Some limitations inherent to static indices of plant competition

Abstract

Various measurements of the abundance, proximity, and stature of neighbouring plants are utilized as indices of competitive intensity experienced by crop trees. These indices can be useful in assessing the desirability of vegetation control and stand thinnings, and in simulating stand development. Static competition indices, however, have fundamental limitations which should be more widely considered. Competition is usually a constraint to growth, not a determinant of growth, so any measurement of competition alone is fundamentally limited in its ability to predict individual crop tree performance. Differences in the microsite effects of competitors and the autecological responses of crop species result in wide variation in the size of the functional competitive arena, and suggest the need for distinct weightings of competitive intensity for different combinations of species and site. Site differences and weather variation, when coupled with species differences in above- and below-ground allocation, will alter the mode and intensity of competition within a stand and from year to year. The phenology of competitors and focal species will likewise alter interference patterns within a year, making one-time measurements sometimes misleading. Suggested alternatives for determining the importance and intensity of competition include the maintenance of competition-free seedlings ("phytometers") in order to gauge the competitive constraints experienced elsewhere in a plantation. Simulation models of tree growth and stand development also show promise, but they must incorporate the dynamics of noncrop vegetation development. In developing guidelines for the assessment of noncrop interference and the advisability of vegetation control, there is still a need to determine, in a generalizable manner, appropriate survey plot sizes and critical competition thresholds for different combinations of site type, crop species, and vegetation complex.