The Tilia Decline: An Anthropogenic Interpretation

The Tilia Decline: An Anthropogenic InterpretationAuthor(s): Judith TurnerSource: New Phytologist, Vol. 61, No. 3 (Oct., 1962), pp. 328-341Published by: Wiley on behalf of the New Phytologist TrustStable URL: http://www.jstor.org/stable/2429814 .

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THE TILIA DECLINE: AN ANTHROPOGENIC INTERPRETATION

BY JUDITH TURNER

Sub-department of Quaternary Research, Cambridge

(Received 6 February I962)

(With 5 figures in the text)

SUMMARY

Evidence is presented from five sites that the decrease in the Tilia pollen frequency seen in many British pollen diagrams is caused by human activity in the woodlands rather than by a change in climate. Analysis of the pollen diagrams makes it probable that there was considerable local variation in the composition of these woodlands and in the nature of prehistoric man's influence on them. Radiocarbon dates show that a major decline in the Tilia pollen frequency has taken place at various times. This means that the VII/VIII zone boundary, as originally defined by changes in tree-pollen frequencies, has little or no climatic significance and the difficulty that has been encountered in defining its exact position follows from the fact that human activity is a much more variable influence upon vegetation than a change in climate.

INTRODUCTION

Much attention has recently been given to the decline in the Ulmus pollen frequency which occurs in many pollen diagrams and which has been taken as the main index of separation of the British pollen-zones VIJa and VIIb, and their west European counter- parts. This Ulmus decline has been many times dated by the radiocarbon technique and values approximating to 3000 B.C. have been rather consistently obtained for it (Godwin, I960). It has been so often associated with cultural remains of the first neolithic settlers, and so evidently accompanied by changes in pollen composition indicative of the opening up of hitherto undisturbed forest, that this elm pollen decline has now been accepted as in all probability due, in large part or wholly, to selective human destruction of that species.

It is clear, however, that much later than this first and major decline in elm, there have followed decreases in the pollen frequency of Tilia. This is to be seen, for example, in the early diagrams from Tregaron (Godwin and Mitchell, I938), from Whixall and Whattall Mosses (Hardy, I939), and from later diagrams such as those of the Somerset levels (Godwin, I956) and of Helsington and Nicol's Moss (Smith, I959). This decline in the Tilia pollen is often associated with an increase in the Betula pollen frequency. Godwin (I956) considered that these alterations in pollen frequency were probably caused by a climatic change and that the Tilia decline in England and Wales was the clearest expression of the revertence which had been described by von Post (I946) from European pollen diagrams. Indeed, the original zonation of the Late and Post-glacial periods is based upon the assumption that the changes in vegetation which are so similar throughout north-west Europe were climatically caused, and the changes associated with the decline in Tilia pollen were therefore used to define the VII/VIII zone boundary of Godwin's scheme (Godwin, I940).

328



The Tilia decline 329

However, from Thrang Moss, Oldfield (I960) recognized a horizon, the U.T. horizon, at which there are several pollen frequency changes, including a decrease in the Tilia frequency, which he interprets as due to woodland clearance by prehistoric man.

METHODS

Peat samples from across previously known levels of the Tilia decline were collected for pollen analysis from the following areas, Whixall Moss in Shropshire, Shapwick Heath in Somerset, and Thorne Waste in Yorkshire. Commercial peat cutting at each of these sites made it possible to obtain samples in the form of a vertical series of blocks cut from existing peat faces, rather than by boring. Pollen samples were taken from the peat monoliths at very close intervals. Samples were first counted at i6 cm intervals then 8 cm, 4 cm intervals, etc., until the changes in the pollen frequencies were defined in sufficient detail to give a clear picture of the main vegetational changes. The level of critical horizons in the monolith could be identified and from each of them a thin horizontal slice of peat was cut (i to 2 cm thickness) for radiocarbon assay.

Five hundred total land-plant pollen grains were counted at each level and the results have been plotted as percentage of total land pollen.

