Environmental reconstruction based on sediment and pollen analysis in south western England.

1) Introduction

Shapwick Heath is a 390.0 hectare national wetland nature reserve situated in the Avalon Marshes, on the Somerset Levels a few miles west of Glastonbury, within the county of Somerset; it is home to many species of wildlife and provides a wide variety of ecological niches and habitats (see fig 1 and 2)(wessexwater.co.uk, 2014)(Natural England, 2010). Shapwick Heath is an example of a physical landscape that has

experienced environmental change since the mid-

Holocene, 7 ka to 3.5 ka, the current interglacial period due to factors such as sea level

rise and climate change (Wilkinson K and Straker V, 2008). The aim of this report is to interpret data from palaeoenvironmental study techniques including palynology, the study of ancient or modern pollen, in order to investigate whether the environmental change Shapwick Heath has endured since the mid-Holocene was due to natural climate change or human influenced land-use change. This

Figure 1- A map of Shapwick Heath, obtained from Digimap and edited on Adobe Illustrator, at a scale of 1:40000

Figure 2- A map of the UK with Shapwick Heath, indicated with a black box, located within the country of Somerset. Made using cartographic techniques on Adobe Illustrator

could lead to guidance for current or future environmental management of the Somerset Levels and Shapwick Heath.

2) Methodology

Pollen analysis was first developed as a palaeoenvironmental technique following von Post’s seminal paper, in 1916, and proved to be efficient in reconstructing floristic, and vegetational, history, and hence past climatic changes, at a variety of spatial and temporal scales (Birks H and Birks H-J, 2000). Pollen analysis involves collecting pollen samples directly from the site being investigated, in the investigation of Shapwick Heath pollen samples were taken at 8cm intervals from a peat core, extracted using peat coring, that was approximately 460cm in length and contained over two metres of dark, well humidified peat. The extracted pollen samples were then processed in the lab by removing the humic acids, clay and silica before breaking down the cellulose; the pollen was then dehydrated and mounted onto a slide for microscopy (Head K, 2015). Microscopic analysis of the pollen grains involved using a high definition light microscope, a light source with a co-ordinated system of lenses arranged so that the viewer sees a magnified image of the specimen (Greenberg G, 1994), until 100 individual pollen grains had been counted, identified and classified- using a pollen classification guide that included each type of pollen grain known. This process involved using different degrees of magnification levels ranging from x10 to x40 and x100; as well as adjusting focus in order to view the pollen grains. These results were then further analysed before a pollen diagram was produced.

3) Results

The pollen diagram, located in the next page, created from the pollen samples extracted show the number of pollen grains of certain types of flora over four different zones that each represent four time zones that range from 4,017 cal BC to 2,673 cal BC. The pollen diagram also shows the depth of the peat where each pollen grain was extracted from and the lithology of the peat, including silt and herbaceous fragments, leaves and stems if present. Each pollen grain that was extracted was classed and identified so that the pollen diagram now shows how the distributions(in %) of trees and shrubs, herbs, dwarf shrubs, epiphytes, aquatics, spores and aquatic bryophytes changed over time at the Shapwick Heath nature reserve.

In terms of trees and shrubs, through zones 4 and 3(4017 cal BC to approximately 3800 cal BC), where peat depth ranged from 232cm to 124cm (representing dark, humidified peat), there were minor distributions of birch, alder, hazel, oak and elm trees that fluctuated from 2% to 10% distribution. However, through zones 2 and 1(3,505 cal BC to 2,673 cal BC), where peat depth ranged from 124cm to 16cm, the distribution of hazel trees dramatically increased to fluctuating from 30%-37% and the distribution of birch trees rose to 30%

distribution midway through zone 2 before dropping again whilst the other species of trees slightly rose in distribution but continued to fluctuate.

In terms of epiphytes, the only species of epiphyte listed on the diagram is ivy; there were no distribution of ivy present apart from a slight rise in distribution (up to 2%) in zone 1, 2673 cal BC, with a peat depth of 72-64cm.

