growth and the numerous roots in the surface soil, reduces the 

 moisture to such a degree that slow-growing annuals or perennials 

 are seriously retarded in establishment, growth, or survival. In 

 Idaho only a few seedlings of the shrub Purshia tridentata are able 

 to survive the competition o^ Bromus tectorum during the first sum- 

 mer, being better able to compete with broad-leaved summer 

 annuals than with BromusJ^^ One reason for the success of 

 crested wheatgrass, smooth bromegrass, and other species that 

 are used in reseeding rangelands, is the rapid development of 

 roots after germination. The top inch or two of soil is likely to dry 

 very soon after a rain, so the seedlings with roots several inches 

 long that penetrate into moist soil have a great advantage in 

 competition over other, shallow-rooted plants. 



Geographic location may influence competition, as in southern 

 Idaho where Agropyron spicatum and Artemisia tridentata compete 

 severely, while in central Washington they seem, instead, to be 

 complementary. ^^ 



The competitive advantage of one species over another may be 

 caused by greater cation-exchange capacity or by greater adapt- 

 ability in modifying this capacity in relation to the soil. Experi- 

 ments have shown that plants such as wheat, beans, tomato, and 

 peas vary in capacity to adapt to different concentrations of 

 cations and anions in the soil. The root CEC, operating in ac- 

 cordance with the Donnan equilibrium, is not the only factor 

 governing ion uptake, for plants also differ in the second kind of 

 passive absorption, diff^usion; and also in capacity to carry on 

 active absorption against a concentration gradient and the Don- 

 nan equilibrium gradients at the expense of respiratory energy.^"^ 



Seedlings of grasses and forbs encounter great difficulty in be- 

 coming established in grasslands that are in good condition, but 

 do so readily in bare areas caused by drought, overgrazing, or 

 other disturbance. The competitive relationships are often intri- 

 cate, as, for example, in New South Wales, Australia, when seeds 

 o^ Bothriochloa ambigua, a grass of low grazing value, and Danthoma 

 spp., with higher grazing value, are both disseminated into a bare 

 area. Here the latter species have only a limited effect on the 

 former, not enough to prevent Bothriochloa from becoming estab- 



Ecological CHaracteristics of Species & Populations • 31 



