VEGETATION STRUCTURE

VEGETATION STRUCTURE OF PLANTS

VEGETATION STRUCTURE

Vegetation structure is defined as the organisation of individuals in space that constitutes a stand of plants. Introduced exotic plant species or clearing of this vegetation, may influence the vegetation structure(organisation), proportional abundance of species, guilds or growth forms in that community.

Vegetation structure is defined as the organisation of individuals in space that constitutes a stand of plants. Introduced exotic plant species or clearing of thisvegetation, may influence the vegetation structure (organisation), proportional abundance of species,  guilds or growth forms in that community. Communities are commonly assessed using species diversity, but change occurs at different levels of community organisation. For example, species turnover does not necessarily imply change in the physiognomy of a plant community, similarly a turnover indiversity of plant guilds or growth forms also does not necessarily illustrate a change in community diversity (Bredenkamp & Theron 1985). Hence changes to the community, brought about by clearing or invasion, may not be reflected in community diversity, but may be evident in vegetation structure. Information on vegetation structure may reflect the extent of invasion and modification to the community  caused by disturbance, and potential implications thatthese hold for community function and regeneration (Bredenkamp & Theron 1985; Breytenbach 1991). Vegetation structure also has an important role to play in the stability and diversity of the community (Shugart & Seagle 1985). A disturbance, such as invasion by exotic species, may influence species entering the community (Geldenhuys et al.1986). A loss in vegetation height class for example, may result in an alteration of under canopy species abundance and diversity. It is known that shade tolerance is importantin community structure, as shade tolerance is the primary criterion used for arranging vegetation into a competitive hierarchy (Horn 1971; Keddy & MacLellan 1990). Hence, a change in canopy cover could alter community characteristics, both structurally and in terms of composition. Possibly the greatest impact of altered vegetation structure has been observed within the eastern escarpment silvicultural zone. Here, an increased woody element formed by invasion and/or plantations has formed thickets changing the appearance and character of the landscape (Henderson & Wells 1986). Although vegetation structure may be used to measure vegetation change, vegetation biomass has been found not to be a good measure, as indigenous vegetation may have a similar biomass to exotic vegetation (Versfeld & van Wilgen 1991).

Distribution ofPlant Species with respect to Environmental Variables

Ecology is the study of distribution and abundance of living organisms resulting from abiotic and biotic interactions in nature. Plant growth is determined by abiotic factors that construct its fundamental niche, defined as the range of conditions and resources within which individuals of a species can persist. If a plant species exhibits patchy distribution and plot size behavior, this may illustrate global or local variations in distribution of abiotic factors within the habitat. These abiotic factors included the following: soil and water pH, soil moisture, dissolved oxygen, redox potential, and phosphate and nitrate concentration in the soil.

Classification of environmental variables

Austin (1980) classified environmental variables based on their biophysical importance, identifying three basic types of environmental variables: “resource variables, direct variables, and indirect variables.” Resource variables such as light, water, and soil nutrients are those that are consumed by plant species. Direct variables are those that are not consumed but have a direct physiological influence on a plant species. Indirect variables have no physiological effect on the plant species but are in some way correlated with variables that do. Models based on direct and resource variables will be the most robust and widely applicable, but it is very difficult to provide a continuous digital representation of these types of variables (Austin and Smith, 1989). Using these variables for predictive mapping of species distribution is difficult and is still extremely rare (Austin and Smith, 1989).

Temperature:

          The temperature of soil and air also has been found to have an indirect effect on plant communities. As the temperature increases, especially night time temperature, there are decreases in soil moisture, depth, and nutrient loads (Lewin, 1974), all of which have a great impact on plant growth (Nichols, 1923).

Light Intensity

            Light intensity must also be taken into consideration because its role in photosynthesis makes it one of the most important factors in a plant’s growth. Voicu & Comeau (2006) have shown that a decrease of light near aspen stands due to shade decreased the ability of young spruce trees to grow.

Angle of Slope

            Angle of slope plays a major role in the variation of plant communities structure.The angle of the slope also greatly affects how much water infiltrates the soil and subsequent runoff and erosion, which influence both soil type and growth of vegetation (Butler et al., 1986). While these have an impact on plant growth, light, soil, moisture, and temperature have been shown to have the greatest effect on plant growth (Butler et al., 1986; Huebner, Randolph & Parker, 1995; Thomas & Anderson, 1993; Nichols, 1923).

Moisture

            A number of studies support for the idea that the type and moisture of soil have great influence on plant growth. McCarthy & Small (2005) found that nutrient retentions of soil affect the type of soil. Plant communities then reflect those differences in soil type and moisture (Voicu & Comeau, 2006). For instance, climax vegetation very often is an indicator of soil moisture regimes (Daubenmire, 1968). Soil type has also been found to be affected by sedimentation and erosion (Butler et al., 1986) which could be of influence with a steep elevation gradient.