In terms of dwarf shrubs, there were no distribution of heather or vaccinium present until zone 1, 2673 cal BC. A distribution of vaccinium briefly grew at the beginning of zone 1 and peaks at a peat depth of 56cm and 8% distribution before going extinct mid-way through zone 1. Heather also began to grow at the start of zone 1, peat depth of 68cm, reaching a peak of 20% distribution, peat depth, of 48cm, before it decreased down to a 10% at a peat depth of 16cm by the end of zone 1.

In terms of herbs, Shapwick were dominated by grasses, peaking in zone 2 with 90% distribution, at a peat depth of 120cm, but decreased by zone 1 to 38% at a peat depth of 16cm. The only other species of herbs present were sedges through zones 4 and 3.

In terms of aquatics, there were no species of aquatics present throughout any of the 4 zones. However in terms of aquatic bryophytes, sphagnum moss peaked in zone 2 up to 45% distribution, at a peat depth of 88cm, before it went extinct and peaked again in zone 1 to 35% distribution at a peat depth of 35cm, before gradually decreasing again by the end of zone 1.

In terms of spores, the only species to exist were monolete and undifferentiated ferns which grew at the start of zone 1, peaking in zone 2 with a distribution of 80%, and a peat depth of 88cm, before decreasing and going extinct by the end of zone 3.

Discussion

Overall, the results show that overtime, from 4017 cal BC to 2673 cal BC, the percentage distribution of most flora present at Shapwick Heath has either increased, decreased, or has at least fluctuated. This means that from 4017 cal BC to 2673 BC, environmental change must have occurred as each flora requires certain biological conditions relating to light, temperature, humidity, availability of nutrients, particularly nitrates, phosphates and potassium; water concentrations, and CO2 concentrations (Poorter H and Soba-Perez M, 2001). All species of flora are photosynthesising organisms meaning carbon dioxide and light are absorbed in order to produce ATP(adenosine triphosphate), releasing oxygen in the process(Poorter H and Soba-Perez M, 2001). This means that flora are particularly reliant on these conditions and dramatic environmental changes that could have taken place from the beginning of the Holocene could be a possible cause for the extinctions and growth of certain species of flora, over the four zones of the pollen diagram. This case could be investigated with dendrochronology, the study of tree rings, as tree rings show past environmental

conditions in terms of the thickness of the ring; this could show whether there was a variance in atmospheric CO2, (Schweingruber F.H) which would then explain the fluctuation in the percentage distribution of flora species in Shapwick Heath nature reserve.

Another possible explanation for the fluctuations in percentage distribution of flora in Shapwick Heath is sea level rise during the Holocene. The Holocene is a geological epoch which began 11,700 years ago and continues to the present day (Holden, 2012). Since the Holocene began, the continental ice sheets gradually began to melt over a period of 7,000 years; the meltwater from the glacial melt returned approximately 44 million km3 to the oceans (Holden, 2012). The eustatic sea level rise caused lots of land to be drowned, particularly low-lying areas such as the Somerset Levels, and Shapwick Heath, which would have caused significant environmental change and could have caused Shapwick Heath to become the wetland environment that is present today(Holden, 2012). This theory could be supported by the 35% distribution of aquatic bryophytes, shown by the pollen diagram, which occurred in Zone 1, 2,673 cal BC, but could also be contradicted as there were no specific aquatic flora present throughout any of the zones shown on the pollen diagram.

One final explanation could be human-induced effects and land-use change causing certain species of flora to go extinct through land clearage for agricultural purposes. This could have been occurring from the 4th zone to the 2nd zone as grassland was dominant, showing possible evidence of agriculture. However, there was no evidence of burning, charcoal would be present, and grassland began to decimate and was replaced by woodland and other vegetation as nature was reclaiming Shapwick Heath.

