Plant Detectives Seek Sources Of Invasive Trees

Barbara A. Schaal, Ph.D., Washington University professor of biology and her graduate students use DNA sequences to reveal information on historical events. Schaal has traced the origins of cassava using molecular techniques, and now is using systematics and phylogeography to document the role of hybridization and introgression in the evolution of Phlox species and to trace the Eurasian source of invasive Tamarix species in the United States.

The Tamarix species, commonly called saltcedar, are environmental threats that have invaded the arid southwest and are contributing to the drying up of creeks and streams in that water-threatened area. Over a million acres are now infested with saltcedar monocultures along streams and riverbeds. The salt cedars' long taproots suck up salty ground water and drop salt-crusted leaves on the soil surface. This makes it almost impossible for native plants to take root. The loss of native plants also decreases the insect and bird biodiversity.

Schaal's graduate student, John Gaskin, has used DNA sequences to identify which species are here and to document hybridization. Their DNA analyses also help them pinpoint where the plants may have originated in Eurasia. Earlier USDA studies show that an Arizona saltcedar will not be eaten by certain Asian insects known to like saltcedars; these insects, instead, prefer plants that grow in Texas or New Mexico. So, there are different kinds of the plants in different areas.

So far Schaal and Gaskin have found that the most common invasive here is a hybrid of two species that do not grow in the same areas of Asia, and thus is a novel plant genotype. These results will help USDA biological control researchers determine which insects to import in the future to help control the invasion, but caution that any novel hybrid plants may prove to be unpalatable to species-specific insects, since they did not evolve with them.

Schall and Gaskin published their results in the Proceedings of the National Academy of Sciences (PNAS), the week of Aug. 12-16, 2002.

Tamarix is the second most evasive plant in the United States. Purple loosestrife (Lythrum salicaria) is number one and a big problem in northern areas. Invasive plants are second only to habitat loss in contributing to loss of biodiversity.

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Related Links:

For more information on Tamarix or the current biological control project, see http://www.invasivespecies.gov/profiles/saltcedar.shtml and http://wric.ucdavis.edu/saltcedar/grant/research.html

Missouri Exotic Invasive Plant Page: http://www.mobotgradstudents.org/~mobot/MOEPPC/MOEIPP1.html

Gaskin website: http://www.mobotgradstudents.org/~mobot/Gaskin/johnpage.html