History Of Mechanical Vegetation Management Environmental Sciences
Vegetation management has been around fairly recently in terms of river management plans. However, since its introduction to river management techniques , vegetation management has become increasingly popular due to its less expensive and shorter time scales in comparison to other techniques. It is worth noting that vegetation has different aspects and characteristics. Examples of this include; overbank & bank side plants, emergent plants, floating plants, submerged plants and algae. Vegetation is important within river systems as the primary food source but excessive plant growth can cause problems within the river system. These problems include threats to livestock and people as floating weeds can disguise deep water, alien species cause losses to native plants. As well as this dense weed and floating weeds can cause blocked weirs and silts, dense weed can also cause impeded flow. Because excess weeds can interact and interfere with flood defences and controls, human activities and habitats of native species there are various techniques which have been put in place in an attempt to counteract this growth. There are four main categories that are used to provide vegetation management, these include mechanical factors such as cutting or dredging, biological factors including animal grazing, chemical factors like herbicides and environmental factors such as nutrient stripping. These techniques will be analysed throughout this essay.
Cutting is mainly used for emergent, floating and submerged plants along river systems. By using the cutting method the river gains hydraulic improvements and can help stimulate re-growth of other native plants. This method should be used in parallel with the river and wildlife needs surrounding the vegetation, if it is removed too early or too late some specie numbers may fall drastically due to predation and lack of cover. Generally, mechanical cutting is performed in summer whilst plants are closest to the river surface, the plants are then cut below the surface, towards the bed of the river (Washington State Department of Ecology, n.d.). This will then create floating debris which is collected in order to reduce the chance of the plants re-rooting. By using this method open areas of water are created almost immediately and habitats for water species are retained provided the plants are not cut short. However, as plants have only been cut it is a certainty that the species will grow back and some can be difficult to cut in the first instance. If the debris of plants are not fully collected there is a possibility that alien species could form (Government of Tasmania, 2013).
Another example of mechanical vegetation management is dredging. Dredging is mainly used to combat emergent and submerged plants. One way in which submerged plants can be dredged is by using experienced divers which does not require any previous controlled selection and it known to effectively remove most submerged vegetation (U.S. Army Corps of Engineers, 2012). However, because the selection is uncontrolled some aquatic species could be removed as their habitats will have disappeared through the dredging action. Using divers for dredging is mainly to tackle invasive plant species, also by doing this action sediment along the river bed is managed and relocated. This method is known to be successful within the Washington state of America and is also being used by the U.S Army Corporation of Engineers. Usually dredging is used within river systems to manage flow, velocity and sedimentation, the removal of the submerged plants will enable the management of aquatic plant growth. Although dredging has advantages, there are also concerns about the high cost of implementing the method as well as investigations of potential re-growth of invasive species downstream. The diver dredging method also has disadvantages such as the decreased visibility of divers and their safety (U.S. Army Corps of Engineers, 2012).
As well as the U.S. Army Corps in Washington, America, another example of how dredging has been effectively used is in Wales. Dredging took place within the Gwent Levels which was monitored for two years after the dredging took place (Wade, 1993). Wade found that the species present before dredging took place had a strong influence on vegetation remaining once dredging took place, as well as the vegetation that grew back. Wade (1993) found dredging to be an extreme management action for aquatic plants and sediments and therefore sought to expose any effects dredged vegetation had on the development of future aquatic life. In 1984 seventeen sites along channels of the Gwent Levels were identified for dredging (Countryside Council for Wales Landscape and Wildlife, n.d.) and preliminary observations were taken by Wade (1990) in order to determine percentage vegetation cover, water depth and sediment depth. All seventeen sites were dredged within a four month period using a hydraulic dredger. After a two year period Wade revisited the sites and observed the same tests before the dredging (Wade, 1993).
It was noted that the dredging operation had successfully removed vegetation and sediment across each of the sites, and two years after the dredging had taken place the same number of species had returned to the sites, however it was also found that different species had re-colonised. Wade (1993) found that across the seventeen sites 61% of the post-dredging vegetation were similar species to that of the pre-dredging vegetation. Most of the species that reoccurred after dredging were floating aquatic plants, most of which were found to be invasive species which could have become dominant due to the removal of accumulated sediment within the channel and therefore provided optimum conditions to grow and reduce non-invasive species (Wade, 1993).
