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# Is there a relationship between the girth of a tree trunk and the percentage cover of lichen?

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Introduction

Is there a relationship between the girth of a tree trunk and the percentage cover of lichen? Experimental Hypothesis: The greater the girth of the tree trunk, the greater percentage cover of lichen. Null Hypothesis: The girth of the tree trunk will have no impact on the percentage cover of lichen. The independent variable is the girth of the tree trunk. It will be measured using a measuring tape as it can be wrapped around the tree trunk to get an accurate measurement, whereas a ruler cannot be wrapped around the tree trunk. The independent variable will be changed by examining different trees, as each tree will have a different girth, therefore changing the independent variable. The dependant variable is the percentage cover of lichen on the tree trunk. It will be measured using a ten square by ten square, acetate quadrat. This type of quadrat has been selected because it can be manipulated to follow the shape of the tree trunk. Also, having one hundred intersections in the ten square by ten square quadrat gives a particularly accurate result and automatically provides the result in a percentage. Using a metal quadrat would be limiting as it could not be manipulated to follow the shape of the trunk and using a ladder quadrat would give less accurate results as there are less intersections, or squares to count from. ...read more.

Middle

44.00 936 20 0.429 36 14 27 21 24.50 1105 21 0.782 34 29 32 17 28.00 776 22 1.233 47 34 32 33 36.50 649 23 0.993 39 31 18 29 29.25 483 24 1.803 73 51 42 45 52.75 1273 25 1.104 46 40 27 31 36.00 611 Analysis The statistical test of choice was Spearman's Rank. This is because the null hypothesis "the girth of the tree trunk will have no impact on the percentage cover of lichen" needed a statistical test that would investigate the relationship between two variables - girth if the tree trunk and percentage cover of lichen - and also find whether the two variables were correlated. The value calculated from the Spearman's Rank statistical test was 0.7925. The critical value for a Spearman's Rank test for a one tailed experiment with a sample size of twenty five is 0.3365, and the calculated value of 0.7925 is greater than the critical therefore the null hypothesis can be rejected, and the experimental hypothesis accepted. Therefore the girth of the tree trunk does affect the percentage cover of lichens and the results gained are not purely down to chance, but down to cause and effect. Girth of a tree trunk can be used to measure the age of the tree. The larger the girth of the tree trunk, the older the tree. ...read more.

Conclusion

Using a quadrat with thinner lines defining where the intersection points were could have reduced this source of error. Much of the lichen on the trees was a similar colour to moss and algae. This presented a problem with distinguishing between the species. Some algae or moss could have been counted and equally some lichen could have been classed as moss or algae and therefore left out. A way to get around this problem would be to use a magnifying glass to look at the species more closely and therefore making it simpler distinguishing between the species. Knuckles on the tree trunk were another issue. It was difficult to get an accurate measurement of lichen cover when the quadrat was not placed against the trunk due to a knuckle causing the quadrat to fold awkwardly. There is no real method to counter act this problem. The only thing to do was to attempt to measure as accurately as possible given the conditions. Even with the difficulties of the investigation, the overall influence was still relatively low. The results gained were still accurate and the data and statistical test has shown them to be valid as the larger the girth of the tree trunk, the greater percentage cover of lichen. This was evident when carrying out the investigation. Without the sources of error, the results would be more accurate, however it is likely that the null hypothesis would still have been rejected, therefore the results are still reliable even with the sources of error. ?? ?? ?? ?? Joshua Kearsley 2137 ...read more.

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## Here's what a star student thought of this essay

4 star(s)

### Response to the question

Very well done piece of coursework. The first thing I notice is that there is no introduction to this piece which should set out the scientific context and reasoning behind the investigation and what the candidate is setting out to ...

### Response to the question

Very well done piece of coursework. The first thing I notice is that there is no introduction to this piece which should set out the scientific context and reasoning behind the investigation and what the candidate is setting out to do. Main body of the text is very good, as is the evaluation. Should have included a separate conclusion. The response overall to the investigation is to a very high level and the response from the candidate is clear to read.

### Level of analysis

Consider the different variables and how it will measure than well. Considers the use of different quadrats and discusses why they have chosen the method they have. Attempts to remove any confounding factors that could influence the results by measuring them. Uses an adequate statistical test. The experiment was successful and proved the experimental hypothesis which was then explained scientifically and was supported by the results and the statistical test. Also expands on each answer by determining other affects such as light intensity and aspect that could affect the lichen growth on the tree which demonstrates a deeper analysis and understanding from the candidate. Uses the evaluation to point out errors and suggest improvements which also supports a deep level of analysis. Could have expanded and improved the analysis by using more than one type of statistical test and using graphs to support the data. It would also have been a good idea to devise a conclusion separate to the evaluation.

### Quality of writing

Punctuation, grammar and spelling all to a very high level.

Reviewed by skatealexia 11/03/2012

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