The effect of light intensity on diversity of planrs on woodland floors.

ONONUJU OKONKWO

INTRODUCTION

THE EFFECT OF LIGHT INTENSITY ON DIVERSITY OF PLANTS ON WOODLAND FLOORS.

The investigation is about determining how diversity of plants in three woodlands is affected by light intensity. To do this I will have to find a way of varying light intensity, since this is not possible; I will do my investigation in Park Hurst forest, which has three different types of woodland. Park Hurst forest is one of the oldest forests recorded in the country. The woodlands in the forest have different light intensities so in effect I am varying the light intensity in my investigation.

The woodlands present in Park Hurst forest are coppice woodland, pasture woodland and conifer plantation.

COPPICE WOODLAND

Coppicing is one of the oldest forms of woodland management and has been practiced in Britain for many centuries. Coppicing is the process of cutting down deciduous trees and allowing them to regrow for some 7-25 years before harvesting again. It is a sustainable way of cropping timber from woodland and is actually beneficial to lots of fast growing new shoots. Many species of trees in Britain are coppiced, including Oak, Ash, Hazel and Maple. The resulting stumps are called stools while the stems, which re grow, are called rods, pole or logs depending on their size. The speed of regrowth varies with species but is always fast; Oak can regrow by 2 metres in a season and Sallow up to 4 metres. Traditional uses for coppiced timber include bean sticks, firewood, baskets, tool handles and brooms. Many centuries ago, coppicing provided people with the timber they needed to live. If coppicing is done on a rotational basis in woodland it increases biodiversity by allowing ground flora to flourish in freshly coppiced areas, which get more sunlight at ground level. As the trees regroup, other areas of the woodland are coppiced so there is always a mix of dark and light areas at ground level.

The coppiced woodland in park Hurst forest consists of Hazel Underwood with tall timber trees. The timber trees are Oak and Beech. The ground layer and herb level consists of plants like Bracken, Ivy and Oak Saplings.

PASTURE WOODLAND

This occurs where the woodland is used for grazing animals and growing trees. Trees grown in pasture woodland are thinned and pollareded i.e. the branches are cut above the browsing height of grazing animals. This allows more light to penetrate the canopy and encourages grass growth on the forest floor for animals. The ground floor consists mainly of holly as it survives grazing because it is not palatable for animals.

CONIFER PLANTATION

Coniferous plantations cover large parts of the countryside in certain areas of Britain. Coniferous forests are usually planted for the important softwood timber crop, which they will produce. Conifer trees include many different species, but most are fast growing and are ready to crop 6 times faster than hardwood trees such as oak. There are only a few native species of conifer in Britain; these include Scots Pine, Juniper and Yew. All the others are introduced for their fast growing qualities.
Because most conifers are evergreen and have leaves all year round, densely planted mature conifer plantations tend to hide sunlight from the ground so that little else grows on the woodland floor.

From my description of the various woodlands I can say that the coppice woodland should have the highest light intensity and conifer plantation the lowest light intensity out of the three woodlands. Coppiced areas have the greatest profusion of woodland plants that can be seen, this is because when they are coppiced, sunlight is able to get to more plants, which use it for photosynthesis.

Areas, which have not been coppiced and where the trees cast a dense shade such as conifer plantation, tend to hide sunlight, so have very little growing on the woodland floor. So places like, ...

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Areas, which have not been coppiced and where the trees cast a dense shade such as conifer plantation, tend to hide sunlight, so have very little growing on the woodland floor. So places like, conifer plantations do not support the same diversity of wildlife, as do deciduous or coppice woodlands. Areas such as pasture woodland which are pollarded tend to have large amounts of sunlight entering it but grazing animals eat up seedlings, grass and woodland floor plants. This leaves quite a lot of the species not eaten by animals but less of those eaten by animals.

Light intensity has a great effect on the diversity of plants in a place; this is because of its importance in photosynthesis. Photosynthesis is a process carried out by plants. Plants have a pigment in their leaves, which is usually chlorophyll, which absorbs light.