From the beginning of the neolithic period onwards, once man had begun to have a marked effect on his environment, one expects there to have been more rapid and complex changes in vegetation than before. The nearer the present day the peat formed, the more difficult it becomes to define these changes. It is hardly surprising, therefore, that at the level of the Tilia decline not only were changes found in the pollen frequencies of genera other than Tilia, but in addition to this, at Whixall Moss and Thorne Waste, more than one level could be recognized at which there were changes in the frequencies of several pollen types. These levels have been shown by breaks in the pollen diagrams. The samples counted are thus divided into a number of groups, lettered A, B, C, etc., each group presumably representing a period of time during which the pollen rain at that place was, broadly speaking, of fairly constant composition. As such groups are very small, however closely the peat is sampled, it is inevitable that one is working extremely near the limits of what can justifiably be regarded as changes in pollen frequencies caused by real alterations in the vegetation, rather than as the normal variations which are found even when the vegetation is stable.

In order to be sure that the groups do represent periods characterized by different vegetation the mean pollen frequency of at least one genus has been calculated for each group, and Duncan's multiple range test (Duncan, I955) used to determine whether there is a real difference between the means or not. Only levels at which a change in the pollen frequency of one or more genera can be recognized in this way, have been dis- cussed with the results.

Pollen of genera such as Tilia and Fraxinus is often present in such low frequencies that this test will not give a significant result with the few samples which can be counted, and the pollen of other types, for example, Gramineae and Plantago, sometimes increases or decreases more or less gradually from one group to the next, and probably for this reason a significant result cannot be obtained for them with the multiple range test, but in both such instances it seems likely, even so, that the changes observed do in fact represent alterations in the vegetation. Such changes have been listed, together with those which are statistically significant, at the beginning of the discussion of the various levels.

Dr. Vishnu Mittre had already prepared diagrams from samples taken at close intervals G N.IP.



330 JUDITH TURNER

from two sites in East Anglia, Holme Fen and Trundle Mere, and he has very kindly allowed me to use his original data and to express his counts from the level of the Tilia decline in the same way as those from the other three sites.

RESULTS

i. Whixall Moss (SJ 492368) North Shropshire, an area of low relief, geographically an outlying part of the Cheshire plain, has been much affected by glaciation and a large part of it is now covered with glacial gravels, sands and boulder clays. Whixall Moss lies in an area of boulder clay between Ellesmere, Whitchurch and Wem. It has been well drained and is being cut for peat commercially. The stratigraphy has been described by Hardy (I939).

The results of the pollen counts are given in Fig. i. The pollen frequencies have been divided into ten groups, A to J, in stratigraphic sequence, so as to characterize more clearly the vegetational changes associated with the Tilia decline.

Table i. Changes at the A/C level Results of Duncan's multiple range test

Mean pollen Mean pollen Smallest frequency frequency Difference in significant during A during B means range (5 %)

Tilia 8.5 2.25 6.25 > 4-39 Plantago i6.2S 2.25 14.0 > 7.68 Pteridium 72.0 14.0 58.0 > 45-12

Other changes Mean pollen frequencies A C

Ulmus 3 .75 I' }e5e Quercus I o5. 75 8 i.o decre'se Fraxinus 2.75 I.5 Gramineae 9-25 31I.25 S Betula 19.5 31.5 Jincrease

The radiocarbon date determined for the level of the Tilia decline between sections A and C is as follows:

Q. 467 87-88 cm I277 B.C.? II5 years

In many cuttings near the site of sampling, there are pine stumps in the peat, all some distance below a clearly marked stratigraphical horizon, in local terminology, the grey/ black peat boundary. There is a maximum in the Pinus pollen frequency at the level F/G and also a pine stump in the peat. The pine-stump layer has been dated by Godwin and Willis (I960) to 348 B.C. ?IOO years. There is some evidence, which indicates that the pine stump layer at F/G represents a dry phase on the bog surface and on the basis of comparisons with the Somerset levels and other places where the Grenz-horizon has been dated to 600 B.C., it is possible that the pine-stump layer at Whixall may also be referable to this stage of bog stratigraphy. A further radiocarbon date would help to clarify the matter. If one accepts the 348 B.C. date for the F/G level and the I277 B.C. date for the A/C level, the 30 cm of peat between the two formed in about goo years. Apart from the pine stumps the peat is fairly uniform in humification between these levels, and if one assumes growth at a more or less constant rate, the time between



The Tilia decline 33I

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332 JUDITH TURNER

successive samples is about 6o years throughout most of the diagram and 30 years at the level of the Tilia decline. If the pine-stump layer really represents 600 B.C., these figures must be reduced somewhat. Although this time scale is obviously imprecise, it is extremely useful for showing the magnitude of the time periods involved. Each sample represents i cm thickness of peat and therefore contains the pollen from areas around the bog over a period of about 20 to 30 years. Changes at the A/C level are shown in Table i.

