Water Flux/Soil Hydrology and Invasive Species

Water Flux/Soil Hydrology and Invasive Species

            

Water flux and soil hydrology refers to the means by which water in all three forms, liquid, solid, and vapor, circulate through the biosphere.

Saltcedar (Tamarix) is a woody plant native to Eurasia that has had a dramatic effect on the ecosystems of southwestern riparian habitats because of changes in water flux.  Introduced in the 1800’s for bank stabilization and windbreaks, it has spread rapidly into not only all wetland and river system of the southwest, but also northward to Utah and Montana.  Human activity, such as river damming, has altered the natural flooding regime of the ecosystem.  Past clearing of woodlands for agriculture also disturbed the ecosystem.  Saltcedar is much better adapted to exploit the new abiotic characteristics of the areas than native species (Lovich 1998).

 Saltcedar is an aggressive invasive species because of several traits: it is prolific (500,000 seeds/year); it can reproduce vegetatively by resprouting when damaged; it is resistant to drought, fire, flood, and high salinity; its salty leaf litter prevents germination of its competitors like Cottonwoods; its high water uptake and evapotranspiration rates make the environment xeric and unfavorable to native species; its dense groves shade out native species (Lovich 1998).  A natural spring with marshland in Death Valley became devoid of surface water after it was invaded by saltcedar.  After the trees were removed, the surface water reappeared (Vitousek 1986).  By trapping and stabilizing alluvial sediment, saltcedar can reduce channel width and impede a channel’s ability to adjust to increased flow, and leads to more frequent floods.  The drought-tolerant deciduous saltcedar produces a high fuel load leading to more frequent fires, following which saltcedar is more resistant, and quicker to recover than native shrubs (Lovich 1998).

                                                                             Saltcedar

Another invading plant that alters soil hydrology is the Yellow starthistle (Centaurea solstitialis) of California’s grasslands.  This plant has displaced annual and perennial native grasses.  Yellow starthistle has a deep taproot and longevity throughout the summer.  Its high evapotranspiration rate significantly lowers the total soil water content and inhibits competitors with more shallow roots (Enloe 2004).

                                                                    Yellow Starthistle

Invasion of South African native shrublands (fynbos) by non-indigenous plants has reduced watershed runoff and caused rivers to dry up.  Native plants on these lands provide an ecosystem service necessary to insure proper function of the watershed.  These plants are adapted to summer droughts, nutrient poor soil, and periodic brushfires that are of moderate intensity due to the low biomass. With their low biomass, these shrubs are able to prevent erosion and consume relatively little water, so there is always runoff for the rivers and streams that benefit habitats below the watershed. Woody plants introduced for lumber such as eucalyptus, Pinus pinaster, Hakea sericea, and Australian Acacias have invaded these lands to the detriment of water resources.  These plants have high biomasses and consume much more water than the native shrubs.  Runoff from the watershed is reduced or eliminated and wildfires tend to be much more intense because of the greater biomass (VanWilgen 1996).

                   P. pinaster                                                  Hakea sericea

                                                Acacia                                                            Eucalyptus

Invasive plants change ecosystems by their effect on hydrology. Changes of rate, timing of evapotranspiration, or runoff of the region, are due to differences between invasive and native plants with respect to the transpiration rate, phrenology, biomass of photosynthesis, or rooting depth (Levine 2003).

References

1. Ecology of Biological Invasions of North America and Hawaii. (1984). Springer-Verlag: New York.  p 163-173.

2. Enloe, Stephen R., et al (2004). Soil Water Dynamics Differ among Rangeland Plant Communities Dominated by Yellow Starthistle (Centaurea solstitialis), Annual Grasses, or Perennial Grasses. Weed Science. 52:929-935.

3. Levine, Jonathan M. et al (2003). Mechanisms Underlying the Impacts of Exotic Plant Invasions. Proceedings of the Royal Society of London. 270:775-781.

4. Lovich, Jeffrey E. and Roland C. DeGouvenain. (1998). Saltcedar Invasion in Desert Wetlands of the Southwestern United States: Ecological and Political Implications. Ecology of Wetlands and Associated Systems. (The Pennsylvania Academy of Science). Chap 30. P. 447-467

5. VanWilgen, Brian W., Richard M. Cowling and Chris J. Burgers. Valuation of Ecosystem Services. BioScience. 46:184-189.