During photosynthesis energy from sunlight is transferred to chemical potential energy in the form of glucose and other carbohydrates. This is the substance required by plants for their growth. Plants convert simple inorganic compounds such as carbon dioxide and water into complex organic compounds such as glucose, using energy trapped from sunlight.

Photosynthesis occurs in two stages: the light- dependent reaction and the light independent reaction. The light- dependent reaction requires sunlight, while the light independent reaction can take place in the presence of sunlight but sunlight is not necessary.

In the light dependent stage, ATP and reduced NADP are formed. ATP is a chemical that contains energy. While in the light independent stage carbon dioxide is converted into carbohydrates. This conversion uses energy from ATP and hydrogen is got from the reduced NADP. Photosynthesis involves a step wise series of reaction before it is complete below is a summarised version of the photosynthesis reaction which shows the importance of light.

Sunlight

CO2 + H2O C6H12O6 + O2

Chlorophyll

We can therefore see that in places such as conifer plantation which have low light intensity will not have enough sun light getting to the plants on the floor so the plants compete for the little sunlight present. Those which cannot tolerate these low levels of light die off

When doing the procedure we measured the percentage cover as well as counting the number of different species. If we had counted number of individuals we would have used another statistical test but since we were trying to compare diversity we used the Mann-Whitney U test. I used the as the statistical test to analysis my results. This test is appropriate for my investigation because it enabled me to make comparisons with my data so I could determine whether there is a significant difference between the medians of two of the areas

ALTERATION TO PLAN

I used 10m × 10m instead of 5m × 5m. this is because when I got to the woodland I realised that 5m × 5m which I wrote in my plan was too small.

In my plan I said I would count on the plants on the woodland floor on getting there I decided to also count the plants in the herb layer of the woodland because in some areas counting only the plants on the woodland floor meant counting only grass and moss.

I took five repeat readings instead of four as stated in the plan, for the light intensity, relative humidity and temperature because I was covering a larger area.

CONCLUSION

I measured the light intensity of the different woodlands. From my results, the coppice woodland had the highest light intensity while the conifer plantation had the lowest light intensity. This that the coppiced woodland has the most amount of light while the conifer plantation has the least amount of light. From the result it was also seen that the coppice woodland which has the most light had the largest amount of species of plant present, while the conifer plantation which had the least light had the smallest amount of species of plants present.

I measured the relative humidity of the woodlands. From the result, the coppice woodland has a high relative humidity, the pasture woodland has a low relative humidity and the conifer plantation also has a high relative humidity. I also took soil samples and was able to find the water content and PH of the soil in the different woodlands.

The PH of the soil in the three woodlands is all basically the same, so I assume they do not affect the diversity of plants in the woodland. On the other hand the water content of the soil in the three different woodlands varied. The conifer plantation had the highest water content while the pasture woodland had the least. From this pattern I can assume that water content of the soil does not affect the diversity of species in the soil. This is because the woodland with low diversity has the most water so this means water is not a problem.

I drew a bar chart using the mean percentage cover. This is so I can compare the percentage cover of the plants in the three different woodlands i.e. is to see how much more there is of one plant in one woodland than there is in another. From the graph I was able to see that the coppice woodland had the most species of plants and plants with the highest percentage cover. While conifer plantation has the least number of species and the lowest percentage cover of the plants present. This shows that because the coppice woodland has the highest light intensity it has high plant diversity. While from the chart the pasture woodland has high plant diversity but low percentage cover of the plants compared to the plants in the coppice woodland. Lastly the conifer plantation has low diversity and the plants have a very low percentage cover. This is because light does not enter the conifer plantation easily because of it dense trees so only areas where old trees have fallen have a profusion of plants growing. The only plants that grow are those that can survive on low levels of light. Also put standard deviation values from the mean in my graph.