The two samples, each i cm thick, that intervene between sections A and C have been labelled B to indicate their transitional character, but initially at least it is proposed to consider together the changes from A to C.

These changes must collectively represent a considerable decrease in the amount of woodland and an increase in the amount of open land. The presence of open areas is suggested by the increases in the pollen frequencies of Gramineae sp. and Plantago lanceolata and the associated increase in the amount of herbaceous pollen at that level. The high frequencies of Pteridium aquilinum spores is perhaps associated with the edges of clearings.

The percentage decrease from A to C in the mean pollen frequency of the different tree genera is as follows:

Tilia 73.5% Ulmus 6o.o% Fraxinus 45.3% Quercus 23.4%

There was therefore a greater decrease in the production of Tilia pollen than in that of Ulmus or Fraxinus and a greater decrease in Ulmus and Fraxinus than Quercus pollen.

Morrison (I959) has drawn attention to a decline in the Ulmus pollen frequency unaccompanied by any in Quercus pollen during prehistoric clearances in north-east Ireland. He contrasts this with the phenomena shown in Iversen's diagrams where each constituent of the mixed oak wood shows a similar decrease. He discusses two possible explanations, one in terms of selective felling of a particular species of a homogeneous woodland, the other in terms of clearance of one of two different types of woodland, both of which contribute to the pollen rain at the bog site analysed.

At Whixall, although we are concerned with a period later than the early neolithic clearances, the problem is basically the same in that there is a difference in the behaviour of the various tree genera. The sampling site is some distance from the bog margin and must have received pollen therefore from a considerable area of woodland. If, as seems likely, the higher frequencies for Gramineae sp. and Plantago lanceolata pollen found after the decline in the tree-pollen frequency indicate the presence of open land rather than areas of partially thinned woodland, the greater percentage decrease in Tilia pollen cannot be due to selective felling, and must be the result of more extensive clearance in near-by woods containing abundant Tilia than in others containing little or no Tilia.

2. Thorne Waste (SE 723144)

Thorne Waste is an extensive area of raised bog lying south-west of the confluence of the Humber and the Trent. Most of the surrounding land is covered with alluvium. Some distance to the west there are outcrops of Magnesian limestone and Carboniferous coal measures, whereas further to the east there are small outcrops of Coniston flags and various oolitic and lias series. Beyond these lies a wide outcrop of chalk.

The results of the counts are given in Fig. 2. The successive samples have been divided into four groups, A to D, which are not intended to be equivalent to those from other



The Tilia decline 333

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334 JUDITH TURNER

sites similarly lettered. As at Whixall Moss, an approximate time scale can be given to this diagram. The radiocarbon date for the C/D level is as follows:

Q.479 83-84cm 368 B.C.+IIoyears

The dates for the B/C level are as follows:

Q. 48I 95-96 cm I2IO B.C.+ I I5 years Q. 482 96-97 cm 98I B.C.? II5 years

The time between successive samples is therefore of the order of iOO years and each sample represents the duration of about 50 years. Changes at the A/B level are shown in Table 2.

Table 2. Changes at the A/B level Results of Duncan's multiple range test

Mean pollen Mean pollen Smallest frequency frequency Difference in significant during A during 13 means range (50,/)

Quercus I 50.0 86.2 65.8 > 47-75 Ulmus 12.75 5.0 7.75 > 7.2I Fraxinus 34.25 6.2 28.05 > 21.0

Other changes Mean pollen frequencies

A 13 Coryloid type 104.25 I69 Gramineae 9.75 20.0 increase Plantago 2.25 8.2 J

Table 3. Changes at the B/C level Results of Duncan's multiple range test

Mean pollen Mean pollen Smallest frequency frequency Difference in significant during B during C means range (5%)

Tilia 7.6 2.833 4.766 > 4.i8 Gramineae 20.0 56.33 36-33 > 33.6


B C Plantago 8.2 17. I increase Rumex o.8 5-33 Coryloid type i69.0 120.0 decrease

As at Whixall Moss, the increases in the Gramineae and Plantago sp. pollen frequencies suggest a certain extent of completely open areas replacing some of the woodland, although the increased coryloid frequency at this site shows that Corylus became more important at the time of the clearance than it did at Whixall. The change in the coryloid pollen type is attributed to Corylus rather than Myrica because of its association with the other changes at this level.

