An introduction to plant demography with special reference to New Zealand trees

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Abstract
An outline of the scope and origins of plant demography emphasises the link between numerical and evolutionary approaches. The rules governing thinning in monocultures and some general features of plant competition in mixtures are described. Plant demography can be applied at several levels — that of the genetic individuals or that of the modular components of which plants are usually composed. This dual population structure puts a new emphasis on the morphological changes occurring in competing mixtures. Various approaches to the definition of strategies are discussed. Plants with short life cycles and abundant production of small seeds are contrasted with long-lived species producing fewer larger seeds. This ‘r-K’ continuum is used as a broad framework, although Grime's (1979) two dimensional strategy model may be more realistic. The concept of the ‘regeneration gap’ has been a persistent theme in forest ecology in New Zealand, and its explanation has generally been presumed to lie in past climatic change. However, population age and size frequency distributions reflect demographic processes, which must be properly explored before extrinsic causes can be ascribed to particular structures. The steady state climax theory is an inappropriate concept on which to build an understanding of the demography of canopy trees in New Zealand. A kinetic model, in which disturbance is accepted as a selective force to which different tree species have become differentially adapted, accepts regeneration gaps localised in space and time as demographic phenomena. A model of cohort structure in kauri is used to illustrate the dangers inherent in drawing conclusions about population ‘status’ from a small sample of diameters and approximate ages. The difficulties of applying transition matrix models emphasises our lack of basic demographic information for most New Zealand trees. The errors inherent in determining tree population age structures are addressed. The regeneration strategies of the beeches (Nothofagus spp.) and the podocarps (Podocarpaceae) are outlined primarily in relation to seed size and seed production periodicity. The beeches conform well to the predator satiation theory of mast seeding, but the podocarps (and other bird-dispersed trees) present additional problems. More information on the interactions between tree fruiting behaviour and the avifauna is needed for satisfactory evolutionary accounts. The existence of ‘advance growth’ seedling populations in New Zealand forests is well known, but there is little information on dormant buried seed banks. Some information about the sizes of seed banks in New Zealand forests is provided, and it is concluded that they are probably of similar magnitude to those in other temperate forests, and warrant more rigorous study.