Finally from my investigation and results I can conclude that plants need light for photosynthesis and they need photosynthesis to grow. In most woodlands the trees grow very tall. In coppice woodland the tall trees do not form a canopy over the forest which means that light is able to enter the woodland and reach the plants on the floor. This is why the light intensity is high near the forest floor. The same can also be said for pasture woodland. The trees are pollared and so light gets to the plants on the floor. This means they have enough light for photosynthesis. Lastly in the conifer plantation the trees form a dense canopy over the forest floor. This means that most of the sunlight is absorbed by the tall trees and very little enter the woodlands so there plants on the floor receive very little sunlight. From the results taken I can verify all I have written above because the coppice and the pasture woodlands have high intensity while the conifer plantation has a very low light intensity.

Using the readings from the logIT meter, the comparisms from the Mann-Whitney U test and the bar chart, I can say that the diversity of plants on woodland floors is affected mainly by light intensity. But there is a major problem, which arises with most ecological investigations. This is that although we try and keep all other factors the same as possible we cannot control the environment the way we can control our environment in the laboratory, so other factors affect the diversity of the plants slightly what I will do is to try and find out if other factors affect the diversity of woodland plants on the forest floor. One of these factors is temperature. Luckily we measured the temperature of the three woodlands on the day of the investigation and they were both the same so we can assume they have no effect on the investigation. Water content of the soil was also measured. It showed that the pasture woodland had the least amount of water while conifer plantation had the most amount of water. But from our bar chart, conifer plantation had the least plants and so we can say that water content of the soil is not a limiting factor in the diversity. Finally the relative humidity of the air was measured. Although they varied I feel they do not affect the investigation.

The three woodlands have a variety of plants, but a few are common to all or at least both of them. Ivy is common to all three of the woodlands this is because it is able to survive in areas of low light intensity and so can survive in conifer plantation and it is not eaten by plants because it is poisonous so therefore can survive in pasture woodland. It has very green leaves and so can absorb small amounts of sunlight. This is its adaptation for living in areas with low light intensity.

Many woodland plants are adapted to cope with the lower levels of light. Most of these plants have adaptations for living in areas with lower level of light, Some, like the woodland rose, and bluebell flower and grow most prolifically early in the spring before the tree foliage is fully developed. Such species grow from bulbs, tubers or rhizomes where food stored from the previous season can be used early in the year to promote leaf growth. Other species such as the honeysuckle and oak saplings can collect as much light as possible by growing large leaves.

Many, such as woodland sedge, woodland spurge and broom produce seeds which can survive dormant for many years until coppicing lets in the light and encourages germination. Others like enchanter’s night shade have seeds which can easily be carried on the wind to other non-shady areas.

EVALUATING EVIDENCE AND PROCEDURES

The major source of error in the experiment is that it is not possible to control the environment. So for instance we cannot keep the light intensity constant for the whole day. So in this case we try and work around it.

Errors in my procedure

When doing the investigation, I used random sampling but before carrying out the sampling I had to mark out an area where the investigation would be carried out. When looking for an area I tended to be biased by looking for a place which had a lot of plants. This added error to my procedure.

I took ten samples and should have taken more because this increases the accuracy of my sampling. More samples means my investigation covers a wider area and reflects the diversity of the woodland better.

The results of this experiment will vary according to the time of year. I did it in august and so the plants which would be present in spring will not be there. So this is a source of error in the procedures.

I took soil samples from places outside my marked out areas and so this might not be a good reflection of the water content.

Distinguishing between plants which look alike is difficult so I may have given some plants a name it wasn’t.

There were no anomalies in the investigation everything had an explanation. The major limitation of the experiment is not being able to control the environment. This always leaves a degree of error in the investigation.

To improve this procedure I would carry out this investigation at different times of the year, so I am able to get as many species of plants present. I would sample more than one area of the woodland so for instance in a coppice woodland I would take samples in three different areas. Finally, I would take more repeat readings.

BIBLIOGRAPHY

Peterken, G.F. (1981) Woodland Conservation & Management

Information sheets from the Medina Valley Center