As previously mentioned, mechanical control techniques are not the only ways of managing vegetation. Chemical techniques such as the use of herbicides have become increasingly common in terms of river management, but can also be reviewed as a potential solution. Because aquatic vegetation can cause problems for flood control and recreational activities it is essential to keep monitoring their growth rate. Within the UK, 63% of river vegetation is being managed by mechanical and chemical controls (Brooker and Edwards, 1975). Chemical controls tend to be cheaper as herbicides are readily available from many companies. Within the UK only eight herbicides have had permission to be used in freshwater habitats from the Pesticides Safety Precaution Scheme (DEFRA, 2008). Brooker and Edwards (1975) created a study in order to observe initial and secondary effects herbicides have on aquatic plants. They found that after the herbicides were used, aquatic plants would become poisoned and would die individually, this raised concerns about the water supply being contaminated with poisonous chemicals, but it was found that unless consumed in excessive amounts, there would be no danger to humans or animals. If herbicides are used for submerged aquatic plants, there is potential for the river concentration to alternate and therefore resilient, invasive species could become more popular within the surrounding area (Brooker and Edwards, 1975). This was then noted to have a direct effect on aquatic fauna and the diversity of the river system was reduced.
A method of management, with similar popularity to that of the chemical control is a biological control; grazing. Grazing became a popular method for managing vegetation because it is affordable and continuous. Targeted grazing is specifically named for the controlled grazing of livestock, for a chosen purpose (University of Idaho, 2012). It is usually more common in the control of invasive species, but it is not becoming a popular management technique for river bank preservation. Livestock can only be used to control emergent and submerged plants, but only if the river is shallow. Targeted grazing is an effective technique if cattle are used as it can be as effective as both chemical and mechanical control methods, especially for invasive vegetation management. It is also seen as an environmentally friendly solution to other methods as it can be used along road less areas, such as river systems, as well as this the method doesnââ‚¬â„¢t leave any herbicide residue, it can be removed almost immediately if no longer required and could improve the biodiversity of the river system. Having said this the effects of targeted grazing are slow although continuous (The Riparian Project, 2011). Usually at least a minimum of three years is needed before differences in vegetation is noticeable. Although the grazing process is slow, this method of biological control is key when vegetation is low-lying along the river and if carefully managed, the technique can restore a balanced ecosystem within the river community. Grazing has also proved to be useful in reducing the growth of invasive vegetation along the riverbank and increase the rate of colonisation amongst native species (Launchbaugh and Walker, 2006).
The final method of control this essay will take into account is an environmental aspect. The term shading refers to the plants ability to tolerate low light (Canham, 1989). This method is not only ecologically sound but it is the more natural choice for river vegetation management. Vegetation shading became part of the Willamette River flow and temperature models that were used whilst assessing the effects of restoring riparian vegetation on the Willamette River. Other effects that were monitored included tree-top elevation and the distance from the centre of the river to the riparian margin (USGS, 2007). Studies such as that of Dawson and Haslam (1983) on effects of aquatic plants and both natural and artificial shade proved that low levels of light can be a good measure for vegetation management. It was demonstrated that the reduction of light affected submerged aquatic plants . This was later proven to be true for both artificially shaded and naturally shaded areas. However it should be noted that whilst studying this theory Dawson and Haslam (1983) found problems occurring in areas mainly within open or densely shaded spaces amongst the ecological stability of species. In addition to this, if there is a recurring absence of aquatic plants native to the area through mechanical, chemical or biological methods there could be problems concerning deoxygenation (SEPA, 2009). After this deoxygenation was observed it was later acknowledged by Dawson and Haslam (1983) that natural shade created by trees, bushes or marginal plants is the most advantageous river vegetation management.
Vegetation management has a variety of reasons for its validity . These range from minimising channel roughness and maximising carrying capacity of a river to aiding the stability of banks and controlling species and habitats. By using various methods, it has enabled man to control the growth of vegetation to protect the natural environment and enhance flood controls. Some solutions are more effective than others such as shading being more effective than the use of herbicides. This is evident for a number of reasons such as ecological sustainability, as well as shading being a long term presence rather than a short term. To summarise, there are many different courses of management that can be used when controlling vegetation, however, from the examples within this essay it is important to review both the positive and negative impacts each method has, as well as the general area which is to be controlled before opting for a specific type. By doing this, flood control and vegetation management will become easier to work with and therefore more sustainable.
Article name: History Of Mechanical Vegetation Management Environmental Sciences essay, research paper, dissertation