- Email: [email protected]
Fine resolution pollen analysis of vegetation history in the Lough Adoon Valley, Co. Kerry, western Ireland
Review of Palaeobotany and Palynology, 64 (1990): 235-245 Elsevier Science Publishers B.V., Amsterdam
235
Fine resolution pollen analysis of vegetation history in the Lough Adoon Valley, Co. Kerry, western Ireland J.R. Dodson
School of Geography, University of New South Wales, Kensington, N.S.W., 2033 (Australia) (Received September 7, 1988; revised and accepted January 25, 1990)
ABSTRACT Dodson, J.R., 1990. Fine resolution pollen analysis of vegetation history in the Lough Adoon Valley, Co. Kerry, western Ireland. Rev. Palaeobot. Palynol., 64: 235-245.
The Lough Adoon Valley on the north of the Dingle Peninsula, in southwest Ireland, has been occupied by humans, perhaps continuously, since Neolithic times. Since about 5000 B.P. the valley has been essentially deforested, apart from local reinvasions of Betula, and much of the landscape has been covered by blanket bog and heath. Time series analysis of pollen and charcoal influx from two sections of peat sampled at contiguous I cm intervals was carried out to explore the vegetation shifts associated with Betula decline and charcoal dominated phases. Cereal cropping and fire were important vegetationalcontrols resulting in an interplay between heath, wetland, grassland and woodland but forest decline was not a result of fire. The number of pollen taxa interrelationships differed between the phases with a greater number of interrelationships in the Betula decline phase than when fire was more prominent in the environment.
Ireland. Throughout the text Webb (1977) is used as the authority for species names.
Introduction Most pollen analytical investigation in Ireland has been directed toward documenting the processes of vegetation and environmental change since the late-glacial. The broad nature o f this is well-understood but much detail awaits to be filled in. Only recently has the power of pollen analysis been extended to investigate important questions associated with forest dynamics at the stand level or with the nature of change such as the Ulmus decline (Molloy and O'Connell, 1987). Investigations at all scales have identified an important role for h u m a n activity in forest dynamics and clearance for agriculture during the Holocene. Indeed the spread of blanket-bog in m a n y areas has been ascribed, at least in part, to h u m a n activity (Mitchell, 1976; Lynch, 1981). The aim o f the present investigation is to examine the detail of vegetation change associated with h u m a n activity, especially fire, in a small valley in southwest 0034-6667/90/$03.50
Site selection The Dingle Peninsula (Fig.l) has recently been the subject of a detailed archaeological inventory (Cuppage et al., 1986) and has a h u m a n occupational history, perhaps continuous, extending back at least 6000 radiocarbon years ( W o o d m a n et al., 1984). The Lough A d o o n Valley on the northern side of the Peninsula was chosen for study because of the abundant archaeological evidence within its confines. These include a collection of settlement and ritual/funery sites including several hut sites, enclosures, standing stones, rock art, several burning-pit sites (fulachta fiadh), a fortified island, several cist graves and a wedge t o m b of possible Bronze Age. It is probable that people have occupied the valley since Bronze Age time. There is possibly no area on the Dingle Peninsula with such
© 1990 - - Elsevier Science Publishers B.V.
236
J.R. DODSON
N
D
Standing stones
\
Kerry
Vlegalithic~ tomb
XS t ~
-/
) "i
':
•,LO
doon~
,
)
X Core Site
,
" :, )
488rn Slievenagower
F--~---', "~'~ A
,'';;'"
;/
~
Scree
o
30o
I
I metres
Fig.1. Location of Lough Adoon valley, western Ireland and pollen sampling site. a concentration and variety of archaeological material.
Physical and biological setting Lough Adoon and Lough Camclaun are corries in Devonian Group purple-red and grey-green conglomerates, sandstones and mudstones (Horne, 1976). The corries sit hard against valley headwalls. The soils are largely blanket-bog peats along the valley floor or shallow peaty-gleys amongst boul-
der strewn fan deposits around the steep corrie walls. The vegetation is mainly heath and rough pasture. There are a few scattered low trees which are mainly Crataegus monogyna in the upper basin and llex aquifolium on the blanket-bog.
