Table 3 — Levels of soil nutrients on the study plots 2 years after 

 fertilization 



Variable Fertilized Not fertilized 



N0 3 -N (mg/kg) 



'5.8 



+ 



0.22 



5.8 



+ 



0.14 



NS 2 



P (mg/kg) 



26.7 



+ 



2.74 



18.0 



+ 



1.31 



0.01 



K (mg/kg) 



66.1 



+ 



4.08 



66.3 



+ 



3.37 



NS 



'Values are mean 



±S.E..n 



= 9. 













'Significance levels for differences between treatments from ANOVA's. 



DISCUSSION 



This section discusses the three null hypotheses I set 

 out in the Introduction section. Also, reclamation strate- 

 gies based on the research may prove helpful to land 

 managers. 



Hypothesis 1 



Increasing density of introduced grasses did not lower 

 establishment or growth of the native species. These 

 findings are contrary to those of studies in which higher 

 seeding densities have decreased species diversity due to 

 increased interspecific competition and competitive exclu- 

 sion of less "vigorous" native species (Holecheck and oth- 

 ers 1981). The results of the current study may be partly 

 explained by the poor establishment of seeded species in 

 general. Densities of the introduced grasses and natives 

 may have been insufficient for significant competitive 

 interactions to occur. The colonizers did exhibit decreases 

 in standing crop with increases in grass seeding density 

 and may have been more sensitive to the competitive 

 effects of the introduced grasses than the native species. 

 In turn, the abundance of the colonizer, alkali sand- 

 spurry, in year 1 may have negatively affected establish- 

 ment of both introduced grasses and native species. 



The high variability in species establishment may have 

 been related more to establishment requirements and site 

 characteristics than to introduced grass seeding density. 

 The saturated soils of the study area early in the growing 

 season may have favored germination and establishment 

 of species adapted to moister edaphic conditions. Tufted 

 hairgrass, muttongrass, and western yarrow occur on 

 both disturbed areas and semiwet meadows near the 

 study site and established in relatively high densities. 

 Species such as spike trisetum, antennaria, silky 

 phacelia, Saskatoon serviceberry, and mountain big sage- 

 brush exist as colonizers on disturbed areas but almost 

 never occur in semiwet meadows. The poor estabishment 

 of these species may indicate a low tolerance to saturated 

 soils. 



Hypotheses 2 and 3 



Fertilization did not have a larger positive effect on the 

 3-year establishment and growth of the introduced 

 grasses than of the native species. Because increasing 

 introduced grass density had no effect on native species 

 establishment, hypothesis 3 could not be tested. Intro- 

 duced grasses, native species, and colonizers all showed 

 large initial responses to fertilizer. However, after levels 

 of soil nutrients had decreased in the fertilized treat- 

 ments, introduced grasses exhibited a larger relative 

 decrease in standing crop biomass than native species, 

 and colonizers had the largest decrease of all. In the un- 

 fertilized treatment, native species had greater increases 

 in standing crop biomass than introduced species. Peren- 

 nial introduced grasses frequently exhibit rapid growth 

 rates and high nutrient uptake efficiencies but maximum 

 expression of these characteristics is obtained only at 

 relatively high nutrient levels (Chapin 1980). Thus, these 

 species show large responses to fertilizers but require 

 sustained nutrient levels to maintain high levels of pro- 

 ductivity. Many native species are adapted to lower nu- 

 trient levels and exhibit slower growth rates and lower 

 nutrient uptake efficiencies (Chapin 1980). In general, 

 they show smaller responses to fertilizers and can main- 

 tain productivity, albeit at lower levels, with lower levels 

 of nutrients. 



The pattern of rapid invasion followed by precipitous 

 decline exhibited by the colonizer, alkali sandspurry, has 

 been observed in other ecosystems for different species 

 (Allen and Allen 1984; Allen and Knight 1984; Redente 

 and Cooke 1986). Immediately following disturbance, 

 availability of nutrients, especially N, can be high depend- 

 ing upon the type of disturbance and the stage of soil 

 development (Tilman 1985; Vitousek and others 1979). 

 Fertilization during reclamation can exacerbate these 

 results. Many early successional species exhibit rapid 

 establishment and growth during this phase, but are 

 rapidly excluded as levels of nutrients decrease and com- 

 petition from later successional species increases. 



The rapid decline in plant available N and K was 

 largely determined by soil nutrient dynamics. A portion 

 of the nutrients added to the site through fertilization 

 may have been taken up by the colonizer species in year 1 

 and tied up in their undecomposed litter in years 2 and 3. 

 However, the soil on the site had inherently low nutrient 

 retention capacity as characterized by a high portion of 

 coarse materials (33.5 percent), low cation exchange ca- 

 pacity (3.2 meq/lOOg), and low amount of organic matter 

 (0.4 percent). Precipitation in the study area is relatively 

 high and may contribute to low retention of mobile nutri- 

 ents. Fertilization of these soils resulted in a pulse of 

 nutrients to the system during the first growing season 

 followed by a rapid decline in the mobile nutrient N and 

 to a lesser extent K. Phosphorus is less mobile and higher 

 levels were maintained over time. 



7 



