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Null hypothesis H0 definition:
The statistical hypothesis that states that there are no differences between observed and expected data.
The Null hypothesis H0 for my investigation will be:
There is no difference in the amounts of moss coverage on the north side of a Yew tree and the north side of an Oak tree.
The alternative hypothesis Ha for my investigation will be:
There will be a difference in the amounts of moss coverage on the north side of a Yew tree and the north side of an Oak tree.
Why the North side?
As moss has no external waxy cuticle, and the leaves are very simple and are usually one cell thick, moss is found on the north side because it is out of direct sunlight so there is less of an issue with water retention and it will not shrivel up and die. Without a waxy cuticle, mosses dry out fast when the air dries.
There will be many biotic variables to this investigation I have researched which can limit or encourage moss growth, which is what I will be analysing.
To case a point, Yew trees are coniferous trees, so they keep their foliage all year round and they have dense foliage, which is what also makes the trees suitable for use as hedging. Green plants need light to photosynthesize so with insufficient light availability, I would think the conditions are not particularly suitable for moss to grow in. The dense foliage also limits rainwater infiltration, so with reduced rainwater and moisture levels in this area, it will discourage moss from growing.
Ethical considerations and safety points
During my investigation it is important to conserve the natural environment and ecosystems of the forest and leave habitats undisturbed. When sampling data, I will be careful where I walk to ensure I do not trample any living plants. I will try to make sure I do not break any branches or damage the plants and their surrounding areas in any way. I will be careful where I place the equipment and be sure not to leave any litter behind. If I move things I will make sure I put them back so I leave the natural environment unchanged.
I will work in groups when collecting data, and make sure I am in other people’s view at all times and not wander. I will wear the appropriate clothing such as walking boots, to ensure I don’t slip on wet rocks for example. I will make certain I am aware of the location of the first aid kits in case of any accident.
Implementing
Method of collecting and recording data
In the classroom I created a results table which I would use to record the data I collected in the forest. I then gathered together all the equipment I would need to carry out a successful investigation and boarded a minibus. When we reached the forest I joined my group and located areas to start collecting the data (the percentage of moss coverage on Yew trees and Oak trees). The data I collected would be used to prove or disprove my hypothesis and null hypothesis. I sampled and recorded the data for the Oak trees first, 15 Oak trees in total. I was able to identify the Oak trees by the distinct rough bark on the trunk. I recorded the data for the 15 Yew trees second. I was able to identify the Yew trees by their green pine needles. The way in which I went about locating the trees was to find each tree one by one by looking out for their distinct features. The Yew trees were all located around a certain dark area which was very shaded. Most of the Oak trees I located were close to the footpath in areas with plenty of light and openings in the canopy.
When I found an appropriate tree I measured the girth of the trunk by wrapping a length of string around the trunk measuring roughly 1.5 meters, to standardise the procedure. Then, I opened the compass and located the north side of the tree. I then looked in the book to check what I was looking at was moss, and not lichen for example which is also green in colour and fury. I then placed the meter ruler against the north side of the trunk and held the quadrat above it against the trunk, with the top right hand corner of the ruler touching the bottom left hand corner of the quadrat. I then calculated the percentage moss coverage by counting how many of the 100 squares of the quadrat were placed on top of moss. Once I had a percentage I recorded it in the results table under the appropriate column. I repeated this procedure for every sample. In total I had 30 moss coverage results, 15 for the Oak trees and 15 for the Yew trees.
Accuracy and validity of results
To ensure the accuracy of my results I took 15 results for both tree types to obtain sufficient data for statistical analysis and to eliminate any anomalies. In the forest I double checked my moss coverage calculations to ensure I hadn’t counted them incorrectly. When sampling the data I will also try to refer back to the plant species pamphlet to ensure I was not confusing moss with other similar species such as lichen. In the classroom I checked my Mann Whitney U test a few times to ensure I hadn’t made any mistakes. In the field I will steady the compass for a minute or so to ensure I am sampling from a northern direction.
Analysing Evidence and Drawing Conclusions
The main trend and outcomes of the results
The main outcome of my investigation illustrated by the results table shows clearly there is much greater percentage moss coverage on the Oak trees than the Yew trees. Many Yew trees in fact have 0% coverage and the highest is 20%, compared with a highest of 100% on Oak.
Therefore, not only does moss grow more on Oak trees, the outcome is that moss growth is exponential on Oak trees compared with Yew trees. Using the bar chart (graph 3) to calculate the average moss coverage, Yew trees have a mean of 2.93% moss coverage and Oak trees have a mean of 72.4%. So there is a significant difference between the groups of results.
