Weedy competitors

Posted on September 26, 2009

Although the competitive success of coexisting plant species depends on above-ground and below-ground effects, the ways that these types of competition operate together aren’t well understood.  In aquatic plants the issue is especially complex because, while the shoots of aquatic species compete for light and the roots compete for nutrients (as in land plants), the shoots also compete for dissolved carbon and nutrients taken up from the water.  Hydrilla verticillata and Myriophyllum spicatum are both invasive aquatic weeds that readily displace native plants.  In mixed-species experiments Hydrilla grew faster, branched more and shaded out Myriophyllum,  while at the same time reducing the root growth of the other species.  Therefore, in this case both above-ground and below-ground interactions were positive for Hydrilla .  However, Hydrilla was limited by shoot competition with other plants of its own species.  In dense weedbeds, water pH increases and this restricts the source of inorganic carbon to bicarbonate.  It’s therefore likely that the invasive ability of some aquatic plants (such as Hydrilla) is partly due to their ability to adapt to low light levels and high levels of bicarbonate.

Reference: Wang, J-W.., Yu, D., Xiong, W. & Han, Y-Q.  2008.  Above- and belowground competition between two submersed macrophytes.  Hydrobiologia 607,113–122.