Sampling and analysis strategy Since the aim of the study was to unravel the history of the vegetation in relation to human activity, especially fire, it was essential to choose sediments likely to contain pollen and charcoal
237
FINE RESOLUTION POLLEN ANALYSIS
collected from the same source area. Thus a core from a small raised bog in a relatively confined setting on the northeast flank of Lough Adoon was sampled. The bog is surrounded by moraine and lake on its western edge, scree slope on the east and down valley, outside the corrie walls, by extensive blanket bog and heath. Adjacent to the site are several hut and prebog field systems and enclosures.
Sampling, preparation, stratigraphy and dating The bog was cored in its deepest section using Russian (Jowsey, 1966) and Dachnowsky corers (West, 1968), the latter to extract the basal few centimetres of sediment. Samples were extruded on-site, measured and wrapped in plastic film and aluminium foil, labelled and stored under refrigeration after return from the field and before sampling. Samples for pollen and charcoal analysis were 1 cm thicknesses of sediment. Throughout most of the core samples were collected at 5 cm depth intervals; avoiding the ends of core sections where possible, as it was noted these sometimes included possible contaminants. For two sections of sediment later analysis involved finer sampling with contiguous 1 cm samples. Each subsample of core was collected by peeling back the core surface and removing 0.5 cm 3 with a fixed volume plunger into a 15 ml centrifuge tube. One or more Eucalyptus pollen stock tablets (Batch No. 106720) containing 13,550_210 grains was added to obtain an estimate of pollen concentration for later influx calculations. Each sample was dispersed in 10% HC1 then diluted with distilled H 2 0 and centrifuged and decanted. The preparation procedure otherwise followed that described by Moore and Webb (1976) with a standard NaOH washing, Erdtman's acetolysis and dehydration and mounting a further subsample in silicone oil. The stratigraphy of the core was as follows: 0-2.5 cm: living plant mat of Calluna vulgaris, Ulex gallii, Sphagnum spp. and bryophytes 2.5-14 cm:dark-brown turfa with abundant rootlets
14-22 cm:
dark-brown turfa, less fibrous than above 22-46 cm: dark-brown turfa, lighter than above 46-247 cm: dark-brown turfa, darker than above, some wood flecks at 156-157 cm and below 210 cm. The Dachnowsky corer hit rock in the deepest section but as no soil was recovered it is assumed that the basal peat is flush with the rock. The core therefore records a history of peat growth initially amongst some woodland, but later through to the heathland and bog of today. Four samples were dated using radiocarbon analysis (Table I). These show that peat growth began before 3700 B.P. and that the sedimentation rate has increased toward the present day. Pollen was counted until a minimum of 300 grains of terrestrial taxa had been identified. This sum included Cyperaceae but not aquatic taxa or fern spores. Charcoal influx was estimated by the point-count method of Clark (1982). The pollen diagram (Fig.2) shows percentages for selected taxa based on total pollen excluding aquatics and spores. The diagram also shows charcoal plotted as cm2/cm 3 of sediment.
General vegetation change The diagram is generally dominated by Poaceae, Cyperaceae and Calluna pollen but has a distinct basal peak dominated by Betula and accompanied by other woody taxa. In the middle of the diagram there is a pronounced charcoal peak. The occurrence of single peak values for Sphagnum, Plantago lanceolata and Rumex, among others, is taken to reflect that local pollen dominates the spectra. TABLE I Radiocarbon age determinationsfrom LoughAdoon Hillside Bog Sample depth ( c m )
Laboratory number
Age (B.P.)
78-85 105-115 215-220 240-245
SUA 2732 SUA 2733 SUA 2734 SUA 2735
770_+ 80 1180_+80 2960_+ 70 3650_+ 70
238
J.R. DODSOb
LOUGH ADOON HILL BOG Pollen Sum: Age {yr B.P)
fofol ferresfriol pollen
Depth (m)
OC. . . . . . . . . . . . . . . . . . . . . . . . . . . .
770 ± 80
i<
1180 ± 80
i i
r
I
~
D
F--
g
_
2 C,2 9 6 0 + 70 •
--~-__-_--_-
3650 ± 70.
2.5. - Betula
Age ( y r E~.P.)
Pinus Ulmus Quercus Fraxinus AInus
Depth r ,
LA_H 1
[lex CaIluna saliz Corylus Hedera Rosaceae (undiff.) Filipendula
C CI q - -
7 7 0 4- 8 0
'
a--
~.
