TAILIEUCHUNG - Springer Old Growth Forests - Chapter 7

Chapter 7 Biosphere–Atmosphere Exchange of Old-Growth Forests: Processes and Pattern Alexander Knohl, Ernst-Detlef Schulze, and Christian Wirth Forests are important agents of the global climate system in that they absorb and reflect solar radiation, photosynthesise and respire carbon dioxide and transpire water | Chapter 7 Biosphere-Atmosphere Exchange of Old-Growth Forests Processes and Pattern Alexander Knohl Ernst-Detlef Schulze and Christian Wirth Introduction Forests are important agents of the global climate system in that they absorb and reflect solar radiation photosynthesise and respire carbon dioxide and transpire water vapour to the atmosphere Jones 1992 . Through these functions forests act as substantial sinks for carbon dioxide from the atmosphere Wofsy et al. 1993 Janssens et al. 2003 and sources of water vapour to the global climate system Shukla and Mintz 1982 . Since old-growth forests differ in age structure and composition from younger or managed forests see Chap. 2 by Wirth et al. this volume the question arises whether these characteristics also result in differences in the biosphere atmosphere exchange of carbon water and energy of old-growth forests. This chapter reviews studies using two contrasting experimental approaches the eddy covariance technique and paired catchment studies. The eddy covariance technique is a micrometeorological standard method to directly quantify the exchange of trace gasses between forest ecosystems and the atmosphere by measuring up- and down-drafts of air parcels above the forest Baldocchi 2003 . Fluxes of scalars such as carbon dioxide water vapour as well as sensible heat can be inferred from the covariance between scalar and vertical wind speed Aubinet et al. 2000 . The advantages of this approach are that no disturbances or harvests are needed to assess fluxes and that the eddy flux tower typically integrates over a flux source area of approximately 1 km2. This approach however assumes that the underlying surface . the forest is horizontally homogeneous which is typically the case over managed even-aged forests. Old-growth forests however are often characterised by a dense and structured canopy including canopy gaps and a diverse range of tree heights see Chap. 2 by Wirth et al. this volume Parker et al. 2004

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