Comparison of establishment of Cottonwood and Saltcedar

Cottonwood (Populus spp.) is native to several temperate and boreal regions of North America where decrease in floodwaters exposes minerals in the soil surfaces. Cottonwoods are ecologically important to arid regions of western and southwestern North America because of the absence of other native tree species as their replacements (Rood and Mahoney, 1990; Cooper et al. 1999). Cottonwoods are prone to high invasion in riparian ecosystems as a result of large amount of hydrological disturbance in such areas (Tabacchi 1995).

Cottonwood trees can be either male or female both of which contain flowers clustered in catkins (Braatne et al. 1996). Seeds produced by both flowers are reported to be greater than twenty five millions per tree annually (Schreiner 1974). Since the seed viability is very less lasting for 1-2 weeks under natural conditions and 2-3 days under wet conditions germination of seeds must occur in short period of time. Cottonwoods can also reproduce asexually through crown breakage and tree fall during wind storms and flooding events. The broken branches after being buried in sediment sprout and develop strong shoots. Species, climatic condition and drainage basin of the area determines whether the seedlings are established more from sexual reproduction or asexual propagules may outdo seedlings in growth and competition (Braatne et al. 1996).

Cottonwood seedlings are poor competitors and their establishment depends on temporal and spatial variation in suitable sites. If sufficient rainfall occurs in growing season seedling establishment is substantially increased even in disturbed non-alluvial environments (Braatne et al. 1996). Cottonwood seedlings are shade intolerant (Braatne et al. 1996) and seedlings are established due to large, relatively infrequent floods (Scott et al. 1997). Both light level and soil water availability are important for establishment of cottonwood seedlings in arid regions and late summer survival of seedlings depends on soil moisture provided by flooding (Cooper et al. 1999). Seedling death can be caused by rapid decline of water table as the seedlings are prone to dry soils (Cooper et al. 1999). Nature and timing of fluvial processes largely determine seedling survival rates as well as life history traits of riparian cottonwoods (Rood and Mahoney, 1990).
Saltcedar (Tamarix spp.) is an exotic shrub which has spread widely in large portions of riparian areas of southwestern United States (Robinson 1965). Saltcedar has substituted dominant stands of native cottonwood (Populus fremontii) in a number of regions due to which habitat of Neotropical migrant birds has reduced substantially (Anderson et al. 1977) and fluvial processes have altered (Blackburn et al. 1982).

Saltcedar produces large number of seeds over a length of time (April through October). Germination percentage of the plant depends on seed collection and varies from 19% to 51.3% (Merkel and Hopkins, 1957). When the seedlings get established they grow faster than other native plants although their recruitment is very slow. Sometimes in absence of seeds, broken stems and branches of plants which are partially buried in sediment can produce roots by sprouting (Hulett and Tomanek, 1961). Waisel (1972) found that cuttings under different concentrations of salts result in a significant drop in the development of aerial parts above 0.3 M NaCl. Plants usually grow in soils with an average salinity of 6000ppm and can survive in sites with salinity from 18,000 to 36,000 ppm (Glenn and Nagler, 2005). Seed germination and seedling establishment of saltcedar is higher in areas where environmental conditions such as salinity and high temperature control soil water potential (Gadallah 1996). Saltcedar is disadvantageous once it gets established because it can spread salt in soil surface and subsurface. Upon establishment of saltcedar, native riparian vegetation can be replaced due to vast changes in channel morphology and hydrologic regimes which directly provides negative impacts biodiversity of aquatic macroinvertebrates and habitat for wildlife (Graf 1982).

Saltcedar commonly approaches monoculture in the areas where it grows as the understory environment due to its presence can only be tolerated by extremely xeric or halophytic species of plants. It also brings salts to the surface of the soil and wins over native plants in competition for sunlight, moisture and nutrients (Schopmeyer 1974). High rates of invasion of saltcedar have been attributed to clearing of sottonwood stands and availability of high amount of saltcedar seeds historically (Horton and Campbell 1974).

Cottonwood and saltcedar both have identical germination requirements; however, cottonwoods often grow rapidly once the seedlings are established after germination (Merkel and Hopkins 1957). Saltcedar is found in higher densities than native species of cottonwood in semi-arid and arid altered floodplains although growth of saltcedar is slow compared to cottonwood because conditions in such areas favor its growth. In an experiment carried out in green house settings about analyzing the competition between cottonwood and saltcedar at seedling stages, Sher et al. (2000) suggested that within certain densities saltcedar is dominated by cottonwood. Bhattacharjee et al. (2006) in similar study in field settings of Rio-Grande Valley concluded that for native cottonwood seedlings to compete and grow faster than exotic salt seedar seedlings favorable germination conditions such as natural or stimulated flood pulses to closely emulate historical hydrograph must be established in floodplains. After a certain density, establishment and growth of native cottonwood seedlings is affected more by increase in competition within themselves than competition with exotic saltcedar (Bhattacharjee et al. 2006).

Hence, cottonwood seedlings are very important to be looked after for survival and establishment to ensure reestablishment of these native dominants of riparian forests. On the other hand, it’s urgent to apply management implications using proper techniques to limit the establishment of saltcedar seedlings as best as possible to avoid rapid spread of exotic saltcedar. Therefore it’s important to understand the role of both biotic and abiotic factors due to which establishment of invasive species gets affected and to apply efforts to control such invasions which can ultimately help in proper management of native species.

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