Springer Old Growth Forests - Chapter 7

Chapter 7 Biosphere–Atmosphere Exchange of Old-Growth Forests: Processes and PatternAlexander Knohl, Ernst-Detlef Schulze, and Christian WirthForests are important agents of the global climate system in that they absorb and reflect solar radiation, photosynthesise and respire carbon dioxide and transpire water
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Springer Old Growth Forests - Chapter 7Chapter 7Biosphere–Atmosphere Exchangeof Old-Growth Forests: Processes and PatternAlexander Knohl, Ernst-Detlef Schulze, and Christian Wirth7.1 IntroductionForests are important agents of the global climate system in that they absorb andreflect solar radiation, photosynthesise and respire carbon dioxide and transpirewater vapour to the atmosphere (Jones 1992). Through these functions, forests actas 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 andcomposition from younger or managed forests (see Chap. 2 by Wirth et al., thisvolume) the question arises whether these characteristics also result in differencesin the biosphere atmosphere exchange of carbon, water, and energy of old-growthforests. This chapter reviews studies using two contrasting experimental approaches:the eddy covariance technique, and paired catchment studies. The eddy covari-ance technique is a micrometeorological standard method to directly quantify theexchange of trace gasses between forest ecosystems and the atmosphere by mea-suring up- and down-drafts of air parcels above the forest (Baldocchi 2003). Fluxesof scalars such as carbon dioxide, water vapour as well as sensible heat can beinferred from the covariance between scalar and vertical wind speed (Aubinet et al.2000). The advantages of this approach are that no disturbances or harvests areneeded to assess fluxes and that the eddy flux tower typically integrates over a fluxsource area of approximately 1 km2. This approach, however, assumes that theunderlying surface, i.e. the forest, is horizontally homogeneous, which is typicallythe case over managed, even-aged forests. Old-growth forests, however, are oftencharacterised by a dense and structured canopy including canopy gaps and a diverserange of tree heights (see Chap. 2 by Wirth et al., this volume; Parker et al. 2004).Additionally, in many parts of the world, old-growth forests occur mainly incomplex often sloped terrain of mountain ranges, which are less favourable oraccessible for anthropogenic land use [see Chaps. 15 (Schulze et al.) and 19 (Franket al.), this volume]. This raises the question of how these characteristics of old-growth forests affect the direct measurement of biosphere atmosphere exchange ofC. Wirth et al. (eds.), Old‐Growth Forests, Ecological Studies 207, 141DOI: 10.1007/978‐3‐540‐92706‐8 7, # Springer‐Verlag Berlin Heidelberg 2009142 A. Knohl et al.carbon, water, and energy. With the second approach, i.e. paired catchment studies,only water exchange is quantified. This is done by comparing the streamflow oftwo catchments that are similar with respect to soil, topography and climate but ´differ in land use or vegetation cover (Andreassian 2004). The method is suited tothe study of differences in evapotranspiration and water yield between contrastingland-use types, forest developmental stages, and management strategies. Topo-graphic complexity per se does not pose a problem. However, this comes at theexpense of a lower temporal resolution and the need for multi-year calibrationperiods. In this chapter, we summarise results from studies in old-growth forests acrossthe globe in order to (1) describe structural characteristics of old-growth forestsrelevant for biosphere atmosphere exchange (Sect. 7.2); (2) show how thesecharacteristics influence net ecosystem carbon fluxes (Sect. 7.3); (3) investigatethe interplay between canopy structure, water, and energy fluxes (Sect. 7.4); and(4) study the absorption of radiation, particularly of diffuse radiation in old-growthforests (Sect. 7.5).7.2 Characteristics of Old-Growth Forests Relevant for Biosphere–Atmosphere ExchangeWhen forest ecosystems advance in age they typically undergo changes in theirstructural properties (see Chap. 2 by Wirth et al., this volume). Old and large treesare more at risk to external forces such as disturbance by wind or by rotting of the ˆheartwood due to fungal attack (Dhote 2005; Pontailler et al. 1997). As a conse-quence, individual trees, or parts of trees, sporadically die resulting in small scalecanopy gaps (Spies et al. 1990). These gaps then supply light to lower parts of thecanopy that were previously in shade. With this light supply, individuals previouslylimited by light are able to enhance their growth and finally close the canopy gap. Inold-growth forests gaps are typically very dynamic, leading to ongoing changes incanopy structure, light environment, and hence species composition (see Chap. 6 byMessier et al., this volume). The spatial extent of canopy gaps and speed of canopyclosure is likely to depend on species, site conditions and disturbance intensity, andvaries greatly among biomes. For old-growth forests in the Pacific Northwest of theUnited States canopy gaps were reported to remain open for decades (Spies et al.1990). Even in cases where canopy gaps in old-growth deciduous forests caused by,e.g., storms were closed within a few years, the light quantity and quality reachingunderstorey vegetation may remain dynamic for decades or even longer (see Chap.6 by Messier et al., this volume). As a consequence of these gap-phase dynamics,old-growth forests typically form a canopy consisting of diverse age classes andalso varying heights of individual trees and canopy parts. Older and tall trees mayact as shelter for younger trees. The 450-year-old Douglas fir/Western hemlockforest at the Wind River Canopy Crane Research Facility (WRCCRF) consists of7 Biosphere Atmosphere Exchange of Old Growth Fore ...