Abstract: Competitive opportunities for native plants Ecosystems are often made vulnerable to invasion when disturbance regimes are altered; any change may create conditions inhospitable to natives, thereby opening a niche for invasion. The invasion by Tamarix ramosissima along dammed and channeled rivers may be such an example, since reduction in flooding prevents establishment by native tree species, including cottonwood, Populus deltoides subsp. wislizinii (Salicaceae). Although this problem suggests management by reinstating flooding, flooding carries the risk of promoting more Tamarix that will also germinate after flooding; establishing Populus seedlings must therefore potentially compete against co-establishing Tamarix. Fortunately however, research findings suggest that the success of Tamarix is not due to superior competitive ability as a seedling. Studies of field populations have found that Tamarix seedlings can have high mortality rates and slow growth rates in areas where native species' seedlings are dense. It was found, in fact, that seedling density was a more important factor than soil nutrients, salinity, or texture for explaining growth or mortality patterns of Tamarix. The hypothesis that Tamarix is a poor competitor against native Populus as a seedling has been further tested with two pot studies. These experiments varied both relative density of the two species to each other, as well as total density. These studies found that, as observed in the field, aboveground growth of Tamarix is strongly suppressed by the presence of other seedlings, especially by Populus. In contrast, competitive effects of Tamarix against Populus could only be seen at very low Populus densities. Competitive interactions were also tested across different soil types and with either a draw-down of the water table (indicative of a healthy riparian hydrograph) or a stagnant but shallow water table. Tamarix was competitively suppressed in every substrate tested, with the weakest response in sand with no draw-down, where growth of Populus was poorest. Together, these results suggest that stream flow management that promotes Populus establishment could also aid in controlling Tamarix invasion across a range of substrates by creating an opportunity for competitive displacement by the native.

Further reading:
Sher, A.A. and D. L. Marshall. 2003. Competition between native and exotic floodplain tree species across water regimes and soil textures. American Journal of Botany 90: 413.
Sher, A.A., D.L. Marshall, and J. Taylor. 2002. Spatial partitioning within southwestern floodplains: patterns of establishment of native Populus and Salix in the presence of invasive, non-native Tamarix. Ecological Applications 12:760-772.
Sher, A. A. ,D. L. Marshall., and S.A. Gilbert. 2000. Competition between native Populus deltoides and invasive Tamarix ramosissima and the implications of reestablishing flooding disturbance. Conservation Biology 14:1744-1754.
Sher, A.A. and L.H. Hyatt. 1999. The Disturbance-Invasion Matrix: a new framework for predicting plant invasions. Biological Invasions 1(3-4):109-114.

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