The Effect of Light Intensity on the Abundance of White Clover - Coursework Example

?The Effect of Light Intensity on the Abundance of White Clover (Trifolium repens) This investigation looks at the effect of light intensityon the abundance of white clover (Trifolium repens)., using variations in cover provided by the canopy and exterior to the canopy to determine this. A transect was run from underneath a maple canopy for 20 metres and measurements of the coverage of clover as well as light intensity were made every metre, in addition, measurements of control variables were made every five metres A significant relationship was found between light intensity and clover abundance, and this relationship did not appear to be influenced by any of the control variables. Thus, the amount of available sunlight plays a significant role in the growth of clover and as a consequence, its growth may be able to be controlled by modifying the amount of light it receives. Introduction: White clover (Trifolium repens) is a perennial, low growing plant with trifoliate leafs consisting of three oval shaped leaflets, normally found in grasslands . It is very common, and is prevalent in lawns and pasture. It is often considered a weed, and as such there are many commercially available options for controlling it, such as the use of weed killers . However, one problem with controlling the growth of clover in this way is that it also has effects on any other plants that are growing in the same area, which may be undesirable. The role of white clover is not always a negative one. It is an important species in much of the world as a consequence of its ability to grow alongside grass, producing pastures in which both species are present . It has the ability to fix atmospheric nitrogen, which enhances the growth of the grass species . White clover is often used to suppress the growth of weeds while allowing the growth of desired crops such as mustard . As a consequence, the factors which control the growth of clover are important to understand, whether the desire is to enhance or repress the growth of the species. One important factor in the growth of any species is the availability of light. Plants produce nutrients for growth and reproduction through the process of photosynthesis. This process occurs within chloroplasts which are present in plant cells, but not in animal cells . The amount of sunlight required for the maximum level of growth and reproduction differs between species of plants. This can be illustrated by the concept of ecological succession. Here, a community progresses from one form into one that is different in both structure and composition over a significant period of time. Succession occurs either through creating a community in a new unoccupied habitat, or following a disturbance. In both cases the first step is the recruitment of plants that thrive under high light conditions. These are often called the pioneer species, and are often grass-like species. Pioneer species are generally intolerant of shade and are eventually outcompeted by species that are more shade-tolerant, i.e. require less light in order to grow . As a consequence the presence of clover inhibits the growth of some species which require high levels of light, and facilitates the growth of haters that are tolerant of shade. It is likely that the same applies of clover itself. White clover has been shown to respond differently to the quality of light that it was grown under in the laboratory , so it is likely the amount of light would have some effect on white clover growth in the natural environment. The aim of this study was to use a naturally varying range of light to determine whether the amount of light available had any effect on the abundance of white clover. Plan Hypothesis: There will be a significantly positive relationship between available light and the abundance of white clover. This is because of the increased amount of light that is available to the clover as it emerges from the tree canopy allows for increased growth and hence increased abundance, while under the canopy the lack of light decreases growth and hence abundance is also decreased. Null hypothesis: There is no relationship between available light and the abundance of white clover. Dependent variable: The abundance of clover is the dependant variable. This was measured as the frequency of clover within a quadrat. Independent variable: The amount of sunlight and the amount of canopy that is present Abiotic Control variables: Soil pH Wind Soil Compaction Humidity All of these variables occur naturally in the environment, and as a consequence cannot be controlled directly. However, they must either be held constant or variation within them be taken into account, in order for the experimental test to be valid. All four of these variables were measured and their values noted every five metres along the transect. If their values differed in to an extent that was likely to be significant, then measurements would have been made for them every metre and they would have been included in an ANOVA along with the main variables to determine whether their variance had any significant effect on the outcome. Risks/hazards: Risk