.=,--
-
1180±80"
LAH 2 ~-
:541
:,,._._
L ;
-I
2960 + 70 " 3650 ± 70 -
Age
(yr B.p.)
2 5
--I
Depth
~. . . . .
.
.
.
.
.
,55
~5o+7o
Poaeeae
(re)
:.
.
.
.
.
.
.
.
•
770 + 8 0
2960 + 70
Sphagnum
Potentil!a Narthecium Cyperaceae .
I;60_+50
LAH 1
'. . . . . . . . . . .
Ericales (undiff.) Myr/ca
.
.
.
.
~L _
.
.
.
.
.
.
.
.
.
.
.
.
:
-
ii
.
.
,_.,
.
.
.
.
.
.
.
.
.
L
.I
_L .L
~
i.,,- I
--:
LAH2
i
~"
-
'
"
I
.
: "
'
i . . . . . . . . . . .
~- _
_ _
~
~.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
ii1~: Asteraceae (Tub) Caltha
Po~amogeton Plan~ago major,/media Mw'ica Pter'idium Asteraeeae (gig) Monolete spores Cerealia Pediastrum coenobia Urticaceae Botryococcus PoIygonum Polypodiaceae
0.54 770 ± 80
2 9 6 0 _+ 7 0 3650
_+ 7 0
:,04
i ,
.
I
Plantago lanceolata Ranunculaeeae Apiaceae Succisa Age ~eD:~ Rumez (yr B.P.) (m',
118o±6o
.
LAH
_
2
_ -.
.
.
.
.
.
.
.
.
.
.
L
A
~
§Charcoal (em2/'cm3)
Fig.2. Percentage pollen diagram for selected pollen taxa from the whole peat section from Lough Adoon Hill Bog.
LAH 1
FINE RESOLUTION POLLEN ANALYSIS
The record clearly suggests that a bog- heathgrassland complex has dominated the site for much of the record following the decline in woodland. The early woodland phase was dominated by Betula and Alnus but some Quercus, Ulmus and possibly Pinus was present. The sediments of the early phase contained wood, mainly Betula, attesting to the local occurrence of woodland. This was replaced by heath, grassland and bog around 3080 B.P. The charcoal input was low. The high charcoal influx phase began around 1100 B.P. and occurred during a period dominated by heath grassland and bog. Toward the end of this phase there was an increase in bogland. A likely scenario is that early bog growth began as woodland clearance was taking place. During the charcoal phase conditions became wetter and probably boggier since Cyperaceae, Potamogeton, Narthecium and Pediastrum are present and increase. A more detailed description of the record will be given elsewhere (Dodson, in press) and the following analyses concentrate on vegetation dynamics within the Betula woodland and charcoal dominated phases. Figures 3 and 4 show pollen influx estimates for major taxa during sections of the two phases. These phases are identified as Betula and a younger charcoal series. The charcoal phase is here interpreted as being a period of more intensive and sustained use of fire by people.
Numerical analysis of the pollen influx data The following section analyses the influx data to detect significant trends in vegetation cover and interactions between taxa and fire within each series. It should be borne in mind that analyses of the whole diagram would result in a different time scale of changes being examined. The contiguous samples of 1 cm thickness have mean pollen accumulation rates of 27.6 and 13.7 years in the Betula and charcoal series respectively. The time separation of samples therefore also has a mean of these values. The method of binary splits is designed to measure effective presences and absences of taxa within a series (Walker and Wilson, 1978). The method identifies sections with relatively constant
239
occurrence or absence of pollen taxa. The analysis was performed on major taxa (Figs.3 and 4). Figure 5 shows positions of significant splits for taxa in each series. The Betula series had 25 binary splits for the major taxa with Ulmus (4), Pinus (2), Fraxinus (1) and Ericales (2) represented amongst woody taxa; the other splits were mostly herbaceous taxa, with Plantago and Asteraceae species and cereal pollen, plus one for charcoal. When these are analysed in conjunction with the influx diagram (Fig.3) the record shows a decline in tree taxa and replacement by Myrica, Calluna and Poaceae, then a small increase in tree taxa toward the end of the phase. The split analysis suggests part of the trend was due to cereal production with the main period between c. 2990 and 3540 B.P. Probably loss of trees was accompanied by increasing cereal production which later decreased due to increasing bog as shown by an increase in Cyperaceae, Narthecium and possibly Sphagnum around 2990 B.P. For the charcoal series 17 splits were found with those for Pinus (3) probably not significant ecologically due to the low influx values. The main trend of the influx diagram (Fig.4) shows a decrease in Poaceae and Plantago accompanied by a rise in Betula, Quercus and Alnus which later decline and are replaced by Cyperaceae, Potamogeton and some Poaceae. Cereal pollen was present throughout and presumably increasing wetness was a factor in tree decline. Cross-correlograms (Figs.6, 7) compare two series by calculating correlation coefficients at zero and all possible lags between the series. The vertical scale represents the correlation coefficient from + 1.0, through zero to - 1.0. The value at the origin gives the correlation coefficient between the series at all data levels. Values to the right give the correlation coefficient for the first named series against the second successively lagging in time whilst values to the left are for the second named series against the first. On the diagrams are given the 0.01 significance level of the correlation coefficients and also a time scale. The 0.01 level of significance was chosen since the series were about 30 samples long and spurious peaks were more likely to arise by chance if the 0.05 level was used.
