Interactions between neighbouring plants
This thesis addresses the issue of interactions between neighbouring plants by looking at
theory and models used in the study of wild plants with a view to applying them in the
design of agroforests. In Part I difficulties of designing mixed crop stands are considered.
Grime's model of the ecological strategies pursued by wild plants is described and its
potential usefulness pointed out. Experimental results are presented suggesting that stresstolerant
trees may interfere less with an understorey component than do competitive trees
through the different patterns of spatial occupation manifested by their roots.
Part II describes a statistical method to determine how any measurable attribute of one
plant depends on the proximity and/or other characters of neighbouring plants. The method
overcomes the need for unfeasibly large numbers of treatments encountered by
conventional field trial methods.
The method has the potential to offer a firm basis for the design of optimized plant
production systems; and will also allow ecologists to detect and quantify interactions
between wild plants in the field.
Part Ill, using concepts of Evolutionary Game Theory, examines the question of
cooperation in plants: both between the bJfeen plant and its associated vesicular-arbuscular
mycorrhizal (VAM) fungi; and between neighbouring bJfeen plants connected by common
VAM fungal hyphae. Exploring the implications for plants ofAxelrod and Hamilton's
(1981) Game Theoretic approach to the evolution of cooperative behaviour, it examines a
logical problem in the view that the relationship between a green plant and associated
VAM fungus is mutually beneficial. That the association is of mutual benefit would be
insufficient to explain its persistence, and the fact that it does persist tells us something
about its structure.
It is shown that no logical paradox exists in postulating the simultaneous existence of
competition for resources and of resource-sharing cooperation between a given pair of
neighbouring green plants; and that at least a certain minimal type of cooperation may be
deemed to exist between connected plant neighbours. Putting together findings from two
fields - direct nutrient transfers and biotic specialisation - solves problems for both, and
provides evidence for inter-plant cooperation. Possible evolutionary stages through which
cooperation could have passed are discussed. Given this theory it can no longer safely be
assumed that plants do not cooperate, and experimental results are presented suggesting
that they may be able to.
Scientific and economic implications are indicated for all three areas covered. Fertile
ground exists for further research and suggestions are made for directions and methods.