Examine the correlation between soil moisture at different heights up the slope and the length of the longest leaf of a Bracken Fern

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Ketan Singh

Research Question: What is the correlation between soil moisture at different heights up the slope and the length of the longest leaf of a Bracken Fern?

Introduction: Slapton Wood, situated in the North-western corner of the reserve, is an Ancient Woodland (an area that has been wooded since 1600AD - Slapton Wood is mentioned in the Domesday Book), and as a result has very little active management. In practise what this means is that the only work that goes on in Slapton Wood is to maintain the network of footpaths and steps, which includes the clearing of dead, dying or dangerous trees when they pose a threat to public safety. The wood itself is a National Nature Reserve and is mostly composed of Sweet Chestnut and Oak, with a reasonable amount of Ash and Beech also present. An understory of Hazel and Holly can be found throughout most of the wood.  

Hypothesis: I think that there will be a correlation between the length of the longest leaf of a Bracken Fern and soil moisture at different heights up the slope. This is because soil moisture will decrease up the slope as the soil gets further and further away from the river, at the bottom of the slope. The importance of the river is that some of the water it contains is absorbed by the soil, thus meaning moisture levels in the soil closest to the river would be higher than soil anywhere else. Furthermore, when it rains, the rainfall would flow down the slope due to gravity and the slope angle. This results in the soil closer to the bottom of the slope being exposed to more water as the rainwater collects at the bottom of the slope, thus meaning that soil moisture would be higher here (at the bottom of the slope) than it would at the top of the slope as rainwater doesn’t collect here and the soil is therefore not in contact with the water for as long as the soil lower down the slope is.  Therefore, as soil moisture decreases up the slope, the Bracken Fern up the slope will be progressively smaller as there is less water available in the soil for growth, thus resulting with the largest Bracken Fern consisting at the bottom of the slope. Due to this, I therefore predict that the length of the longest leaf would decrease up the slope as the Bracken Fern get smaller, because if the plant is smaller, its leaves should also reduce in length. We will determine whether this hypothesis was correct using Spearman’s rank because we are measuring the correlation between the soil moisture and the longest leaf, and whenever correlation is measured, spearman’s rank is used. This experiment will be carried out in Slapton Wood in June.

Null Hypothesis: There will be no correlation between the length of the longest leaf of a Bracken Fern and soil moisture at different heights up the slope.

Independent variable: Soil moisture at different heights up the slope up the slope is our abiotic factor and therefore our independent variable. The independent variable being the variable that is changed in order to deduce whether there is a correlation between soil moisture and Bracken Fern leaf length. The soil moisture will be measured with the soil moisture probe, where it’s inserted into the soil of each of the 7 Bracken Ferns being used, every 5 metres along the transect starting at 0 metres and finishing at 30 metres, for 30 seconds (in order to allow the reading to stabilise). A measurement is then taken after the 30 seconds. This measurement will be repeated three times in order to guarantee a reliable reading. In Ecology we can’t control our independent variable because for example in this case, the value of the soil moisture in the forest is always changing due to different weather and different seasons. However, by measuring and changing the area at which it’s being measured, we’re able to control it to an extent because we are deciding where it’s being measured and we’re also making the measurements along the same 30 metre transect, in the same part of the wood, along the same section of the valley, so we therefore know that the results should be concordant. This is our method of controlling the independent variable and why we’re controlling it.

Dependent variable: Length of the longest leaf of the Bracken Fern is our biotic factor and is therefore our dependent variable. This will be measured using a metre ruler from the start of the Bracken Fern leaf, from where the stem branches off into the leaf, until the tip of the leaf (the last possible point of the leaf at which a measurement can be taken). In order to determine the longest leaf, all leaves must be measured because if they’re not all measured, our recording is therefore based upon perspective which is unreliable as everyone’s perceptive of what may be the longest leaf may be different. Therefore by measuring each leaf there would be no debate as to which leaf is longest. The leaf with the longest length should then be recorded. We must ensure that this measurement is done carefully and precisely in order to guarantee a reliable reading. The measurement is made to identify if leaf length is affected by soil moisture and therefore if there is a correlation between the two.

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There are other possible control variables; however, these are the variables that I’ve identified as being most important to the data I’m collecting.


Digital thermometer

Soil moisture probe

Soil PH probe


Whirling Hydrometer


Metre Ruler

30 metre measuring tape for transect

Bottle of water (for Whirling Hydrometer)

Pen and paper to record results

Towel/cloth (for cleaning soil moisture probe)


1) From the river lay out a 30 metre transect up the slope using the measuring tape. This should be done by leaving one end of the measuring tape at the bottom ...

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