Conclusion

To conclude this report, having interpreted data using palaeoenvironmental techniques including palynology and a pollen diagram, the environmental change that Shapwick Heath has experienced since the mid-Holocene, 7 ka to 3.5 ka, could be due to:

Eustatic sea level rise from continental glacial melt from the start of the Holocene, 12,000 years ago, which caused extensive drowning of land, especially low-lying land such as the Somerset Levels and Shapwick Heath, and could have turned Shapwick Heath into the wetland of present day.

Changes in the concentration of atmospheric CO2 which due to different flora requiring specific conditions, could have caused certain species to go extinct whilst others would thrive. This could be further studied with dendrochronology which would show whether atmospheric CO2 levels had changed or not.

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104112120128136144152160168176184192200208216224232

Dep

th (c

m)

2673 cal BC

3505 cal BC

4017 cal BC

Dat

es

20

Alnus (

Alder)

20 40

Betula

(Birc

h)

20 40

Corylus (

Hazel)

Fraxin

us (A

sh)

Pinus (

Pine)

20

Quercu

s (Oak)

Salix (

Willo

w)Til

ia (L

ime)

Ulmus

(Elm

)

Hedera

(Ivy)

20

Calluna

vulgari

s (Hea

ther)

20

Vaccin

ium

20 40 60 80 100

Poace

ae (G

rasse

s)

Cyper

aceae

(Sed

ges)

Caryoph

yllace

ae

Chenop

odium

undif

f.

Filipe

ndula

(Mea

dowsw

eet)

Plantag

o lan

ceola

ta (R

ibwort p

lantain)

Ranunc

ulus a

cris-t

ype (

Buttercu

p)

Saxifra

ga (S

axifra

ge)

Nymph

aea a

lba (W

hite w

ater-lily)

Typha

latifo

lia (C

ommon

reed

mace)

Polypod

ium (P

olypod

y fern

)

Pteridium

(Brac

ken)

20 40 60 80

Pteridop

hyta

(mon

olete) u

ndiff.

(Fern

s)

20 40

Sphag

num (S

phag

num m

oss)

20 40 60 80 100

Trees

and sh

rubs

Epiphy

tesDwarf

shru

bs

Herbs

Aquati

cs

20 40 60 80

Spore

s

20 40

Aquati

c bryo

phytes

Zone

Zone 4

Zone 3

Zone 2

Zone 1

Percentage pollen diagram for Shapwick Heath, Somerset Levels (Katie Head 2012)

Dwarf shrubsEpiphytes

Trees and Shrubs Herbs Aquatics Spores Aquatic bryophytes

Lithology

Dark, well humified peat (Thh4) Dark, humified peat (Th3) Herbaceous fragments, leaves, stems (Dh1) Fine silt (Ag1)

References

Birks H and Birks H.J.B (2000). Future uses of pollen analysis must include plant macrofossils. Journal of Biogeography Volume 27(1) pp 31-35.

Greenberg G, (1994). Illumination system and method for a high definition light microscope.

Head K, (2015). An introduction to Sediment and Pollen Analysis. Holden J, (2012). An introduction to Physical geography and the Environment.

Pearson Education Limited. Natural England, (2010). Shapwick Heath-National Nature Reserve. Available

at: www.naturalengland.org.uk/publications [Accessed 11 Mar. 2015] Poorter H and Soba-Perez M, (2001). The growth response of plants to

elevated CO2 under non-optimal environmental conditions. Oecologia Vol 129(1) pp 1-20. Springer-Verlag

Schweingruber F.H, (1988). Tree rings- basics and applications of dendrochronology.

Wessexwater.co.uk, (2014). Shapwick Heath case study - Wessex Water. [online] Available at: http://www.wessexwater.co.uk/environment/threecol.aspx?id=1278 [Accessed 11 Mar. 2015].

Wilkinson K, Straker V, (2008). Neolithic and Early Bronze Age Environmental Background. Atlantic.