My bar graph (graph 1) and scatter graph (graph 2) further illustrate this, as they show that the moss coverage percentages are very high for the Oak trees (predominantly 70-100%) and very low for the Yew trees (mainly 0-4%). The scatter graph shows strong groupings, with the Oak tree readings concentrated towards the top end and the Yew tree readings are concentrated at the base of the graph.
To case a point, if we look at trees number 9 on my bar graph, the Oak tree here has 100% moss coverage while the Yew tree has 1% moss coverage. If we look at trees number 15, the Oak tree has 98% moss coverage, while the Yew tree has 0% moss coverage.
Using my Mann-Whitney U test calculation, I can safely reject my null hypothesis. To reject it my result had to be less than or equal to 64, my result came out as 4. Therefore, according to the table of critical values of U at the 5% level, I can be 95% certain I can reject the null hypothesis.
Using the evidence supplied in my results, I can conclude significantly that the preferred habitat of moss is on the north side of an Oak tree rather than the north side of a Yew tree.
This agrees with my alternative hypothesis: There will be a difference in the amounts of moss coverage on the north side of a Yew tree and the north side of an Oak tree.
Why is the north side of an Oak tree preferred to the north side of a Yew tree?
I believe there can be many reasons why the preferred habitat of moss is on the north side of an Oak tree instead of the north side of a Yew tree.
Firstly, the branches of deciduous trees are shaped to funnel water down their trunk, encouraging ideal moss habitat which is damp and moist. Coniferous trees branches are shaped to shed snow and flake off, discouraging moss growth. So it is not surprising there was often 0% moss growth on the Yews.
Green plants need sunlight to some extent in order to photosynthesise to synthesise organic molecules, to which respiration uses the energy in organic molecules for other processes in an organism. With mosses, the cells of the developing sporophyte soon become photosynthetic, therefore making it able to or at least partially synthesize its own nutrients. The spores of many moss species require light before they can germinate.
Yew trees are coniferous trees with dense foliage and the areas underneath them are very dark and shaded all year round. So with respect to this, there may not be sufficient light levels for the moss to be able to grow properly which may explain the lack of moss on the trunks of the yew trees. The 20% moss coverage anomaly on one of the Yews could be explained by an opening in the canopy due to dead branches, or a location where the bark had not yet shed off. The Oak trees are deciduous and they grew where there were quite a few openings in the canopy due to leaves falling off. So there would be sufficient light levels for the moss to photosynthesise, but not dry them out because of the direction they were situated, away from direct sunlight, hence the high moss levels.
Moss itself requires moisture to survive because of the small size and thinness of tissues, lack of cuticle (waxy covering to prevent water loss), and the need for liquid water to complete fertilisation. When there is enough moisture present, mature antheridia (the male sex organs) release their sperm, some of which may swim to the archegonia.
Since mosses and similar species such as lichens lack roots, surface absorption of rainfall through their tiny leaves by osmosis and diffusion is the only means of obtaining vital nutrients which are dissolved in rainwater. Instead of roots, they have a system of multicellular rhizoids, simple root like structures whose major role is that of anchoring the plant. Their lack of vascular tissue restricts mosses to moist or wet habitats. The rhizome tends to be fuzzy; this helps the moss absorb water and nutrients.
As mentioned, Yew trees are coniferous and have a dense canopy cover. Therefore this will very much limit infiltration of rainwater and prevent the moss receiving much of the vital nutrients which it needs to survive. The soil will also be dry and hard so there will not be a damp, moist environment, this limiting factor could account for the very low 2.93% average moss coverage. The Oak trees on the other hand were situated where there were many openings in the canopy.
The Oak trees grow lighter areas, more open to the weather, whilst there is still quite a lot of canopy cover. This creates the ideal conditions for moss as moist shade is its preferred habitat, where it is cool, damp and out of direct sunlight, particularly on the north side. These conditions must have been ideal as one of the moss readings was 100%. Because moss lacks traditional vascular structures of true leaves, stems, and roots, its growth is limited to moist locations.
The deeply furrowed surface of Oak bark is a convenient place for mosses and similar species to grow. They attach to both living and decaying trees. In contrast Yew trees have flaky bark, which regularly peels and shreds. This provides a less stable growing surface for moss. Mosses and Lichens use tree bark only as an anchor, obtaining their nutrients from the air.
There is further explanation why moss grows mostly on deciduous trees (the Oak tree in this case). Also, the forest we visited had limestone underneath, which is alkaline. Yew trees prefer chalky soils, and limestone soils, so the pH of the Yew tree bark could be more alkaline as a consequence. The optimum more acidic pH for most moss to grow in is around 4.0-5.0. So the initial physical conditions may not be suitable for the moss to grow on the bark. Changes in pH will alter ionic charges and alter the tertiary structure of the enzymes, possibly causing denaturation. Therefore, moss would not survive in conditions away from the optimum.
