1 Bouteloua gracilis and Buchloe dactyloides are dominant species in the sh
ortgrass steppe. Previous studies have suggested that these species have si
milar resource requirements and. as there is no temporal segregation in gro
wth, we suggest that competition for below-ground resources should be inten
se. Classical competition theory predicts that, under stable conditions. co
existence of species with similar requirements can occur if intraspecific c
ompetition is more intense than interspecific competition. We therefore com
pared the competitive abilities of the two species under both inter- and in
traspecific conditions, another determinant of coexistence in plant communi
2 A 3-year experiment was conducted in a small homogeneous area and a separ
ate 2-year experiment was conducted in a larger area. In both experiments,
a hexagonal planting design was used to achieve all possible combinations o
f species and to control plant size and neighbourhood asymmetries at the st
art of the experiment. Half the target plants were grown within steel cylin
ders to reduce below-ground competition.
3 We found similar results in the two experiments. Plant biomass and seed p
roduction were always higher in plants growing with reduced competition, al
though seed production was quite variable. Relative competition intensity d
id not differ between intraspecific and interspecific competition.
4 Our results suggest that competition between these coexisting shortgrasse
s is intense, but do not support the prediction of a difference between int
ra- and interspecific competition. We discuss alternative explanations.