There is a greater percentage decrease in the Fraxinus and Ulmus than in the Quercus frequency:

Fraxinus 82.oo0 Ulmus 6o.7?0 Quercus 43.8%



The Tilia decline 335 'This is a further point of similarity with Whixall and probably indicates, as there, that Fraxinus and Ulmus were more important in the woodlands that were cleared than in those that were unaffected by clearance. Changes at the B/C level are shown in Table 3.

These changes once again show a considerable increase in the amount of open land in the region and quite clearly, as the Tilia frequency is the only tree frequency that decreases, the newly created open areas must have been at the expense of woods con- sisting of almost pure Tilia.

The fact that similar increases in Gramineae and Plantago sp pollen at both the A/B and B/C levels occur at the same time as decreases in Fraxinus, Ulmus and Quercus at the one level, and Tilia at the other, provides convincing evidence that, as at Whixall, there were at least two different types of woodland near Thorne Waste at that time.

Section C There is no significant change in the pollen frequencies for about 700 years during C.

'Fhe absence of an increase in tree pollen must mean that the land which had been cleared was occupied by man throughout this period, roughly speaking the latter part of the Bronze Age. Changes at the C/D level are shown in Table 4.

Table 4. Changes at the C/D level Results of Duncan's multiple range test

Mean pollen Mean pollen Smallest frequency frequency Difference in significant during C during D means range (5%)

Betula 47.5 30-33 17.17 > i6.8 Fraxinus 6.5 3.0 3.5 > 3.45 Gramineae 56-33 124-33 68.o > 37.0 Plantago 17.1 36-33 19.23 > 13.51


C D Rumex 5.33 i6.o Pteridiun, 26.o 47.0 f increase Tilia 2.833 0.333 1 Alnus 104.83 62.66 decrease Coryloid 1I20.0 74.0

At this level there is a change in nearly all the pollen frequencies. There can be little doubt that the changes in the Gramineae, Plantago, and Rumex pollen and Pteridium aquilinum spore frequencies indicate clearance as at the A/B and B/C levels. The dis- appearance of Tilia pollen at this time suggests that the remaining woods containing Tilia were amongst those cleared.

The Carboniferous coal measures and Magnesian limestone outcrops west of Thorne Waste which today carry different types of woodland, oak-birch woods on the coal measures (Moss, I9I3) and woods with abundant ash and some Tilia on the limestone (Jackson and Sheldon, I949), provide at least two contrasting habitats where the different types of woodland indicated by the clearances in this area might possibly have grown.

3. Trundle Mere (TL 200900)

rrhe results of Dr. Mittre's counts are given in Fig. 3. The frequencies have been divided into two groups excluding the sample at I35 cm which is regarded as representing a transition stage between A and B. Changes at the A/B level are shown in Table 5.



336 JUDITH TURNER

These counts are based on 200 total tree pollen rather than 500 total land pollen. There- fore the change in each tree-pollen frequency in Fig. 3 is a change relative to the other tree-pollen frequencies and the changes in the non-tree-pollen frequencies are relative to the change in the total tree-pollen frequency.


Mean pollen Mean pollen Smallest frequency frequency Difference in significant during A during B means range (5'%,)

Tilia 11.21 1.14 10.07 > 7.i88 Gramineae 29.36 86.71 57.35 > 53.31


A B Plantago 10.57 30.4 } Quercus 72.85 87.0 increases Betula 8.57 13.71

There is an increase in Gramineae and Plantago pollen and as at Thorne Waste and Whixall Moss these increases probably mean the creation of open areas in the neigh- bourhood. The relative decrease of Tilia pollen suggests that this was largely at the expense of Tilia-dominated woods, rather than of those containing abundant Ulmus or Ouercus.

