References

1. FOREST STRUCTURE AND THE CANOPYLIGHT ENVIRONMENT IN THE TAPAJÓS NATIONAL FOREST • Geoffrey G. Parkera, David R. Fitzjarraldb, and Irene Cibelle Gonçalves Sampaioc • aSmithsonian Environmental Research Center, Edgewater MD, parkerg@si.edu • bUniversity at Albany, SUNY, fitz@asrc.cestm.albany.educLBA-ECO, Santarem, cibelle@lbasantarem.com.br Atmospheric Sciences Research Center Abstract To understand the interaction between canopy structure and light environment in primary moist forest at km 67 in the Tapajós National Forest, Brazil (2°51' S, 54°58' W), we combined continuous high-frequency pyranometer and quantum sensor measurements above the canopy and with an array of sensors on the forest floor with observations of canopy structure made with a portable LIDAR system deployed from the forest floor. We describe the whole canopy budget of Photosynthetic Photon Flux Density (PPFD), the canopy reflectance and understory penetrance of PPFD. We examine the dependence of penetrance and canopy reflectance on solar elevation angle, season, and sky conditions. To describe the response of the light environment to sky conditions (“brightness”) and incoming PAR flux we constructed transfer functions for canopy reflectance and penetrance based on these variables, on a half-hourly basis. With these functions and a record of incoming radiation we constructed canopy radiation budget. From the distribution of local maximum heights (the hypsograph), obtained with the ground-based LIDAR system, we estimate the mean vertical pattern of within-canopy penetrance and absorbance. This forest receives only 71.2% of the potential light – clouds, smoke and haze are important considerations. Very little PAR is reflected from the canopy (2.1%) or transmitted to the understory (0.8%), thus PAR absorbed by the canopy is very high, 97.1% (10,709 mol m-2 ). Finally, because of the porous and rugose nature of the canopy structure, we estimate that the mean transmittance profile is gradual and PAR absorption occurs over a wide height range. Photosynthetic Light Environment Atop the km 67 tower quantum sensors facing up and down and at the forest floor below a pentagonal array of 16 pyranometers and a PAR sensor acquired measurements every 5 seconds. All pyranometers were intercalibrated and calibrated for PAR. We formed half-hour averages from the above-canopy sensors and the understory array to construct transfer functions for estimating canopy penetrance and reflectance as functions of atmosphere clarity and actual incoming radiation. The brightness categories (similar to the clearness index) are defined relative to modeled clear conditions are 1 (<30% of potential), 2 (30-60%), 3 (60-90%), and 4 (>90%). Using these functions we then estimated canopy reflectance, penetrance to the ground, and absorption (= actual incoming - reflectance – penetrance) for the year of 2003. Date missing from km67 were estimated from the nearby pasture site at km77. Both canopy penetrance and reflectance are somewhat higher for the same level of incoming PAR under lower brightness, such as in cloudy, smoky, or hazy conditions – this supports the suggestion that diffuse light may be more efficient than direct for canopy penetration. However, since these situations are associated with low light, the effect on PAR flux is small. Canopy penetrance functions Canopy reflectance functions Pentagonal array of light sensors (above), with high frequency data collection, located at the base of the km 67 tower (left) annual variation (A) diurnal variation by season (B) Canopy structure A ground-based LIDAR (the Portable Canopy LIDAR, Parker et al. 2004a) was used to estimate the vertical and horizontal distribution of canopy surface area. From these we constructed the cumulative distribution of local maximum heights (the hyspograph, A), whose shape has been shown (Parker et al. 2004b) to be related to the shape of the mean transmittance profile (B). From this curve, we estimated the vertical distribution of absorbed PAR (C). Locations with high PAR aborbance cover a broad vertical range. annual budget (C) C B A The annual variation in potential and actual PAR input (A) is controlled by seasonal patterns in sun angle and sky conditions. The actual PAR input (10,709 mol m-2) is only 71.2% of the potential amount under clear skies. The diurnal variation in mean PAR flux for the major components did not differ much among the dry (days 195-319), wet (15-135), and transition seasons, although the wet season has less flux (B). The diurnal pattern of reflectance follows closely that incoming radiation, however penetrance is limited in the early morning and late afternoon, an interaction between high canopy density and the sun angle. On an annual basis (C), very little PAR reflects from or penetrates through the canopy, almost all is absorbed. References Parker, G.G., Harding, D.J., and Berger, M.L. 2004a. A portable LIDAR system for rapid determination of forest canopy structure. Journal of Applied Ecology 41:755-767. Parker G.G., Harmon M.E., Lefsky M.A., Chen J., Van Pelt R., Weiss S.B., Thomas S.C., Winner W.E., Shaw D.C. and Franklin J.F. 2004b. Three dimensional structure of an old-growth Pseudotsuga-Tsuga canopy and its implications for radiation balance, microclimate, and atmospheric gas exchange. Ecosystems 7:440-453. Acknowledgements This project was part of LBA ECO project CD-03 and was supported with grants from NASA to the University at Albany, SUNY (subcontract NNG-06GE09A and NCC-5692) and by the Smithsonian Environmental Research Center. We thank Luiz Medeiros (Federated University of Santa Maria), Júlio Tóta (LBA Project Office, Manaus), Alexander E. Tsoyref (SUNY Albany), and Ralf Manfred Staebler (SUNY Albany) for help in the field.