Method:
- Select 2 areas of woodland. One with a majority oak canopy cover and one with a majority larch tree canopy.
- Mark out an area 10 meters by 10 meters in each area.
- Measure aspect and slope using a compass and clinometer.
- Using a random numbers table select co-ordinates within predetermined area and place quadrat at the co-ordinate.
- Using the light meter measure the light intensity reading taking care not to shadow the equipment, place the meter in the centre of the quadrat and leave for 30 seconds.
- Using the meter ruler measure the lengths of all the separate bracken shrubs occurring in the quadrat. Note down the number of bracken and the height of each one so that a mean can later be worked out.
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Systematically take the temperature using the soil thermometer probe every 5 quadrats. Place the probe in the centre of the quadrat and leave for 1 minute. Also take a soil sample at every 5th quadrat, using a trowel placed a fair amount of soil in plastic bags making sure you label them. pH will be investigated back at the lab (done by adding barium sulphate to sample along with deionised water and then universal indicator, colour changes will be compared with known results).
- Throw 20 quadrats in each area using the random numbers co-ordinates.
- Mean results will be used for comparison using the students t-test: this statistical test will be used as it is better for analysing results with larger samples.
- Results will be displayed on a bar chart using error bars (the light meter will fluctuate most so the error will be + or- 10 x10LUX. I will measure the bracken to within 0.5 of a centimetre so this will be the error for the heights showed on the graphs).
Safety Notes:
Certain safety precautions must be undertaken when carrying out the plan:
- Remember you will be working on a hillside so wear sensible foot ware and be careful when walking in steep/wet areas.
- Make sure supervisors know where you are at all times and keep in groups when moving to new sites.
Also make sure that you do not trample on any of the bracken you are going to measure and be careful not to disturb or harm wildlife in the area.
Implementation:
From the results gathered for light intensity and height of bracken I calculated a running mean and produced 2 graphs showing both for both sites. As you can see from the graphs (graph 1 and graph 2) my results began to taper as the number of quadrats studies increased, this brings me to the conclusion that I took enough results to have a thorough and accurate experiment. Obviously there were some time limits but I think that the amount of bracken studied gives a good representation of the two areas.
The measurements for pH obtained from the soil samples showed that there was a 0.5 pH difference between the soil pH for each site. However every sample taken at each site remained the same which means that there was a consistency in each site.
The slope and aspect for each site were very similar. At site 1 the aspect was 24˚ and the slope 170˚, whereas at site 2 the aspect was 25˚ and the slope was 174˚. Although again I could not control the angle of the slope they are quite close and should not have affected the results too much.
Analysis:
I have produced 2 bar charts one showing the mean height of bracken in both sites and one showing the mean light intensity in both sites. Both bar charts contain error bars which show the margin of error. For graph 3 the error is +or- 0.5 cm because I noted the height of the bracken to with in 0.5 cm. For graph 4 which shows light intensity the margin of error shown is + or – 10% because the light meter had a tendency to fluctuate quite a lot.
Graph 3 shows that the mean height of bracken in site 2 is higher than the mean height of bracken in site 1, there is no overlapping between the error bars which leads me to believe there is a significant difference between the bracken growth in both sites.
Graph 4 shows the mean light intensity in site 2 is higher than the mean light intensity for site 1, again there is no overlapping of error bars which leads me to believe that there is a significant difference between the two areas.
I have decided to carry out some statistical tests from the results which I have obtained because I believe there is a significant difference in the results from each site. The statistical test I will use will be a student t test. A student t test is best suited to the sample size I did because it is better for showing correlations within larger sample sizes. I will do this for both my light intensity and mean height of bracken results.
I will use the following numbers from my results to complete a student t test:
Mean light intensity site 1: 316.05 (x10Lux)
Standard Deviation site 1: 56.30
Mean light intensity site 2: 323.35 (x10Lux)
Standard Deviation site 2: 45.53
Mean height of bracken site 1: 76.32 (cm)
Standard Deviation site 1: 7.93
Mean height of bracken site 2: 128.67 (cm)
Standard Deviation site 2: 14.73
My null hypothesis in concern with light intensity will be: “there is no significant difference in light intensity between sites”.
My null hypothesis in concern with height of bracken will be: “there is no significant difference in height between sites”
From the results of the statistical tests I compared the values to the critical values table of known results. From this table at 19 degrees of freedom the results must be between 1.33-3.56 or higher to reject the null hypothesis. Therefore based upon my results I can reject the null hypothesis concerning the height of the bracken, meaning there is a significant difference in bracken growth between the two areas. However I must accept the null hypothesis is concern with the light intensity, as no significant difference has been proven between the two areas.
