The physiological mechanisms of growth allocation in trees are complex, involving both molecular-level and genetic control of transport and utilisation of metabolic products at active meristems (Matyssek et al. 2005). Attempts have been made towards mechanistic models of growth partitioning, but from the point of view of stand growth modelling, these are still very much under development (e.g. New Phytologist 2005, 166(3)).

On the other hand, the results of the complex allocation patterns are tree structures which, despite wide variation in form between species and habitats, have been found to manifest striking regularity (McMahon and Kronauer 1976, Shinozaki et al. 1964, West et al. 1999). To what extent can such regularities be used as a basis for modelling growth allocation in variable environments? Regularities in tree structure can be understood as the outcomes of selective pressures under competition within and between species. Some key aspects are (1) balancing different metabolic processes with minimum waste of resources, (2) providing sufficient mechanical support for functioning tissues at minimum cost, and (3) competing successfully with neighbours for available resources.

For modelling allocation, it is important to be able to separate between structures that are largely independent of the environment, and those that are plastic from one situation to another. In the latter case, it is also important to understand the time constants of response to changing environments. This paper describes an approach to growth allocation in a carbon balance framework, where conservative structures are defined by means of (1) the pipe model, and (2) the allometric scaling of crown structure (Valentine and Mäkelä 2005). Acclimations to different competitive situations are described in terms of plasticity in (a) crown ratio and (b) foliage and shoot structure. Acclimations to different growth sites are described by plasticity in (c) fine root to foliage ratio, and (b) nitrogen concentration in foliage. The signifcance of the different components for selective pressures for these constraints are reviewed, and species-specific strategies and coexistence of species in mixed stands are discussed in this framework.

References

Matyssek, R., Agerer, R., Ernst, D., Munch, J.-C., Osswald, W., Pretzsch, H., Priesack, E., Schnyder, H. & Treutter, D. 2005. The plant's capacity in regulating resource demand. Plant Biology 7, 560-580.

McMahon, T.A. & Kronauer, R.E. 1976. Tree structures – deducing principle of mechanical design. J. Theor. Biol. 59, 443-466

Shinozaki, K., Yoda, K., Hozumi, K., Kira, T. 1964. A quantitative analysis of plant form: the pipe model theory. I. Basic analyses. Jpn. J. Ecol. 14 , 97-105

West, G.B., Brown, J.H., Enquist, B.J. 1997. A general model for the origin of allometric scaling laws in biology . Science 276 , 122-126

Valentine, H.T. & Mäkelä, A. 2005. Bridging process-based and empirical approaches to modeling tree growth. Tree Physiol. 25, 769–779