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Abstract
Previous studies have revealed quite an interesting phenomenon that occurs in the semi arid lands in western United States. Sagebrush steppe (Artemisia tridentata) is a foundation plant in the Great basin of USA (Inouye 606). This species is ill adapted to the environment is usually vulnerable to environmental effects such as wild fires and other human activities. The destruction of this species often leads to alteration in the ecosystem thus facilitating the proliferation of exotic species.
This study sought to determine how the removal of herbs and increase in nitrogen affects the abundance herbaceous vegetation. The study was carried out in a designated area that had earlier been divided into different plots that were subjected to different treatments as follows; low nitrogen, high nitrogen, no sage brush and a control plot whose conditions were not altered. The results indicate that the removal of brush and increase in nitrogen deposition leads to increased herbaceous vegetation.
Introduction
Sagebrush steppe is traditionally associated with western United States (Bechtold and Inouye 74). However, change in the vegetation in those areas has been influence by growing of crops and grazing activities. In addition, the presence and movement of domestic livestock has increased the spread of plant species that were introduced to this area (Inouye 606).
Cheatgrass (Bromus tectorium) is one of the annual grass species that were introduced and has now spread to vast areas of the intermountain west (Baker 179). This annual grass species spreads easily because of its ability to complete its “vegetative growth stage early in the summer, leaving behind dry materials that burn easily” (Baker 179, par. 2).
Thus cheatgrass is significantly associated with the cases of wildfires as its litter provides fuel for the wildfires. Several species of shrubs that are found in the sage brush steppe communities and more specifically sagebrush (Artemisia Tridentata) are ill adapted to fire breakouts (Inouye 607 ). Usually after wildfire incidences, sagebrush often disappears leaving behind a plant community that is predominantly composed of herbaceous species (Inouye 607).
Studies have indicated that in various plant communities there is always one or a few species that have more access to water or nutrient cycling compared to other species (Rein and Berendse 64). This species of plants are often referred to as foundation species. Some studies have assessed how the foundation species interfere with other plant species in the community (Prevey, Germino and Huntly 39).
It is usually observed that if the foundation species is removed from a given ecosystem then more nutrients are made available to the other species. The increase in resource availability has been implicated as a mechanism for the invasion and persistence of the exotic species (Prevey, Germino and Huntly 39). The sagebrush (Artemisia tridentanta) is foundation plant is found in the “shrub steppe, cold-desert ecosystems of the Great Basin, USA” (Prevey, Germino and Huntly 39, par. 3).
Wildfires and other effects that result directly from human activities have cleared vast traditional habitats of this foundation species. This has caused the exotic herbs to increase in those areas. In direct evidence has suggested that incidences of extensive wildfires may have occurred at intervals of between 30 to longer than 200 years in the sagebrush steppe before the arrival of European settlers (Baker 179). Currently, outbreaks of wildfires occur more frequently at intervals of approximately five years.
Human activities have also impacted negatively on vegetation cover through increased nitrogen output. This nitrogen mostly results from industrial processes such as the burning of fossil fuels, have released large amounts of nitrogen into the atmosphere (Inouye 607). Normally, nitrogen is often a restraining resource in most terrestrial plant communities that includes those found in arid land (Inouye 607).
An increase in the output of nitrogen may lead to an increase in “plant biomass, shifts in the relative abundance of plant species, changes in plant species richness and it may result in a prolonged period during which annual species dominate disturbed sage-steppe vegetation” (Baker 180, par.4).
Rothamsted Experimental station that is located in the UK has for a long time conducted researches on nitrogen cycling in the Ecosystems. Recent data from the station indicate that there is a larger concentration of nitrogen in the atmosphere during winter as compared to summer. This is attributed to the use of central heating systems and to the variations in the direction of wind.
This study sought to identify how the removal of shrubs influences the abundance of herbaceous vegetation and the community structure of a sagebrush steppe community. It also sought to determine what changes may occur in the sagebrush steppe community under the influence of increase deposition of nitrogen.
Thus the study was carried out in a designated area that was divided into different plots subject to different treatments. This was both a field and lab exercise in which a relationship was to be drawn between the treatment and the effect observed on the plants.
Hypothesis
Null hypothesis: The removal of shrubs and increase in nitrogen deposition increases the abundance of herbaceous vegetation.
Alternative Hypothesis: The removal of shrubs and increase in nitrogen deposition does not increase the abundance of herbaceous vegetation
Methods
Study area
The study was carried out on a designated area on Barton road. The area had previously been divided into thirteen plots which were fenced to minimize human and other forms of interference. The study site is located in Bannock county, area that is semi arid with cold winters, moist springs, and warm dry summers. The study area was used as a grazing ground prior to 1990 and was acquired for research purposes n 1996. The area is dominated the Shrub Artemisia tridentata which is mainly responsible for the sagebrush-steppe ecosystem.
Experimental design
The study site was previously divided into 13 plots and four treatments assigned randomly to the plots.
