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Chapter 6 Functional Relationships Between Old-Growth Forest Canopies, Understorey Light and Vegetation Dynamics Christian Messier, Juan Posada, Isabelle Aubin, and Marilou Beaudet Old-growth forests are characterised by the presence of old trees (200 years of age), considerable amounts of large pieces of dead wood and a complex horizontal and vertical structure | Chapter 6 Functional Relationships Between Old-Growth Forest Canopies Understorey Light and Vegetation Dynamics Christian Messier Juan Posada Isabelle Aubin and Marilou Beaudet 6.1 Introduction Old-growth forests are characterised by the presence of old trees 200 years of age considerable amounts of large pieces of dead wood and a complex horizontal and vertical structure see Chap. 2 by Wirth et al. this volume . These three elements create a unique understorey environment including light that differs somewhat from earlier successional or second-growth i.e. forests that have regrown following harvesting forests. Identifying factors that influence variation in light availability within forested ecosystems represents an important component in our understanding of the complex determinants of understorey vegetation dynamics. Based on an extensive review of the literature on old-growth forests in boreal temperate and tropical biomes this chapter discusses 1 the distinct structural and compositional features that are likely to influence the understorey light environment in old-growth forests 2 the particular understorey light conditions found under such forests and 3 the unique understorey vegetation assemblage that can develop in old-growth forests. We focus as much as possible on shared trends among all three biomes but we also discuss some of the fundamental differences that differentiate them. Comparisons with second-growth forests are also often made to highlight the uniqueness of old-growth forests. 6.2 Structural and Compositional Features of Old-Growth Old-growth forests possess distinct structural and compositional features that influence understorey light environment and vegetation growth and dynamics Chap. 2 by Wirth et al. this volume . Since such forests are generally found where small-scale disturbances predominate their disturbance regime tends to be characterised by gap dynamics see Chaps. 2 Wirth et al. 10 Bauhus 13 Grace and Meir and 19 Frank et al. .
