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Institute of Forest Biometry and Informatics

Faculty of Forest Sciences and Forest Ecology
at the
University of Göttingen


PLANT MODELLING GROUP

PROJECT:

SPATIALLY EXPLICIT, PHYSIOLOGY-BASED MODELS OF WOODY PLANTS



1. Development of a formal language for spatially explicit modelling and simulation of the allocation of carbon and nitrogen and of growth in woody plants

The central aim of the project is the development of a general formal language designed to describe C and N allocation and growth in woody plants, particularly in forest trees. A software which "understands" this language and which generates 3D simulation models of plants will be a natural by-product.

Lindenmayer-systems (L-systems) will serve as a basis. These are string-rewriting systems; each L-system specifies a set of replacement rules describing the evolution of strings which encode 3D branching structures. "Stochastic sensitive growth grammars" are an extended version of L-systems, they are able to represent features of plants like reaction to overshadowing or density, and also already some simple forms of carbon allocation modelling. A software developed at our institute, called GROGRA (Growth Grammar interpreter), can read these grammars from text files and generate time series of morphological (3D) structures representing the plants or the forest stand. GROGRA contains also several output filters for standard graphical file formats (PostScript, HTML, PovRay, DXF, AMAP) and some analysis tools to explore the virtual plants and to compare them with empirical data from measured plants. More information on this preliminary work and descriptions of the growth grammar language and of the software can be found in the publications of the research group.

Of course it is possible to construct simulation models of plant architecture, growth and carbon metabolism directly in some standard programming language, without using a formal description tool like "growth grammars", and this was in fact already done by many research groups in ecophysiology, agricultural or forestry-related projects. However, using grammars as a means for specification has some advantages:

The "stochastic sensitive growth grammars" have still some weaknesses, particularly when the physiological basis of plant growth is to be taken into account. Sensitivity of processes to environmental influences, which is represented in the current grammars by specialized "sensitive functions", should be modelled in a much more general and transparent way. Likewise, the formal tools to describe allocation and plant-internal control mechanisms are not yet optimized.

The exploration of the physiological background and of control mechanisms governing the allocation of carbon and nutrients and the onset of growth in plants is one of the central topics of plant physiology, and there exists already a huge number of diverse modelling approaches. The formalism which shall be developed here should be able to represent several of these models (to allow for precise comparisons). Model experiments and sensitivity tests can then perhaps help to clarify some of the questions concerning mechanisms of allocation discussed in the literature.
Methodologically, a combination between the rule-based and the object-oriented programming paradigm will be necessary for the specification language.


2. Simulation of the reaction of plants to competition and pruning as a result of the interaction of plants and plant organs with each other

The object-based approach, which considers the plant as a complex 3D structure composed of relatively simple units (leaves, internodes, growth units, root segments etc.), has the advantage that changements in architecture which are caused by competitors or by local damage can appear in the model as emergent phenomena, resulting from the interaction of the units, and need not be introduced artificially. Manipulations in forestry as well as the pressure exerted by competitors are cases where spatially explicit models can integrate the knowledge which is currently available at the scale of single plant organs, and where the consequences for the whole plant or for tree stands can be checked against reality. Modelling thus becomes a means to explore the consequences which assumptions introduced at a low level can have at a high level (several interacting plants, a forest stand).


3. Taking into account some fundamental results concerning genetic control of plant morphogenesis

Ecophysiological models of plant growth concentrate normally on the environmental factors which have influence on allocation and growth, and they emphasize the opportunistic reactions of plants to changements in these factors. However, the genetically fixed "blueprints" of the plant organism are equally important. Endogenous control of the activity of meristems during ontogenesis determines the shape of the resulting plant to a considerable extent. This is the reason why we can distinguish e.g. a birch tree from an oak tree by their habit.
Basically, L-systems are designed to represent a development of plant form which is genetically determined. However, until now they do not take some recent results of plant genetic into account. During ontogenesis, some genes are activated in a spatially and temporally limited manner. Here, transcription factors activating or repressing the expression of specific target genes play an important role. The ability to represent regulating networks of such factors would enhance the usefulness and power of growth grammars in the field of plant morphology considerably. It is therefore planned to cooperate with other research groups to extend the formalism of growth grammars in a way which opens the possibility to take such networks into account.


4. Development of statistical and geometric methods to characterize the spatial structure of plants and plant stands and to compare results of models

For a validation of the results of new models, a methodologically reliable comparison of simulation results with empirical data is a prerequisite. Furthermore, the comparison between the results of several models will be of growing importance in the future. Methods of evaluation and comparison which are adapted to the spatially explicit models developed here are subject of a related project.


Start of project: January 1, 2000.
End of project: September 30, 2001.
The project was originally planned for 5 years. Premature interruption occurred because of W. Kurth's call to a tenureship at the Technical University of Cottbus. However, important parts of the project will be continued further, at Göttingen and Cottbus.
Funding: Heisenberg scholarship of the DFG.

Final report (in German)

This project was led by Winfried Kurth.

 



 

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Last modifications: August 13, 2003