Replication of CO2 Flux in Deciduous Broad-Leaved Forests by Multi-Layer Canopy Model

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1 Replication of CO2 Flux in Deciduous Broad-Leaved Forests by Multi-Layer Canopy Model Research Institute Hajime Utsugi, Kenzo Kitamura, Nagaharu Tanaka, Tadashi Sakata, Hiroyuki Tobita (Hokkaido Research Center), Yuichiro Nakai, Tsutomu Watanabe (Department of Meteorological Environment), Shigehiro Ishizuka (Department of Forest Site Environment) Forestry and Forest Products Research Institute Background and Purpose The quantity of carbon dioxide (CO2) absorption of forest ecosystems is a delicate balance between the absorption by leaf photosynthesis and emission by tree respiration (leaf, stem, and root) and decomposition of soil organic matter, and is strongly influenced by the changing climate every second. In order to predict the change of CO2 absorption of forest ecosystems due to climate change, we carried out a year-round observation on CO2 flux (tower flux) between the forest and atmosphere using a tower, and observation on the different processes of photosynthesis and respiration in deciduous broad-leaved forests, which is mostly white birch and Quercus crispula. With the simulation results of CO2 flow obtained by using a multi-layer canopy model, where the processes of photosynthesis and respiration were introduced, we were able to roughly replicate the tower flux. Achievement CO2 Flux between the Atmosphere and Forest With a 40-meter high tower installed at the Hitsujigaoka Experimental Forest of the Hokkaido Research Center, Forestry and Forest Products Research Institute, we measured the tower flux at 30-minute intervals (Figure 1). Beginning from CO2 being absorbed rapidly during foliation and after leaves have grown thick, the quantity of absorption changed with the variation of solar radiation. When the leaves began to fall, the absorption rapidly decreased. During the snow season, a small amount of CO2 was emitted from the forest into the atmosphere (Figure 4). According to this method, the quantity of annual CO2 absorption of the forest ecosystem was about 3.4t/ha with carbon conversion. Estimation of Gross Photosynthesis

2 The number of leaves in the forest changes depending on the height of every tree species coupled with the fact that the lower the leaf layer the darker it is, after the mathematization of the relationship between light and warmth, humidity and photosynthesis rate by tree species, season, and stratum and the simulation of the canopy photosynthesis, the fixed CO2 (gross photosynthesis) was estimated by photosynthesis over a year to come to about 18.5t/ha with carbon conversion (Figure 2). Carbon Emission Rate from Leaves, Stems, and Forest Soils The stems were put in a transparent chamber (Figure 3b) and CO2 emission rate (respiration rate) was measured. We obtained the annual emission amount from the relationship among temperature, growth, and stem surface areas, and estimated that the annual carbon amount released by respiration of leaves and stems were about 3.8t and 2.3t per ha, respectively (Figure 2). On the other hand, CO2 emission rate from forest soils was measured continuously (Figure 3c). From the short time measurement during different seasons at 100 points in 20m intervals, we estimated the annual emission of each stand by considering the horizontal variation (Figure 2). CO2 Budget Simulation by the Multi-Layer Canopy Model Using the multi-layer canopy model, where the processes of measured photosynthesis and respiration were introduced, as well as the simulation results of the flux, we were able to roughly replicate the measured CO2 flux observed in the tower (Figure 4). This result is helpful to estimate and to predict CO2 absorption of forests based on the daily weather information. Moreover, it also helps to predict the absorption variation due to climate change such as global warming etc. However, there were some problems about CO2 emission rate during the night and snow season. So it is necessary to improve accuracy and promote cross-validation in the future. This research was supported by the Research about Parameterization for Terrestrial Ecosystem Model, Research Revolution 2002 (RR2002), Ministry of Education, Culture, Sports, Science and Technology.

3 Figure 1 The Hitsujigaoka Experimental Forest and 40-meter high flux observation tower Figure 2. Carbon budget model in the Hitsujigaoka Experimental Forest and carbon budget by flux tower observation. (Average value for 2002~2003) The ecosystem respiration is the total carbon emission of the ecosystem estimated from tower observation at night.

4 Figure 3a. Measurement of photosynthesis Figure 3b. Measurement of stem respiration Figure 3c. Measurement of soil respiration

5 Figure 4. Variation of annual CO2 flux (On a day-to-day basis) Negative value represents absorption. DOY: Time that has elapsed since January days represents July 19. Straight line: Tower observation value Dot: Model calculated value