The Imprint of Species Turnover on Old-Growth Forest Carbon Balances – Insights From a Trait-Based Model of Forest Dynamics

Succession is the process that eventually transforms a young forest into an oldgrowth forest. Describing and analysing plant succession has been at the core of ecology since its early days some hundred years ago. With respect to forest succession, our understanding has progressed from descriptive classifications (i.e. identifying which forest types constitute a successional sequence) to general theories of forest succession (Watt 1947; Horn 1974, 1981; Botkin 1981; West et al. 1981; Shugart 1984) and simulation models of forest dynamics that are capable of predicting successional pathways with remarkable precision (Urban et al. 1991; Pacala et al. 1996; Shugart and Smith 1996; Badeck et al. 2001; Bugmann 2001; Hickler et al. 2004; Purves et al. 2008). Although the importance of different factors in controlling successional changes in species composition is still debated – particularly in speciose tropical forests (Hubbell 2001) – a large body of evidence implicates the tradeoff between shadetolerance and high-light growth rate as a key driver (Bazzaz 1979; Pacala et al. 1994; Wright et al. 2003). In contrast, there is no well-accepted mechanism to explain successional changes in forest biomass, much less other components of ecosystem carbon. A range of biomass trajectories have been observed (e.g. monotonic vs hump-shaped curves), and some basic ideas have been proposed to explain these patterns (Peet 1981, 1992; Shugart 1984). However, we are aware of only one systematic, geographically extensive assessment of biomass trajectories (see Chap. 14 by Lichstein et al., this volume). In this data vacuum, it has been difficult to assess the relative merits of different theories or mechanisms. This is especially true for later stages of forest succession, and in particular for old-growth forests. With respect to biomass dynamics, there are at least four non-mutually exclusive hypotheses: (1) the ‘equilibrium hypothesis’ of Odum (1969); (2) the ‘standbreakup hypothesis’ of Bormann and Likens (1979) and its generalisations (e.g. Peet 1981, 1992; Shugart 1984); (3) the hypothesis of Shugart and West (1981), which we term the ‘shifting-traits hypothesis’; and (4) the ‘continuous accumulation hypothesis’ of Schulze et al. (Chap. 15, this volume). Because some of these