|
|
|
| Resource allocation in three dimensional, process-based tree models; bottom up or top down approach? |
|
E. Nikinmaa,
University of Helsinki
|
Resource capture and allocation determines tree performance in given environment. Successful growth strategy places capturing organs in favourable positions without excessive expenditure on growth. Tree architectural designs that bring about favourable results vary according to the resource availability; strategies that are optimal in good resource supply may be detrimental in conditions of scarcity. Three dimensional, process-based models are a developing tool to study the potentials and limits of the whole tree growth. In attempting to bring together realistic growth of tree organs and the physiology of resource capture and allocation, the approach can be viewed also as a workbench for many different tree level studies.
During the last 30 years, eco-physiology has increased our understanding of the plant resource uptake. Botanical description of plant growth has advanced substantially during the last decade up to development of several formal methods of plant architecture development. The challenge for the modelling, and the whole tree physiology as a whole, is to study how the former leads to the latter, i.e. how the resources are distributed and used for growth and why. Modelling offers a valuable tool to link different processes into a whole and testing the outcome of different hypotheses at whole tree level.
Mechanistic modelling of resource partitioning and growth requires description of local uptake processes, transport and local growth processes. At an aggregated plant level reaction - diffusion approach has been used. An improved realism requires a true description of the transport processes. Models of mass- transport in xylem and phloem are currently being developed. The relationship between the cambial activities and resource availability at localities determines the growth but also influences the transport. A lot of evidence suggests the need of including growth regulator substances in the mechanistic approach of growth.
While the mechanistic approach on whole tree growth and physiology is tempting also the challenges are still considerable both computationally but most of all due to lack of knowledge. For some purposes it is sufficient that we describe how the tree structures grow depending of their local environment and position within the whole. The development of the whole is further constrained by the resource uptake of the whole, i.e. we may require that the functional balance within the tree is maintained.
The functional balance approach has been able to produce realistic results. However, it suffers from the inherent strong assumptions of the structural regularities that need to be verified in different conditions.
The presentation reviews the challenges of representing functional realism in tree architecture models. The possibilities and limitations of both bottom up and top down approaches are considered.
|
|
|
|
