Competition and the Formation of Spatial Pattern in Spacing Gradients: An Example Using Kochia Scoparia

Abstract

SUMMARY (1) The presence of pattern in the distribution of dominant and suppressed individuals of the annual plant Kochia scoparia was studied under greenhouse conditions. (2) The experimental design consisted of plants positioned in a polar coordinate grid with eleven arcs and nineteen rays. The distance between arcs increased exponentially from 2-3 cm to 11 1 cm. Rays were separated by an angle of 100. A superimposed triangular array determined that plants were always surrounded by six neighbours. (3) The average size (stem height, stem sectional area and total weight) of plants varied in a regular fashion both across and along arcs. Spatial autocorrelation of the average weight of plants in each arc showed a negative correlation when the lag was one, three or five arcs and a positive correlation when the lag was an even number. When comparisons were made between plants along each arc a negative correlation between a plant and its most immediate neighbour was consistently found. In contrast, the correlations with a lag greater than one did not reveal a consistent pattern. (4) The relationship between mean plant size and spacing did not follow a monotonic function, but showed fluctuations up and down the expected trend. This indicated that interference in this kind of design is not a monotonic function of inter-plant distance. Hierarchy, as measured by the coefficient of variation of plant size in each arc, increased with time, but at a higher rate in inner arcs. For a given recording date, hierarchy within an arc was inversely related to mean plant size. (5) The results suggest that the formation of pattern in the distribution of dominant and suppressed plants is a consequence of a strong 'edge effect' whose transmission proceeds in the preferential direction of the spacing gradient. The overall effect can be described by a kind of wave of constant period (though this constancy need not be the case in every instance). This 'competition-effect wave' makes the definition of individual plant density impossible: the degree of interference experienced by each individual in the design depends on the degree of interference its neighbours have themselves experienced. (6) It is recommended that spatial gradients should not in general be used to calculate yield-density relationships. Plants create their own 'density' according to the space that they capture. Because the emergence of spatial pattern in plant populations is the rule, we suggest the use of spacing gradients to study the transmission of interference effects in modular, sessile organisms.