240
JR.
DODSON
LOUGH ADOONHILL BOG: birch series
Influx diagram Depth
(cm) 0 1000 gr/cm'2/'yr
1' = Scale x 10.00
§ = Scole x 0.10
bL
225
250
Pinus
§Betula
Ulmus
Depth
(~m)
215 225 i
250 ]
Quercus
Alnus
Depth I
£raxi~us
Depth
~yr~ca
Calluna
Covylus Salix Hedeva Ilex
(~m)
25o ~' Sphagnum
II
•
Ericales (undiif.)
Potentilla Cyperaceae
R o s a c e a e (undift.)
Fili~e~dula §Poaceae
Pla~tago lanceolata Ptevidium Plantago majo%/media Rumex Narfhe cium Cerealia t C h a r c o a l (cm3,/cm ~) Asteraceae (Tub) . Asteraceae (Lig)
Fig.3. Pollen influx diagram for the Betulaseries.
Box and Jenkins (1970) describe the theory and here a program described by Green (1981) was used to perform the analyses. Many permutations of analyses amongst pollen taxa can be performed and a massive data set
arises. What follows is an analysis for specific questions concerning the relationships amongst tree, shrub and heath taxa and of fire. A selection of correlograms (Figs.6 and 7) illustrate the results described below.
241
FINE RESOLUTION POLLEN ANALYSIS
LOUGHADOONHILL BOG: fire series
Influx diogrom Depth (cm) 0_~1000 ~r/cm2/yr
t = Scale x 10.00
100 Betula
Pivt~s Saliz Calluvta ElTn~s Hedera Q~erc~s llez Al~s Rcsaceae (undiff.) Frazivt~s M!frica
Depth
Co~l~s
(cm)
,::1 imt . r
~ P
Ericales (undiff.) Pote~tilla
Depth
(cm)
so]
I00~ Sphag~u~n
Cyperaceae
Depth
(cm)
801 t
1001 Filipe~td~ta voaceae Depth (cm)
100
'° l Plavttago la'ttceolata
Pota~ogeto~ Plavttago ~ajo'rf ~edia Nm'thec~gm Cerealia ~Charcoal (cm'/cm =) Asteraceae (Tub) Rumez Ptev'idi~
Fig.4. Pollen influxdiagramfor the charcoalseries.
242
.I.R. DODSON
kough Adoon Hill Bog Presence / absence changes
80 cm
90
100
110cm
Charcoal series Corylus SalJx
215 cm
Ptnus
Compositae Tubulillorae Potam~2eton Ulmus Pinus Fraxmus Compositae Tubuliflorae Sa#x Pinus Compositae Tubuhflorae Potamogeton Potamogeton Ulmus Corylus Compositae Tubuliflorae
220
230
240
1
247cm
Betula Series Ulmos Ulmus Ericales Compositae Tubuliflorae Ulmus Cereal Composllae Tubulfflorae Compositae Liguhflorae Plantago lanceolata Ulmus Plantago major Composilae Liguliflorae Compositae Tubuliflorae Fraxmus Plantago lanceolata Ptnus Plnus Composilae Tubuliflorae Cereal Ericales Cereal Narthecmm Plantago major Pteodium Charcoai
Fig.5. Binary splits for the two influx series.