Likelihood Severity Ways of Reducing Risk Falling over equipment Medium Low Be alert Slipping Medium Low Don’t work in wet weather Sunburn Low Medium Wear long sleeved clothing Contamination of cuts or scrapes with bacteria from the soil Low Medium-High Make sure all open wounds are covered; Ensure that sanitizer is used after touching anything suspicious Allergic reactions from stings, flora or insect bites Low High Make sure anti-allergy medicine is present when available. Make sure staff are aware of the potential risk. Avoid insects and harmful flora where possible. Injury from use of unfamiliar equipment Low Medium Make sure operator is familiar with all equipment and usage instructions before experimental work. Cuts or injuries from dangerous objects hidden in grass (such as glass) Medium Medium Take care and work slowly when visibility is limited. Methods: Equipment: Field equipment: light meter, soil compaction pin, soil auger, wind watch, identification chart, winding hygrometer, tape measure, metre ruler, 50x50cm quadrat (containing 100 squares) Laboratory equipment: barium sulphate, universal indicator, pH indicator chart (pH colorimeter kit). Method of Sampling There are a number of different sampling methods available, and the decision for which method to use can significantly affect the outcome of an experiment . Some sampling methods may be relevant for a particular situation but not well suited, while others may be not relevant and one or more may be completely relevant. Strong experimental design involves the choosing of a method of sampling that is without bias and can be analysed in a statistically relevant fashion . One method that was available for sampling was random sampling. This is where the sites that are used for sampling are randomly determined, most commonly by the use of a random number generator which is used to determine random coordinates for the placement of a quadrat . While random sampling is often effective, in this situation it does not provide an adequate coverage of the area being studied, as we are looking at a gradient of light intensity and it is important that the varying levels of light intensity from the base of the tree to the end of the transect are all examined. Because of this stratified sampling is a more relevant technique. Stratified sampling involves taking samples at regular intervals, often along a transect , as is used in this study. Using a tape measure, a belt transect was created that was 15m in length. The transect began at the base of the tree which was the centre of the canopy (Figure 1) and ran outwards for 15 metres. The length of 15 metres was used as it was a significant distance that covered areas that were shaded by the tree canopy, partially shaded and in the open. The distance between the beginning of the belt transect to the edge of the canopy created by the tree was 6 metres and 90 centimetres. The edge of the canopy was determined as being the point during which the leaves of the tree stopped providing shade for the vegetation on the day of study (Figure 2). The distance from the edge of the canopy to the end of the transect was nine metres and ten centimetres. The investigation was completed at 3pm GMT in July and as a consequence the tree canopy did not cover a relatively large area. A brief examination of the data collected from this preliminary transect indicated that it was likely not to provide sufficient information. This decision was made because an increase in the abundance of clover was recorded towards the end of the transect, and it was thought that extending the transect an additional five metres would produce a more reliable result. As a consequence, the transect used for the final study was 20 metres long. In addition, the preliminary transect counted the number of individual plants within the quadrat but this was found to be too time consuming, and unreliable as many of the plants were small or partially covered. Thus, the quadrat squares were used to provide an estimate of percentage clover cover. Figure 1: The outer limits of the tree canopy being investigated. Figure 2: The edge of the canopy. Note, the yellow line does not represent the transect but the limits of the canopy, approximately 7 metres from the beginning of the canopy. The option of sampling type was also relevant when determining how to take samples from along the transect. The experiment aimed to determine the amount of white clover that was present at varying light intensities, and as a consequence, the most effective sampling method to use was one which allowed for estimation of abundance. Quadrat sampling is an effective technique that is widely used, as it allows for abundances to be estimated within an area . Once more, systematic sampling is useful as it allows the different levels of light to be adequately represented. As a consequence, systematic sampling along the 15 meter belt transect was used. A belt transect is a form of transect that examines not just the line of the transect but the area on either side of it . For this study, a 50 centimetre quadrat was used to determine the abundance of white clover. Using a transect also allowed for the abundance of clover to be plotted in relation