Moreover, the leaves, bark and seeds of the Yew tree are poisonous. The leaves of the yew are now used to produce a drug which inhibits cancer cell growth permanently, called taxol. This poisonous bark could prevent moss and similar species establishing themselves on these trees.
Evaluating Evidence and Procedures
Limitations of the techniques used and their effect on the data obtained and the conclusion made.
There was confusion at times whether what I was sampling was actually moss or another species such as lichen. So the percentage data could be slightly higher than what it should be. But if there was some other species counted from my results it is clear the differences between what was mostly moss coverage on the Yew and Oak varies to a great extent so this would have very little if no effect on the reliability of the conclusion that there is a great difference in moss coverage. It could account for the anomalous 20% moss coverage result on the Yew tree data however. It would have been better if I had a booklet to hand illustrating the different species I would come across. I did not always have access to these pamphlets.
When counting the percentage moss coverage using the quadrat, we sometimes got different answers when the coverage was counted again. So the data collected may not be strictly precise, however, it is clear that any minor differences in the percentages calculated would not have had any effect on the conclusion which is definite. To combat this, another person should have re-counted it every time to get a second opinion and double check the first person was counting it properly. Ideally a strict counting protocol would be followed such as that one person is assigned to counting the percentage coverage while the other always records the results, and plenty of time would be given when counting.
For each tree, I sampled the moss coverage on the trunk at a height of one meter. However, on the bark of the Yew trees at one meter high there was very often 0% moss coverage, if there was any moss on the tree it was always lower down near the base. Therefore, to improve this technique I could have sampled the moss coverage for all trees at a lower height such as 10-20cm. If I changed the experiment in this way the moss percentage results on the Yew trees would be higher and could possibly alter the conclusion. This would be a good way to show how this factor affected the results.
Collecting more data could be one way in which to improve the technique and accuracy of my results. For example, instead of taking data from 15 Oak trees and 15 Yew trees as I did, I could instead sample 20 Oak trees and 20 Yew trees. This larger data size could give a greater range of results and be more representative of the area, or it could serve to confirm the conclusion which has been drawn with this smaller data set.
Limitations of the apparatus used and their effect on the data obtained and the conclusion made.
I was sharing quadrates with people in my group and other groups and even though they look the same, their size could vary slightly and this could have influenced my results ever so slightly. But again, this would do very little if nothing to alter the trend in my results and the final conclusion. I could use the exact same quadrat every time to combat this.
A meter ruler was used to measure up to 100 centimetres on the tree trunk which is where the quadrat would be placed. But the trees have many large roots and an odd shape which means a straight ruler cannot be places directly from the base to the 1 meter mark so moss may have been sampled at a height of over 1 meter where it will probably be less abundant. This could alter the numbers in the data as there is generally more moss on the trees lower down the trunk. It could have been enough to effect the general conclusion however it doubtful as 1 meter is still a long was up the trunk. A meter ruler could have been stood up on flat ground next to the tree, and another straight ruler or piece of wood placed flat on top to reach a height of 1 meter on the trunk.
I could adopt a more precise technique with apparatus. For example, if I used a device such as an annometer to measure wind speed, I could connect it to a data logger and leave it for a week and not just one day to get averages which would be more accurate. For instance if I visited an area which had a large moss covering and took wind readings on what was a particularly windy day, I could presume that moss can grow in a strong prevailing wind, however, at any other time the area may be relatively sheltered from the wind. Taking averages over a period of time would not give me any misleading data.
I could improve the accuracy of the results if I used other equipment and devices to survey the environment such as a hydrometer for example, which measures wind strength, and devices which measure light availability and moisture levels. Using this equipment would mean I could be very precise in selecting and sampling areas with similar physical conditions and may also enable me to link specific abiotic variables and their influence of the growth of moss.
I believe my results are valid because they are a good reflection of the facts and information I collected on the preferred habitats of moss and how this relates to the features of Oak and Yew trees.
Bibliography
The following sources of information were used to assist in the techniques and explanations as to whether the preferred habitat of moss is on the North side of a Yew Tree or the North side of an Oak Tree.
This lists various facts about mosses.
A wikipedia article about moss with information such as habitat information.
This has Information detailing the appearance and structure of moss and its reproductive structures.
Author: Mark Lefers and the Holmgren Lab
Definition of null hypothesis
Author: United Nations Educational, Scientific and Cultural Organization
Explanation of null and alternative hypothesis