Tree taxa dynamics (a) Betula series There were few interrelationships evident showing that while Betula and Pinus declined there were no impacts as a result o f this on other tree taxa. Ulmus and Quercus were positively correlated at zero lag, showing they increased and decreased together while Alnus positively lagged Quercus by about 30 years and Ulmus lagged Fraxinus by 85 years and Fraxinus also lagged Ulmus by 30 years. The most complex interrelationship therefore appears to be replacement between Ulmus and
Fraxinus.
Tree and shrub dynamics (a) Betula series The most common relationship observed was positive correlation at zero tag between Corylus, Salix, Myrica and Ericales and between Betula/Calluna, Ulmus/ Corylus, Quercus/ Myrica and Fraxinus/ Salix. These data suggest that the abundance of major shrub taxa and for some tree/shrub pairs changes occur in concert. In addition there were 30-60 year positive lags for Fraxinus and Ulmus on Ericales and Myrica on Fraxinus suggesting a replacement series of Ericales to Fraxinus/Ulmus followed by Myrica invasion. The only negative correlation and hence competition or other replacement interaction observed was that between Corylus and Betula.
(b) Charcoal series The pairs Betula/Alnus, Ulmus/Fraxinus, Fraxinus/Quereus and Quercus/Alnus showed positive correlation at zero lag, suggesting most trees increased or decreased together within a single sample interval of 14 years. In addition there was a positive lag of 70 years for Ulmus on Alnus.
(b) Charcoal series Positive correlation at zero lag was found for Corvlus with Salix, Calluna and Ericales and for Salix with Alnus, Fraxinus and Quercus. Thus many shrub and tree taxa increased or decreased together. In addition there were positive lags at
243
FINE RESOLUTION POLLEN ANALYSIS
U/mus against
Quercus against
Ulmus against Fraxinus
Corylus against ;afix
Myrica against Corylus
Fraxinus against
--,,,X,Wv-v~
•
U/mus against
~a~
Fraxinus against
Sa/ix against
h a r c o a ~
~
Cereal against
'V"V
'try" v•
V
Poaceae against
Salix against
Cereal
l IVI" "~,~"
-W' "V
Fig.6. Selected cross-correlograms for the Betula series. The curve shows the position of the 0.01 significance level of the correlation coefficient and the vertical lines either side of the origin give a 200 year lag scale.
about 25 years for Corylus on Calluna, 55 years for Alnus on Corylus and 95 years for Ulmus on Corylus. These latter show longer term tree replacements of shrubs by trees. The only negative relationship observed was Betula on Calluna suggesting a suppression of Betula by Calluna.
Fire relationships (a) Betula series Only Salix, cereal and Plantago major pollen were found to have any relationship with charcoal influx with positive correlations at 55 year lags.
244
JR. DODSON
BetUllnaugains'
against
/
iii
iJtv ,v,v - -
Atnus against Salix
Ulnus against
Quercusagainst
/
v V ~
,qV-~IV-~ vv
V
Corylusagainst /
Calluna
" U V,A/~AaAA V v
\
Alnus against
Betula against
/
/~V Betula against
"It,Jv''v
il Cereal against
Fig.7. Selected cross-correlograms for the charcoal series. The curve s h o w s the position o f the 0.01 significance level o f the correlation coefficient and the vertical lines either side o f the origin give a 100 year lag scale.
Thus fire seems to have had little role in tree and shrub dynamics. It may have been used as a tool by people on a rotation of about 55 years to enhance conditions for cereal cropping or have been the mean natural fire return period. Since Salix and
cereal pollen were also positively correlated at zero lag as were cereal with Poaceae, Corylus and Myrica it would appear that fire probably had the effect of promoting conditions for Salix-Corylus shrubland, grassland and cereal cropping.
245
FINE RESOLUTION POLLEN ANALYSIS
(b) Charcoal series Betula and charcoal were positively correlated at zero lag and cereal pollen influx was negatively correlated at a 14 year lag. In addition charcoal positively lagged Corylus at 55 and 95 year periods. Fire may have therefore been used to clear Betula and Corylus thicket but this appears to have possibly had a negative impact on cereal production, perhaps in part a result of increased boggy conditions created by fire as evidenced by increased Narthecium values.
time interval of a decade; thus clearly where the time scale can be reduced statistical analysis of pollen influx data can undoubtedly provide a valuable perspective on vegetation change.
