Chapter 9 Aboveground and Belowground Consequences of Long-Term Forest Retrogression in the Timeframe of Millennia and Beyond Following the occurrence of a substantial disturbance and creation of a new surface, primary succession occurs. This involves colonisation by new plant species, and their associated aboveground and belowground biota. | Chapter 9 Aboveground and Belowground Consequences of Long-Term Forest Retrogression in the Timeframe of Millennia and Beyond David A. Wardle Introduction Following the occurrence of a substantial disturbance and creation of a new surface primary succession occurs. This involves colonisation by new plant species and their associated aboveground and belowground biota. During this period substantial ecosystem development occurs Odum 1969 and this involves the buildup of ecosystem carbon through photosynthesis and nitrogen through biological nitrogen fixation. The initial colonising plant species are short-lived and often herbaceous but these are replaced over time by those that are larger woody more conservative at retaining nutrients and produce organic matter of poorer quality Grime 1979 Walker and Chapin 1987 . Disturbances that are not sufficiently severe to result in new surfaces being formed can reverse the successional trajectory resulting in a secondary succession that often operates in a broadly similar way to primary succession though from a later starting point White and Jentsch 2001 Walker and Del Moral 2001 . Following the initial development of forest during succession and as trees age there may be a notable reduction in net biomass productivity. The generality of this phenomenon is under debate see Chap. 21 by Wirth this volume but where it occurs the decline is usually apparent in the order of decades to centuries following forest stand development Gower et al. 1996 . The mechanistic basis for this decline is unclear but there are likely to be multiple factors involved see detailed discussion in Chap. 7 by Kutsch et al. this volume . Some proposed explanations have a plant-physiological basis such as increasing hydraulic limitation as trees grow taller shifts in the balance between photosynthesis and respiration and increasing stomatal limitation as trees age. However the evidence for or against each of these mechanisms is mixed and no universal .