- Control (no treatment) plots # 1,5,7 and 11( N=4)
- Shrub removal. Plots # 4,9 and 13 (N=3)
- Low nitrogen addition (6Kg/ha/yr). Plots number 2,8 and 12 (N=3)
- High nitrogen (12 Kg/ha/yr) plots # 3, 6 and 10 (N=3)
The plots used in this experiment measured 20 by 20 meters.
Field procedures
- The Tragopogon dubius plants were located in the plots. The maximum height for each flowering stock was measured by a meter ruler and recorded for all the flowering plants plus the number of flowering heads that were present. Any other thing that is unusual and by be relevant to the study was also recorded. For instance, the presence of aphids, whether the plant has been browsed by any herbivore and the appearance of any disease condition.
- In the same plots that Tragopogon was sampled, the above ground herbaceous biomass was sampled. Two suitable clipping areas were identified on each plot and a pair of scissors used to cut at ground level all herbaceous (no-woody) plant material that was rooted inside the clipping area. Plants that had their roots outside the clipping frame were not clipped. Four sampling sections measuring 1 by 12 meters were created on the outer part of each plot leaving the center intact. The clipping strips measured 15 x 100 cm and the grass and forbs that had their roots located inside the strip were clipped off at the ground level. Shrubs, mosses, cacti or lichens were not clipped. The grass was then placed in one bag and forbs in the other and then labeled appropriately with the following information; date, plot #, group members and the type of plants. The bags were then transported to the lab, the plants sorted out, the weights measured using a scale and the results recorded in grams.
Results
The results for the different parameters were measured and tabulated as follows. The results forbs biomass and number of tragopogon plants from same plot were entered in one table as shown.
Table 1: Control plots
Table 2: Low Nitrogen Plots
Table 3: High nitrogen plots
Table4: No brush plots
A bar graph showing the average number of Tragopogon plants in the different plots that received different treatments.
A bar graph showing the average levels of forbs biomass in the different plots that received different treatments
Height of stalks and the Number of flowering heads
The tragopogon stalk height and the number of flowering heads were measured randomly in one section of the different plots and the results tabulated in a table that is not shown due to its length. The data was subjected to statistical analysis to determine whether the height of the stalks influenced the number of flowering heads.
According to the p-value there is a significant linear relationship between the number of flowering heads and plant height. R2=0.113915, p value= 3.74E-10, slope= 0.029369, n=327. As plant height increases the number of flowering heads increases due to the increasing regression line. The association between the numbering of flowering heads and plant height is weak based on the multiple R value of 0.338. Only 11% of the variation in the number of flowering heads is explained by plant height.
Discussion
The results obtained from the above study indicate that indeed the nitrogen deposition and shrub removal have led to the increase in the abundance of herbaceous vegetation. The results showed that forbs clipped from plots that had low nitrogen tended to have lower biomass values as compared to those from plots that had high nitrogen.
The number of tragopogon plants is also higher in plots that were treated with high nitrogen as compared to those with low nitrogen. This shows the increased nitrogen deposition as a direct result of human activities such as industrial processes is partly responsible for the increase in herbaceous vegetation.
Plots that had no brush also indicated higher number of tragopogon plants compared to those whose sagebrush was left intact. For instance, the control plots had an average of 71 tragopogon plants while those with no brush had an average of 112 tragopogon plants. This shows that the removal of brush, which is a foundation plant, altered the ecosystem and thus making more nutrients to be available for the tragopogon.
The removal of sagebrush (Artemisia tridentata) and increase in nitrogen deposition are phenomena that occur together and thus can be used to explain the colonization of traditional sagebrush steppe areas with exotic herbs. Measurement of stalk lengths versus the number of flowering heads showed that the increase in plant height leads to an increase in the number of flowering heads. The reason may be that a plant with a long stalk has an advantage over others in terms of access to light for food manufacture.
In this study it was revealed that there was more vegetation abundance in the plots that had high nitrogen rates. This reiterates the claim that increased nitrogen deposition resulting from human activities actually affects the ecosystems.
Increased human activities have modified the nitrogen cycle through increased fixation by legumes, by energy and fertilizer production, and by the mobilization of nitrogen from long –term storage pools (Rein and Berendse 65). The extra reactive nitrogen is readily transported through the environment, and there is increasing evidence that ecosystems are changing due to eutrophication and acidification (Baker 179).
Works Cited
Baker, William. “Fire and restoration of sage ecosystems.” Wildl Soc Bulletin (2006): 34:177-185. Web.
Bechtold, Harry and Richard Inouye. “Distribution of carbon and nitrogen after six years of nitrogen addition and shrub removal.” J Arid Environ (2007): 71:122 – 132. 7th December 2010.
Inouye, Richard. “Effects of Shrub removal and nitrogen addition on soil moisture in sagebrush.” Journal of Arid Environments (2006): 65: 604-618. 7th December 2010.
Prevey, Janet, et al. “Exotic plants increase and native plants decrease with loss of foundation species in sagebrush steppe.” Plant Ecol (2010): 207:39–51. 7th December 2010.
Rein, Aerts and Frank Berendse. “The effects of Increased Nutrient Availability on Vegetation Dynamics in Wet Heathlands .” Vegetation (1998): 1: 63-76. Print.
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