A standardized neighbor removal experiment was conducted in 12 plant communities located on three continents to test the null hypothesis that competition intensity (CI) was independent of the amount of plant biomass present. Six plots were chosen in each community to cover the range oflocal variation in plant biomass. In each plot the relative growth rate (RGR) of transplanted Poapratensis (Poaceae) seedlings was compared in the presence and absence of neighbors. Neighbors were removed experimentally using R. J. READER ET AL. Ecology, Vol. 75, No. 6 herbicide. Removing neighbors increased RGR of transplants significantly in most plots. CI increased with an increase in the amount of neighbor biomass present in one community where the range of neighbor biomass was greater than in any other community. In contrast, CI did not change significantly with an increase in neighbor biomass in other communities where the range of neighbor biomass was smaller. For the communities combined, CI was not related to neighbor biomass in a consistent fashion. These results indicate that competition may reduce growth over a wide range of habitat productivity, but the relationship between CI and neighbor biomass may differ among communities.
VAN HULST, R., SHIPLEY, B., and TH~RIAULT, A. 1987. Why is Rhinanthus minor (Scrophulariaceae) such a good invader?Can. J. Bot. 65: 2373 -2379. Rhinanthus minor L. is an annual grassland hemiparasite often found where other annuals are rare or absent. In addition to its invasive abilities, R. minor has other characteristics seldom found in annuals: virtual lack of a seed bank; few seeds per plant; seeds too big to be readily dispersed under natural conditions; and a Deevey type I or 11 survivorship curve. A combination of correlative and experimental studies was performed to investigate the variables influencing fecundity and survivorship in R. minor. Fecundity is positively correlated with surrounding vegetation biomass and negatively with R. minor density. Survivorship is negatively correlated with surrounding vegetation biomass and with R. minor density. Presumably, R. minor is an effective invader in relatively unproductive grassland because its finite rate of increase is a nonlinear function of the biomass of the surrounding vegetation, being maximized when the surrounding vegetation is dense enough to allow effective root parasitism (increasing R. minor fecundity) but not so dense that young R. minor plants are outcompeted for light (decreasing R. minor survivorship). VAN HULST, R., SHIPLEY, B., et TH~RIAULT, A. 1987. Why is Rhinanthus minor (Scrophulariaceae) such a good invader?Can. J. Bot. 65 : 2373-2379. Rhinanthus minor L. est une plante annuelle hCmiparasite qu'on trouve dans des prairies et autres vCgCtations dominCes par des graminkes, souvent dans des stations oh elle est la seule plante annuelle. Outre ses capacitks de plante envahissante, R. minor a d'autres attributs qui sont peu communs pour une plante annuelle : quasi-absence d'une banque de graines, peu de graines par plante, graines trop grosses pour &tre facilement dispersCes sous conditions naturelles et une courbe de survie de type Deevey I ou II. Nous avons conduit des Ctudes de type corrklatif et expkrimental pour Ctablir quels facteurs peuvent influencer les taux de survie et de fCconditC de R. minor. La fCconditC est en corrClation positive avec la biomasse de la vCgCtation environnante et en codlation nCgative avec la densit6 de R. minor. Le taux de survie est en codlation negative avec la biomasse de la vCgCtation environnante et avec la densite de R. minor. Apparemment R. minor est capable d'envahir efficacement des prairies relativement peu productives parce que son taux fini de croissance est une fonction non IinCaire de la biomasse de la v6gCtation environnante, dont le maximum est atteint quand la vCgCtation est assez dense pour permettre un parasitisme racinaire efficace (augmentation de la fCconditC de R. minor) sans que les jeunes plantes de R. minor soient perdantes dans la concurrence pour la lumibre (diminution du taux de survie de R. minor).
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