For this project I aim to investigate if moss coverage has a preferred habitat on the North side of a Yew tree (a coniferous variety) or the North side of an Oak Tree (a deciduous variety). I will undertake this by means of fieldwork in a woodland area and the sampling and collection of data in the natural habitat of the forest in which I will visit.
Appropriate Equipment to be used
For the investigation I will need:
* A compass
* A quadrat
* A meter ruler
* A notepad to record results on to
* A paper bag to protect notepad from the rain
* A pen
* String measured at approximately 1.5 meters long
* Appropriate clothing such as waterproofs and walking boots
On a separate A4 page is a drawing of the table I will use to record my results, showing the readings I am planning to take.
The procedures selected and anticipated methods of collecting data.
I will need to control various techniques and methods of working in order to standardise my investigation.
I will standardise the actual sample taking. I will place the meter ruler against the trunk and place the quadrat at a height of one meter, with the bottom left hand corner of the quadrat touching the top right hand corner of the ruler. I will count the amount of squares placed over moss to derive a percentage. I will do this twice.
Using the compass I will ensure the moss I am sampling is on the north face of the tree, as I specified in my aim. Using the string I measured, I will standardise the circumference of the trees, selecting trees only with a circumference of around 1.5 meters long. I will also try to make sure all the trees are roughly of similar height. These two variations relate to the age of the tree and this is a factor which could influence the amounts of moss growth. I will be certain to use the same quadrat size for the whole investigation. In using this quadrat size I will be surveying a reasonable area of the trunk and it is not too small, something which would probably give an unfair representation of the moss coverage in that area.
I will furthermore try to make certain that the surroundings of each tree are similar, for example some areas may be more open to the elements such as the prevailing wind and this could influence moss coverage. I will try to some extent to ensure there is similar light availability at each area I sample by surveying the amounts of canopy cover for example.
I will take percentage moss coverage recordings from 15 Yew Trees and 15 Oak Trees. I believe a larger data set such as this will lessen the effect of any potential anomalies in my results and give more reliable overall results.
There are numerous factors I will not be measuring but they may have an effect on my results. Light availability (in relation to canopy cover), rainfall, moisture levels, bark texture and characteristics, the season (time of year), the type of forest, soil depth and pH are some examples. I cannot control these but I will be able to analyse their influence on my results.
I will sample in choosing trees in and around the same area so that factors such as light availability, soil types and moisture levels are kept as constant as possible. I will also sample trees with similar characteristics. I will standardise the circumference of the trees by sampling trees with a similar circumference (1.5 meters). This will also ensure the trees I sample are all of similar age and height. This is necessary because these are factors which could influence the amounts of moss growth. For example, a more established tree could have more branches and canopy cover which equates to less light availability for instance.
How the data will be analysed
As well as recording the data clearly in a table, I will draw two graphs: a bar graph and a scatter graph. Both will have the same axes, as shown (diagram 1), and will illustrate the relationship between the type of tree (Yew or Oak) and the percentage moss coverage from each of the 30 sites. I will also draw a bar chart to compare the average percentage moss coverage from both tree species, cumulating all of the readings (diagram 2).
The statistic I will use to asses whether or not my results agree with the null hypothesis will be the Mann-Whitney U test. I will use this statistic because I am planning to collect a series of replicated measurements of one variable. I will need to make sure I collect at least six replicates, and I will compare this set of data against a second set.
* 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.
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.