BP GEOGRAPHIC

A comparison of the abundance and distribution of tree species in three different woodlands at King’s Wood National Nature Reserve, Bedfordshire

Introduction

Population ecology is a sub-field of ecology that describes the dynamics of species populations and how these populations interact with the environment. “[It] focuses on the factors that affect the population size of a given organism, population growth rate, and spatial dispersion of individuals with populations” (McGinley 2014).

“The only factors that can alter population sizes are births, deaths, immigration, and emigration. Population growth rates are affected by interactions with both the abiotic and biotic environment. Climactic factors such as precipitation and temperature can have profound direct and indirect effects on population sizes. Temporal fluctuations in abiotic conditions can be important causes of variation in population sizes. Biotic factors such as competition and predation can directly and indirectly affect population sizes. Population dynamics can be influenced by interactions with members of their own species … and members of different species …” (The Encyclopedia of Earth 2014). “Another factor that limits the abundance and distribution of plants is dispersal. If insufficient time has elapsed a plant may not have been able to reach an area yet” (Krebs, 2008). But if there’s an ocean or a mountain range which forms a barrier the plant species might never be able to reach the other side, comparable to the separation of human cultures, like the Chinese culture which was separated from the rest of the world by mountain ranges.

There’s a report about a research in Southeastern Tibet which came to the result of “little change in the fir tree-line position on the southeastern Tibetan Plateau after 200 years of warming” (Liang et al., 2010). It was an investigation about the influence of altitude and temperature on the growth of the Smith fir. They found out that “although the Smith fir stands at the tree line in the Sygera Mountains have become increasingly dense because of an increasing number of seedlings after the 1950s, the tree-line position has only moved slightly and insignificantly upslope in response to climate warming.” (Liang et al., 2010).

This investigation is about comparing the size distribution and abundance of different tree species in three different types of woodland at King’s Wood National Nature Reserve, Bedfordshire.

Methods

Study Area: King’s Wood National Nature Reserve is 0.5 kilometres north of Heath and Reach village on the outskirts of Leighton Buzzard.

“The site lies on an unusual mixture of soils. Birch, sessile oak, bracken and heather grow in the sandy areas, with pedunculate oak, hornbeam and dog’s mercury on the clay. There are small-leaved lime trees here and a large population of lily-of-the-valley. The site is also home to rare mosses and fungi and there are recent records of the purple emperor butterfly here” (gov.uk, 2008).

Field Methods: All students were asked to form groups of four or five. Each group formed quadrats of 10m x 10m in three types of woodland and collected data inside of these quadrats. To prevent double counting the tutors made sure that each group was in an area which has not yet been recorded. Then they measured the circumference of each tree at chest height and recorded the tree species. If the tree had several stems below chest height every stem was recorded. Additionally the groups estimated the cover percentage of the canopy, shrub, field and ground layers and recorded the soil moisture percentage. All of this was repeated in each quadrat.

Afterwards the data were entered into an Excel spreadsheet and double-checked. To show estimates of the population density of the trees in the three types of woodland graphs were generated from the data. Other graphs were generated to show measures like canopy cover and soil moisture percentage. The variability of the data was displayed using 95% confidence intervals.

Results

The observation showed clear differences in the frequency and size of the three species of trees in the three different woodlands. The first observed one, which was a Oak-Birch woodland, showed a similar or higher frequency of Birch in the ranges of 0-90 cm circumference, more Oak with a circumference between 91 and 150 cm and over 210 cm, and the trees with circumferences between 151 and 210 cm were solely Oak trees.

The mixed woodland wasn’t that much mixed, Hawthorn and Hazel were only found in the 31-60 cm circumference range, this one was also the one which showed the by far largest amount of Aspen. Otherwise Aspen could be found in all ranges up to 150 cm. The stems of Oak trees were either smaller than 30 cm or bigger than 91 cm in circumference.

The Lime woodland actually only deserves its name in the circumference ranges till 120 cm, where Lime can be found, it dominates the ranges between 31 and 90 cm. There’s Silver Birch up to ranges of 210 cm circumference, but it just dominates the smallest range till a maximum of 30 cm circumference. Oak can be found in all ranges, while it slightly dominates the middle ones.

Finally there’s the result of the estimation of canopy cover, shrubs and herbs on trees, ground coverage, and of course leaf litter. All three woodlands have a high amount of leaf litter of about 90 percent, except of the mixed woodland, about 70 percent there. Ground coverage is only in the mixed woodland slightly over 50 percent, the other two areas have only 10 percent. Herb layer is lowest in the Lime woodland with around 10 percent, otherwise it’s between 40 and 50 percent. Shrub layer is generally low between 10 and 20 percent. And finally there’s the canopy cover which ranges from a bit over 50 percent in the mixed woodland, over around 75 percent in the one with Oak and Birch dominating, to about 85 percent in the Lime woodland.

Discussion

The greater average frequency of Oak in the Oak-Birch woodland in nearly all of the circumference ranges (except 31-60 cm, more Birch there) could suggest that there is something specific about the conditions there which is favoured by Oak rather than Birch. It could simply be that it began to grow earlier than Birch in that area. That’s because naturally the trees with the largest circumference are the oldest. The fact that there’s more Birch in the ranges with smaller circumferences could mean that Birch regenerates more. Little amount of both kind of trees with circumferences shorter than 30 cm could suggest that there’s a low regeneration on the site. This could possibly be caused by animals (like dear, caterpillars, rabbits) which have the habit of eating saplings, as it is well-known.

In the mixed woodland Aspen dominated the smaller circumference ranges, while Oak was the only species present in the larger ones. This could simply be because Aspen trees don’t grow so big stems. And Oak maybe dominates the larger ranges because this tree species generally lives longer than others, or it simply arrived earlier here.

The Lime woodland got it’s name from the presence of Lime in the smaller circumference ranges till 120 cm. Over that only Oak and Silver Birch can be found. But Oak dominates these ranges, this could be because the Oak is taller and shading out the Birch, so it stops them to get enough sunlight for photosynthesis, which is the “motor” of a plant for growth. Furthermore altitude and soil moisture could also play a role. The higher altitude and more soil moisture could favour Oak.

There might be a few sources of error in these observations. For example inaccuracies during measurements or the estimation of canopy cover, or even the guessing of some to be measured data to safe time. There could have been more checks of tutors and more instruction, but even if, this can maybe never completely ruled out, as the group members are all students and not fully educated scientists. There could also have been some more observations, like about animals in the area, however this might be difficult if there are so many people present, because animals are shy and would hide.

This investigation showed the differences in three different woodland areas, and for the time given it provided a sufficient amount of data.