4. Holme Fen (TL 2208go) Dr. Mittre's results from this site are similar to those from Trundle Mere (see Fig. 4).


Mean pollen Mean pollen Smallest frequency frequency Difference in significant during A during B means range (5'S,)

Gramineae X8.o 87.0 69.o > 52.6


A B Plantago 6.125 51.0 increase Tilia 11.125 4.5 decrease

As at Trundle Mere these results indicate the clearance of Tilia-dominated woodlands.

5. Shapwick Heath (ST 433398) Shapwick Heath is an area of raised bog which forms part of the Somerset levels within the low-lying region between the Wedmore Ridge and the Polden Hills. The peat is being cut commercially. The results of the analyses are shown in Fig. 5. The series of analysed samples has been divided into two groups, A and B. Changes at the A/B level are shown in Table 7.

The increase in the pollen frequencies of Gramineae, Plantago and other ruderal species is very small at this site, and a total of herbaceous pollen which could be considered as certainly derived from the area around the bog was therefore calculated. The



Thne Tilia decline 337 correlation coefficient for this composite frequency and that of Pteridium aquilinum spores is o.6i6 (significant at the i.o00 level of probability). A total of non-local herbaceous pollen and P. aquilinum spores was therefore used as the indicator of disforestation, although it has become apparent from the results that 'disforestation indicators' is not perhaps the best term to use for this composite frequency.

HOLME FEN i.BETULA UJLMUS QUERCUS TILIA ALNUS FRAXINUS CORZYLUS GRAMINEAE PLANTAGO PTERIDIUM

32 E~E J E~JED E[J B 33 CZJ 0 E CZ E EECJ__ _

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0 2 4 01i2 30 10 20 0 12 30 10 20 0 12 0 20 40 0 10 20 0 10 20 0 2 4 PERCENTAGE OF TOTAL TREE POLLEN

Fig. 4. Holme Fen. Details as in Fig. 3.

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Fig. 5. Shapwick Heath. Details as in Fig. 3.

The changes in pollen frequency at the A/B level, although different in magnitude from those at the other sites described, are sufficiently similar to be interpreted in the same way as due to human interference. The main difference from the other sites is that although the decrease in certain tree frequencies is of the same order of size, the increase



338 JUDITH TURNER

in 'disforestation indicators' is considerably smaller. This implies that the woodlands were affected to a similar extent, but that the nature of the interference was quite different and consisted primarily of a modification of a woodland community rather than the replacement of it by open areas, as was probably the case at Whixall, Thorne Waste and Trundle Mere. It is possible that the small inerease in the 'disforestation indicators' frequency is due to pollen derived only from the actual dwelling places of the people, and not from large areas of woodland modified by their activities.


Mean pollen Mean pollen Smallest frequencv frequency Difference in significant during A during B means range (5'o)

Disforestation indicators 5.8 25.62 I9.8I > 14.96

Other changes

Mean pollen frequencies A B

Tilia 2.75 0.75 decrease Ulmus 10.125 2.875 J

The decrease in the Ulmus and Tilia and not in the Quercus and Fraxinus pollen frequency has similar implications to the differences in percentage decrease of the various tree species at Whixall Moss. As there is no evidence for open areas in the woodlands near Shapwick Heath it is not possible to infer whether this is due to selective modification of a mixed woodland or to activity in a predominantly Ulmus and Tilia woodland, although, as at Whixall and Thorne Waste, there are areas near the bog with sufficient contrast in geological strata and soil type to allow the possibility that there were at least two different types of woodland in the area.

DISCUSSION

Human interference with the natural vegetation is expressed in all the pollen diagrams by increases in the pollen frequencies of Gramineae, Plantago, Rumex and other ruderal species and in the spore frequency of Pteridium aquilinum at the same level as decreases in the pollen of one or more of the tree genera. At every site in which the Tilia decline was investigated each significant decrease in its pollen frequency is very closely associated with these indices of human interference in woodlands and it is quite clear that this interference was the immediate cause of the decline. It is interesting that a similar association can also be seen in some of the continental diagrams, for example, that from Bjarsj6holmssj6n in Schonen (Nilsson, I96I) and that from Gifhorn in Germany (Overbeck, I952).

