rain forest.  *We will use cross-comparisons to assess estimates made by three contrasting, independent approaches: a) scaling up chamber measurements with process modeling; b) estimating forest NEE with eddy covariance techniques; and c) estimating component processes with data from ongoing studies of C cycling (aboveground biomass and biomass increment, fine root stocks, CWD stocks and inputs, fine litterfall, coarse woody litter, soil respiration, SOC stocks) obtained continuously since 1997 in the CARBONO plots.

_**FUNDING SOURCES**_
**NATIONAL SCIENCE FOUNDATION, **
**	**** ****INTEGRATED CARBON CYCLE RESEARCH **
**ANDREW W. MELLON FOUNDATION**

- Fig. 1 Tower sectoins for moblie tower.

- Fig. 2 Hypothetical mobile tower transect locations in old growth  forest.

- Fig. 3. Location of flux tower, windshed,  and biometry plots.

- Fig. 4 Eddy flux tower.

- Fig. 5  Eddy flux data from site, 1998-2000.

- Fig. 6 Long term tree growth record for 6 species of trees at La Selva.

- Fig. 7  Growth, biomass production, and mortality on biometry plots.