Springer Old Growth Forests - Chapter 9

Chapter 9Aboveground and Belowground Consequences of Long-Term Forest Retrogression in the Timeframe of Millennia and BeyondFollowing 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.
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Springer Old Growth Forests - Chapter 9Chapter 9Aboveground and Belowground Consequencesof Long-Term Forest Retrogression in theTimeframe of Millennia and BeyondDavid A. Wardle9.1 IntroductionFollowing the occurrence of a substantial disturbance and creation of a new surface,primary succession occurs. This involves colonisation by new plant species, andtheir associated aboveground and belowground biota. During this period, substan-tial ecosystem development occurs (Odum 1969), and this involves the buildup ofecosystem carbon through photosynthesis and nitrogen through biological nitrogenfixation. The initial colonising plant species are short-lived and often herbaceous,but these are replaced over time by those that are larger, woody, more conservativeat retaining nutrients, and produce organic matter of poorer quality (Grime 1979;Walker and Chapin 1987). Disturbances that are not sufficiently severe to result innew surfaces being formed can reverse the successional trajectory, resulting in asecondary succession that often operates in a broadly similar way to primarysuccession though from a later starting point (White and Jentsch 2001; Walkerand 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 thisphenomenon is under debate (see Chap. 21, by Wirth, this volume), but where itoccurs, the decline is usually apparent in the order of decades to centuries followingforest stand development (Gower et al. 1996). The mechanistic basis for this declineis unclear, but there are likely to be multiple factors involved (see detailed discus-sion in Chap. 7 by Kutsch et al., this volume). Some proposed explanations have aplant-physiological basis, such as increasing hydraulic limitation as trees growtaller, shifts in the balance between photosynthesis and respiration, and increasingstomatal limitation as trees age. However, the evidence for or against each of thesemechanisms is mixed and no universal explanation emerges (see, e.g. Gower et al.1996; Magnani et al. 2000; Weiner and Thomas 2001; Ryan et al. 2004, 2006).Other explanations relate to belowground properties and nutrient supply from thesoil. For example, as forest stands develop and succession progresses, the rate ofmineralisation of nutrients from the soil declines (Brais et al. 1995; De Luca et al.C. Wirth et al. (eds.), Old‐Growth Forests, Ecological Studies 207, 193DOI: 10.1007/978‐3‐540‐92706‐8 9, # Springer‐Verlag Berlin Heidelberg 2009194 D.A. Wardle2002). This is at least partly as a result of a greater proportion of nutrients beingimmobilised in plant tissue and because of the declining quality of plant litter ¨(Hattenschwiler and Vitousek 2000; Nilsson and Wardle 2005). This reduced soilactivity is consistent with changes in the composition of the soil community thathave sometimes been observed during succession (e.g. Scheu 1990; Ohtonen et al.1999). Often the reduction of nutrient availability is driven in part by changes inthe forest understorey composition, such as increased densities of dwarf shrubs(Nilsson and Wardle 2005) and mosses (Zackrisson et al. 1997; Bond-Lambertyet al. 2004), which may lock up nutrients or produce litter of poor quality. Regard-less of the precise mechanisms involved, it is apparent that at least part of thereduction in forest stand productivity in the order of decades to centuries isfrequently associated with the reduced rate of supply of nutrients from the soil,and probably involves changes in the composition of the soil biota as well as thevegetation. In the prolonged absence of major disturbance, i.e. in the order of millennia andbeyond, the decline in forest productivity can be followed by significant declines inforest stand biomass. This decline is often associated with declines in the availabil-ity of soil nutrients that occur during pedogenesis (Walker and Syers 1976;Richardson et al. 2004; Vitousek 2004; Wardle et al. 2004; Coomes et al. 2005).We refer to this situation of long-term decline in forest biomass caused by reductionin available nutrients as ‘ecosystem retrogression’ (Walker et al. 2001; Walker andReddell 2007). This phenomenon is distinct from the shorter term decline in forestproductivity that frequently occurs in the order of decades to centuries and that mayhave a variety of causes (Gower et al. 1996). Significantly, as ecosystems age in theorder of thousands of years without major disturbance, phosphorus availabilitymay become a major factor limiting forest biomass. In a classical investigation oflong-term chronosequences on sand dunes and moraines in New Zealand (spanningseveral millennia), Walker and Syers (1976) showed that as soils age the totalamounts of phosphorus declines significantly (presumably through leaching andrunoff), and that the remaining phosphorus becomes converted to forms that areincreasingly physically occluded or bound in relatively recalcitrant organic com-pounds, and that are relatively unavailable to plants. This type of pattern hassubsequently been shown in other locations and for other ecosystems, e.g. in easternAustralia (Walker et al. 1981) and the Hawaiian islands (Crews et al. 1995;Vitousek 2004). In the long term, greatly reduced availability of nitrogen mayalso occur, partly because of increased immobilisation, partly because of retentionof nitrogen in recalcitrant polyphenolic complexes that are less easily decomposed(Northup et al. 1995, 1998; Wardle et al. 1997), and partly because of leachinglosses as dissolved organic nitrogen (see Chap. 16 by Armesto et al., this volume).These changes in availability of key nutrients during retrogression appear to belinked to both changes in soil biota (Williamson et al ...