The expression immediate cause has been used in order not to exclude the possibility of an indirect climatic influence. For example, climatic changes may have affected the size and movement of populations and even the farming activities of prehistoric man and in this way to have affected the Tilia decline. It is possible that climate affected woodland composition directly during the Bronze and Iron Ages but the results of these investigations do not disclose such effects.



The Tilia decline 339 The radiocarbon dates for the level of the Tilia decline are as follows:

Shapwich Heath Q. 644 72-74 cm 2015 B.C. ? I I5 years Q. 645 74-76 cm I920 B.C. ? II5 years

Holme Fen Q. 403 65 cm 1440 B.C. ? 120 years Q. 404 70 cm 1445 B.C. 1iI20 years

Whixall Moss Q. 467 87-88 cm I277 B.C. i I 5 years Thorne Waste

First decline Q. 48I 95-96 cm 12 1 0 B.C. I I 5 years Q. 482 96-97 cm 971 B.C. ? 115 years

Second decline Q. 479 83-84 cm 368 B.C. i IO years

Allowing for the standard deviation of the radiocarbon dates, the decline at Whixall, Holme Fen and the first decline at Thorne Waste could be regarded as broadly synchronous and of middle to late Bronze Age, but the earlier date at Shapwick Heath, probably late neolithic, and the later date for the second decline at Thorne Waste, early Iron Age, show that the Tilia decline was not in fact a single synchronous event throughout Britain and must indeed be attributed to people of at least three distinct prehistoric cultures. This strongly supports the anthropogenic as against the climatic interpretation of the Tilia decline.

Several other interesting points arise from these investigations. At some of the sites, for example, Whixall Moss, Trundle Mere and Holme Fen, although woodlands replace the cleared areas after a shorter or longer period of time, Tilia never again plays an important part in these woods. It is not clear which environmental factor or factors changed during the disforested period making conditions unsuitable for Tilia to grow as before. There are several possibilities; for example, a change in climate might have been decisively adverse to Tilia at some stage in its life history. In this connection it is perhaps significant that Tilia species do not readily regenerate naturally in Britain today (Elwes, 19I3). A change in climate, particularly the increased wetness indicated in various ways for the Bronze Iron Age transition, might have made the soils unsuitable for the regeneration of Tilia or, on the other hand, a similar effect may have been caused by man's use of them for several centuries.

These results also show that, except possibly at Shapwick Heath, there were different types of woodland surrounding the bog in each area. Something of the composition of these woodlands has been deduced. Near Thorne Waste there appear to have been woods which were dominated by Tilia and Corylus as well as woods with considerable quantities of Ulmus and Fraxinus and some with more Quercus. Near Trundle Mere there were woods dominated by Tilia as well as others with more Quercus, Ulmus and Fraxinus in them. The nature of the woods near Whixall is harder to ascertain, but there also there must have been at least two types, one with greater quantities of Tilia and possibly Ulmus and Fraxinus than of Quercus species. At each site there are contrasting geological strata or soil types which might well have carried different types of woodland in the past. Morrison (I959) first recognized the possibility that the prehistoric woodland of Northern Ireland had a strong mosaic character that determined the pattern of their early exploitation, and this study of the Tilia declines shows that such a mosaic existed also in England and Wales.

As well as drawing attention to the diversity of structure in the forests, the results indicate that there were variations in the effects of human occupation. There is a good contrast between the type of interference with the woodlands at Shapwick Heath and that at the other four sites. This is strikingly similar to the contrast during the neolithic period between the 'landnam' clearances first demonstrated by Iversen and the type of



340 JUDITH TURNER

modification which Troels-Smith has suggested was responsible for the Ulmus decline. Yet a third type of effect can be seen in the diagram from Whixall Moss during sections H to J, where a considerable proportion of the tree pollen is replaced by Corylus pollen and Pteridium aquilinum spores. The twofold nature of these pollen frequency variations as at the G/H and I/J levels, makes a climatic interpretation of them most unlikely. The recognition of different types of human influence gives grounds for the hope that particular patterns may be correlated with specific prehistoric cultures in the way that Waterbolk (1958) has demonstrated the association of neolithic landnam clearances with the graves of the Standvoetbeker culture and the semi-farming culture described by Troels-Smith with those of the Funnel beaker culture.