to the distance from the canopy, information that would not have been available using random sampling methods. Data Collection Every metre along the transect the abundance of clover was measured using a 50x50 centimetre quadrat which contained a 100 squares. The squares on the quadrat allowed the abundance of clover to be measured as a percentage. If clover was present in one square then this was scored as present, if not it was scored as absent. Consequently, the number of squares where white clover was observed was approximately equal to the percentage area of the quadrat that the clover covered. In order to measure the independent variable of light level, a light meter was used at the location of each quadrat to determine the level of light that was present. To ensure that the measure was consistent across samples, the light level was measured at a height of one metre above the ground, measured using a metre ruler. In order to measure the soil pH, a soil auger was used to gather a sample of soil at each five metre interval along the transect. A 1cm3 sample of this was placed in a beaker, to which was added 7ml of barium sulphate and 1.5ml of universal indicator. After waiting two minutes per sample the pH of the soil could be estimated based on the colour that the final solution was, compared to the universal indicator chart. This allowed for the determination of what the acidity present in the soil was, as this can have a potential effect on growth. To measure soil compaction a soil compaction pin was dropped into the ground from the height of one metre every five metres along the transect. The depth the pin was immersed in the soil indicated what level of compaction the soil had. Wind velocity was measured using a wind watch at the height of one metre every five metres along the transect. The measurement was made in the direction that the wind was moving to ensure consistency between samples. The level of wind at each interval was recorded. For wind velocity, soil compaction and light intensity three separate measures were taken, and the mean used for analysis. This was to minimise measurement error. The soil auger was not repeated, but care was taken to ensure that every step in the process was done accurately. Finally, counts for the abundance of clover were done twice and the average taken to ensure that the counts were consistent. Data Analysis Data will be analysed by using regression analysis, determining whether there is any significant relationship between light intensity and the abundance of clover. In addition, the control variables will be examined to determine whether their levels are likely to have any effect on the abundance of clover. Finally a t-test will be used to determine whether there is any significant effect of light on clover abundance. Results Table 1: Prevalence of clover in each sample quadrat in terms of abundance, percentage cover and absolute number of plants (preliminary results). Distance of sample (m) Abundance Clover Cover (%) White clover 1 Rare 0 0 2 Rare 1 1 3 Rare 0 0 4 Rare 0 0 5 Rare 0 0 6 Rare 2 4 7 Rare 4 6 8 Frequent 37 42 9 Occasional 17 33 10 Frequent 53 77 11 Common 69 64 12 Common 80 110 13 Frequent 48 45 14 Frequent 62 57 15 Abundant 92 136 The preliminary data was gathered from a 15 metre transect to determine whether the length was sufficient for study. As described in the methods, it was determined that the transect should be extended a further five metres for the final study, and percentage cover, rather than number of individual plants, be used. Table 2: Relationship between distance from canopy and light intensity, soil pH, humidity, wind speed and soil compaction. Distance of sample (m) Light Intensity (SI) Soil pH Humidity (RH) Wind (m/s) Soil Compaction (Mr) 1 41 4.5 16.2 1.2 5 4 21 5.5 17 0.7 4 9 105 5.2 15.1 0.5 4 14 228 5 15.3 0.6 3 20 309 5 15.5 0.7 7 There was no relationship between the distance of the sample from the start of the canopy to pH, humidity, wind rate or soil compaction. Likewise, there was no relationship between any of those factors and light intensity. The samples taken at one metre from the tree did not match the observed trends for most factors. The light intensity and soil compaction were higher than the following measurement, despite general trends of increase for both factors. Likewise, wind speed was close to double the rate at this sample point than any other. This may be the result of interference from the trunk of the tree, or some other unknown factor. However, while these differences may be relevant, they do not directly affect the hypothesis of the effect of light on cover abundance. When the distance from the start of the canopy was ignored, there was a significantly positive relationship between the percentage cover of clover and the intensity of light (Figure 3). Figure 3: Relationship between clover cover (%) and light intensity (SI) along the transect. A single factor ANOVA confirmed the relationship observed in Figure 3, the ANOVA showed a significant relationship between light intensity and clover abundance (Table 3). Table 3: Single Factor ANOVA testing for significant interaction between light intensity and the abundance of white clover Source