The effect of competition on Bacopa monnieri zonation in an temporarily open/closed tropical estuary

In this paper we investigated the role of competition to determine Bacopa monnieri L. Pennel (Plantaginaceae) zonation in a temporarily open/closed tropical estuary. In this estuary, B. monnieri occupies a given vertical zone in the saline stretch, where it forms dense monospecific stands, but is absent elsewhere. We transplanted turfs to areas outside of their natural occurrence in the estuary, both in the presence and absence of competition: a higher-elevation zone, a lower-elevation zone and a non-saline region. Turfs transplanted between naturally occurring stands served as controls. Turfs transplanted to the lower zone either died or became much smaller. As there was no competition under this condition, we conclude that the absence of this species from this zone is due to abiotic conditions, likely light limitation imposed by the turbid water column. Turfs transplanted to the higher zone under competition died; however, in the absence of competition, they survived. Turfs transplanted to the non-saline zone died, regardless of the presence or absence of competition, indicating an abiotic restraint. Our results indicate that the absence of B. monnieri from higher elevations is related to competitive displacement, whereas its absence from lower elevations and from non-saline areas is related to abiotic drivers.