Besides demonstrating the close association of the Tilia decline itself with human alteration of the woodland, the results illustrate the sensitivity of the other tree frequencies to human activity. Alterations in the Quercus and Ulmus frequencies can clearly be associated with disforestation. This can be seen at the B/C level at Whixall Moss, the A/B level at Thorne Waste and the A/B level at Shapwick Heath. A glance at the Whixall diagram is sufficient to show that at each level at which there is human interference there is also an increase or decrease in the Fraxinus pollen frequency. This is also true of the A/B and C/D levels at Thorne Waste. An increase in the Betula frequency occurs at the same level as human influence at Whixall Moss at the B/C level, and at Trundle Mere at the A/B level. Whilst not suggesting that all the changes in tree-pollen frequencies since the first Ulmus decline have been caused by human activity, these results do show how very many of them can be associated with it.

The original zonation of the latter part of the Post-glacial period was based upon the concept that the alterations in the vegetation, particularly in tree species, were caused by changes in climate. These were assumed to be synchronous. The zone boundary, VIIb/VIII is based upon decreases in the frequency of Tilia and/or Ulmus, increases in that of Betula and an alteration in the ratio of the Quercus/Alnus frequency. As has been shown, all these changes can be and often are associated with human influence. Much difficulty has previously been encountered in defining the exact position of the VII/VIII zone boundary. Godwin's proposal of a transition zone, VIIb/VIII, and Oldfield's reluctance to define the boundary at all in Lowland Lonsdale (I960) illustrate this. As the effect of human influence on tree-pollen frequencies is much more varied both in time and in expression than that of a climatic change the difficulty of defining this particular zone boundary is not surprising. In so far as the recognition of zones VIIb and VIII has focused attention upon the pollen frequency changes occurring during the Bronze and Iron Ages, it has served a very useful function, but its continued use now has the twofold danger of implying climatic changes which may not have occurred and of not giving sufficient emphasis to the variety that exists in the expression of human occupation of an area. Now that radiocarbon dating has made it possible to study this period in so much more detail it will obviously be easier in future to use the absolute time scale which this method provides.

ACKNOWLEDGMENTS

I am most grateful to Dr. Vishnu Mittre for allowing me to use his pollen counts from Holme Fen and Trundle Mere, to Dr. E. H. Willis for determining the radiocarbon dates quoted in this paper, and particularly to Professor H. Godwin, F.R.S., for his patience and encouragement in supervising my work. Miss R. Andrew has kindly



The Tilia decline 34I

advised with pollen identification and other members of the Sub-department of Quater- nary Research have assisted with the field work; I thank them most sincerely. The financial support of the D.S.I.R. is gratefully acknowledged.

REFERENCES

DUNCAN, D. B. (I955). Multiple range and multiple F tests. Biometrics, xI, i. ELWES, H. G. & HENRY, A. (1913). The Trees of Great Britain and Ireland, VJI. The Authors, Edinburgh. GODWIN, H. (1940). Pollen analysis and forest history of England and Wales. New Phytol., 39, 370. GODWIN, H. (1956). The History of the British Flora. Cambridge. GODWIN, H. (I960). Radiocarbon dating and Quaternary history in Britain. Proc. roy. Soc. B, 153, 287. GODWIN, H. & MIrCHELL, G. F. (1938). Stratigraphy and development of two raised bogs near Tregaron,

Cardiganshire. New Phytol., 37, 425. GODWIN, H. & WILLIs, E. H. (I960). Cambridge University natural radiocarbon measurements. II.

Amer. J. Sci., Radiocarbon Supplement. GODWIN, H. & WILLIS, E. H. (I962). Cambridge University natural radiocarbon measurements. V.

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Notiser, I I2, I 85. Moss, C. E. (19I3). Vegetation of the Peak District. Cambridge. NILSSON, T. (I96I). Ein neues Standardpollendiagram aus Bjarsj6holmssj6n in Schonen. Acta Univ.

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The Tilia Decline: An Anthropogenic Interpretation

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