COMPETITION BETWEEN TREE SEEDLINGS AND HERBACEOUS VEGETATION - SUPPORT FOR A THEORY OF RESOURCE SUPPLY-AND-DEMAND

1 We measured competition intensity (CI) between herbaceous vegetation and tree seedlings (Quercus macrocarpa and Q. ellipsoidalis) along an experimental moisture-light gradient. Contrasting theories were teste d by comparing variation in competition intensity to changes in neighb our biomass and resource supply and demand. 2 CI based on survival was inversely correlated with net soil water supply (gross supply minus d emand by herbaceous vegetation). CI was not positively correlated with either gross resource supply or neighbour biomass, contrary to predic tions of Grime's triangular model for plant strategies. 3 Many of the inconsistencies and conflicting results that have characterized the re cent literature on plant competition could be eliminated if changes in competition intensity along a resource gradient are compared with cha nges in net resource supply rather than changes in productivity or nei ghbour biomass. 4 Tree seedling success in savannas and grasslands may be strongly influenced by the intensity of competition from herbaceou s vegetation. Factors that reduce soil water content are likely to inc rease competition intensity (and reduce seedling success) in these env ironments, while factors that increase soil water content will favour seedling success through decreased competition for water with herbaceo us vegetation.