of Variation SS df MS F P-value F crit Between Groups 164224.225 1 164224 28.40673388 4.7198E-06 4.098171661 Within Groups 219684.55 38 5781.17 Total 383908.775 39         Along the transect there were some areas where there was no growth of grass or clover. All of these areas had animal droppings present (Figure 4). Figure 4: Presence of bare patches of soil along the transect line An addition ANOVA with the results where animal dropping were present excluded (Table 4). Despite the removal of plots that followed the trend, and the decrease in the P-value, there was still as significantly positive relationship between light intensity and clover abundance. Table 4: ANOVA of the relationship between light intensity and clover abundance, with plots where animal droppings were present excluded. Source of Variation SS df MS F P-value F crit Between Groups 173816.5 1 173816.5 32.60711 2.52E-06 4.149097 Within Groups 170580.2 32 5330.632 Total 344396.7 33         Discussion One interesting fact that was observed was the prevalence of plots that had low intensity of both light and clover, in many of these plots no clover was present. These plots differed visually from other plots, as in these plots the soil was exposed and there was no vegetation of any species. In addition, there was animal faecal matter present. The low light intensity occurring in these plots was a consequence of them being close to the tree. It appears that the present of the faecal matter had an effect on the soil which resulted in them being unable to support vegetation. This theory has been proposed in other literature also, such as the article by Thompson, who proposed that the excess nitrogen that is in animal faeces represses the growth of plants . In this case both the dominant species of grass and the white clover were completely prevented from any growth in the bare patches of soil. These results were excluded from the final analysis, as it could not be determined whether the cause of the low clover abundance as light or the presence of faeces. Nevertheless, even with results excluded, there was a significantly positive relationship between light intensity and clover abundance. As a consequence the hypothesis that there will be a significantly positive relationship can be accepted, and the null hypothesis of no relationship can be rejected. There were no trends in any of the control variables, with most of them changing little across the transect line. This indicates that the results observed from clover abundance and light intensity are unlikely to be related to these factors. Limitations of the study The measurements for this study were taken in the summer season during July at around 1.30pm. Replicate samples were not taken, nor were the samples taken on multiple days. The levels of available sunlight vary significantly throughout the year as seasons change, and also vary depending on the time of day. The measure of pH used for this study is not definitive and provides only a rough guide as to the pH of the soil being tested. This is because there is a wide range of colours in the spectrum, but only some are represented within the test, and the determination of the colour of the solution is a subjective measure and as a consequence interpretation is likely to be different for different individuals. Determining soil compaction was undertaken by reading the level that the soil reached on the soil compaction pin after it had been removed from the ground. One limitation of this is that determining exactly where the soil reached on the pin was subjective which left the potential for error. Furthermore, the soil often slid down the pin during the process of removal then measurement and as a consequence excess soil could have been missed, resulting in a higher level of compaction being read than was correct. Recorded wind velocity is dependent on the direction in which the wind is moving relative to the wind watch. While every effort was made to ensure that recordings were made in the direction of the wind, this was not always possible. Improvements This study could be improved by increasing the number of transects that were run, possibly running them at different times of the day so that a more accurate image of the effect of light can be determined. Choosing areas to run the transect where animal droppings are minimal to none may also help to obtain more relevant results. Using no replication when doing an experiment test means that there is a high potential for experimental error that will not be identifiable. As a consequence, any statistical tests, such as the ones done for this study will have low statistical power. Conclusion The study showed strong evidence for a significant relationship between the level of light available and the abundance of clover. In addition the study revealed an interesting effect where animal faeces were able to suppress the growth of all vegetation. The biggest problem with this study was lack of replication, and it could be improved by running multiple transects in different areas or in different areas of the day. Nevertheless, as a consequence of the results the hypothesis could be accepted as there was a significant effect. Works Cited Read More