Conclusion:
As my results concerning the height of bracken support my original hypothesis and the statistical test shows that there is very little probability that my results were obtained by chance. I can conclude that there is a significant difference between bracken growth in the two areas, but this was not proven to be due to the light intensity as there wasn’t a significant correlation between the two.
I feel that if the experiment could have been done at the same time on both sites that the light intensity results would have been more significant.
Further Knowledge:
I believe that the higher the light intensity such as that in a larch woodland with little canopy coverage bracken will grow higher because more light allows the plant to photosynthesise more, which would make it more abundant and grow to its full potential. In areas of increasing canopy cover such as an oak woodland bracken will still grow but not to the same heights because light becomes a limiting factor, this means it cannot do as much photosynthesis and will reach a smaller peak height because its cannot produce food to feed anything larger.
Green plants photosynthesise when there is light. The light they need is absorbed by chlorophyll found in the chloroplast cells in the leaves and stems of plants. Solar energy is converted into chemical energy and is used to convert carbon dioxide and water into glucose releasing oxygen as a by-product. The rate of photosynthesis can be limited by light.
Photosynthesis involves light-dependant and light-independent reactions. The light-dependent reaction has 2 products which are needed for the light independent reaction to occur.
During the light-dependent reaction ATP and reduced NADP are produced whilst the chlorophyll molecule absorbs light energy. The process occurs because electrons in chlorophyll become ‘excited’ when exposed to light and are emitted from the leave; electrons then travel between the molecules as they act as carriers, some of the released energy is also released synthesising ATP.
Reduced NADP occurs through the transfer of electrons between chlorophyll molecules, then when water replaces these missing electrons hydrogen ions and oxygen gas become released from the water. Hydrogen ions are then free to bind with the NADP reducing it in the process.
In light-independent reactions ATP and reduced NADP are used with carbon dioxide to produce sugar in the stroma of a plant. An enzyme completes the process by adding carbon dioxide to a 5 carbon sugar forming Gp (glycerate-3-phosphate). The reduced NADP then reduces the Gp using ATP forming glyceraldehydes phosphate. (Called the Calvin cycle).
Competition:
Competition is the striving of different organisms to obtain resources from the environment, this occurs when two or more organisms in the same community compete for some resource that is in limited supply.
The competition between organisms theory could be used to explain the difference in the height of bracken between the two areas of woodland. In site 1 (the oak woodland) I noticed that there was a higher diversity of plants in comparison to site 2 (the larch woodland). A higher diversity of species leads to more competition for resources such as water, food and light. This could explain why the bracken was not able to grow to its full potential because its resources would not allow it, whereas in site 2 there was less competition allowing the bracken to become abundant.
I also discovered that the soil of larch woodland has fewer nutrients than that of oak woodland, and therefore would not be able to sustain as high a diversity of species. So where some species where not adapted for the low nutrient levels the bracken has developed genetically allowing it to take advantage, and become dominating.
Evaluation:
During the course of the experiment there were many limiting factors which may have had an effect on the accuracy of results. Things such as:
- Light intensity is very changeable and I noticed that my results in site 2 although were higher than site 1 were not as high as I had previously expected them to be. This was probably due to taking readings at different times of the day because due to time constraints I was not able to do all of the readings at one time. Site 1 readings were taken in the morning whilst site 2 readings were taken in the afternoon, this could account for some cross over in results as the sun would have moved into a lower position in the sky by the time the monitor was used.
- There was a noticed change in pH between the areas and site 2 was more acidic. From the research I previously did I discovered that bracken preferred to grow on acidic soil, site 2 was more acidic than site 1 which could have affected the accuracy of the results. Therefore the lower pH could account for the higher growing bracken in site 2.
- Another limiting factor is that I may have overlooked some rare species when carrying out my experiment, as I was only searching for bracken.
- Bracken spurs during between June and August and the leaves begin to die out in autumn. I noticed that some of the bracken I measured was wilting which may have affected some of the measurements I have for my results. In future it would be better to measure the bracken during peak season once it is fully grown to get a better idea about the height differences.
- Although my statistical test supported my hypothesis there is still a small probability that my results occurred through chance, as always there is a margin for error to occur so my results can only be an indication of the experiment.
If I was going to do the experiment again I would use a computer with many light intensity monitors to take the light readings at the same time. This would improve accuracy and allow for a better comparison. I would also investigate further the abundance of bracken in relation to the light intensity which may strengthen my own findings.
