Competition as an amplifier of short‐term vegetation responses to climate: an experimental test

Responses to a spring warming treatment were measured on five common herbaceous species grown in outdoor microcosms in Northern England. Although elevated temperature had a beneficial effect on canopy height and plant cover in monospecific cultures of all species, strongly divergent responses to warming occurred in mixtures. 2. We show that the effect of interspecific competition was to modify and amplify the vegetation response to the warming treatment through effects on phenology and morphological development. 3. The observed responses between the species to warming are consistent with predicted differential responses linked to genome size. 4. We conclude that the mechanism of competitive interaction proposed by Boysen‐Jensen (1929 ) coupled with the insights related to genome size provide a basis for explaining and predicting the role of interannual variation in temperature in determining year to year fluctuations in the relative abundance of species in productive perennial herbaceous vegetation.