Acknowledgements I wish to thank my wife Mary for her assistance in the field and for completing the laboratory preparations. The Department of Geography, Trinity College Dublin generously provided laboratory facilities.
Conclusions References In general terms decline in woodland taxa resulted from expansion of heathland and bog. This was possibly related in part to reduced evapotranspiration and such change at Lough Adoon seems to have taken place on time scales measurable in centuries. However there is also interplay at scales of a few decades. Where woodland recovers Salix, Calluna, Ericales, Myrica and Corylus develop. It was noted that Ericales recovery precedes Fraxinus and Ulmus increase and Myrica developed as understorey later. The only negative relationship observed was between Betula and Corylus. Fire was found to have few direct relationships with pollen and while it may have been used to clear Betula and Corylus thicket it was otherwise absent from woodland. It played a role in land management as both positive and negative correlations were observed between cereal pollen and charcoal influx. During the Betula phase fire apparently promoted shrub regrowth as well as conditions for cereal cropping but the negative impact during the charcoal phase may have been due to promotion of boggier conditions. Management for cereal cropping was always accompanied by increased grassland and undoubtedly pasture was always part of the vegetation. Fire was largely restricted to heathland, grassland and bog and there is a suggestion that these formations represent a series of ecological drift resulting from continued burning. Most interactions were found to occur within a
Box, G.E.P. and Jenkins, G.M., 1970. Time series analysis: forecasting and control. Holden-Day, San Francisco, 553 pp. Clark, R.L., 1982. Point count estimation of charcoal in pollen preparations and thin sections of sediments. Pollen Spores, 24: 523-535. Cuppage, J., Bennett, I., Cotter, C. and Rahilly, C.O., 1986. Archaeological Survey of the Dingle Peninsula: Surbhe Seandalaiochta Chorca Dhuibhne. Ordhreacht Chorca Dhuibhne, Ballyferriter, 462 pp. Dodson, J.R., in press. The Holocene of a prehistorically inhabited valley, Dingle Peninsula, Co. Kerry, western Ireland. Proc. R. Irish Acad., Dublin. Green, D.G., 1981. Time series and postglacial forest ecology. Quat. Res., 15: 265-277. Home, R.R., 1976. Geological Guide to the Dingle Peninsula. Geol. Surv. Ireland Guide Ser. No. 1. Geol. Surv. Ireland, Dublin, 52 pp. Jowsey, P.C., 1966. An improved peat sampler. New Phytol., 65: 245-248. Lynch, A., 1981. Man and environment in S.W. Ireland. Br. Archaeol. Rep: Br. Ser. 85, 175 pp. Mitchell. F., 1976. The Irish Landscape. Collins, London, 240 pp. Molloy, K. and O'Connell, M., 1987. The nature of the vegetational changes at about 5000 B.P. with particular reference to the elm decline: fresh evidence from Connemara, western Ireland. New Phytol., 106: 203-220. Moore, P.D. and Webb, J.A., 1976. An illustrated guide to pollen analysis. Hodder and Stoughton, London, 131 pp. Walker, D. and Wilson, S.R., 1978. A statistical alternative to the zoning of pollen diagrams. J. Biogeogr., 5: 1-21. Webb, D.A., 1977. An Irish Flora. 6th edition, Dundalk, 277 pp. West, R.G., 1968. Pleistocene geology and biology. Longman, London, 379 pp. Woodman, P., Duggan, M.A. and McCarthy, A., 1984. Excavations at Ferriter's Cove. J. Kerry Archaeol. Hist. Soc., 17: 5-9.
235
Fine resolution pollen analysis of vegetation history in the Lough Adoon Valley, Co. Kerry, western Ireland J.R. Dodson
School of Geography, University of New South Wales, Kensington, N.S.W., 2033 (Australia) (Received September 7, 1988; revised and accepted January 25, 1990)
ABSTRACT Dodson, J.R., 1990. Fine resolution pollen analysis of vegetation history in the Lough Adoon Valley, Co. Kerry, western Ireland. Rev. Palaeobot. Palynol., 64: 235-245.
The Lough Adoon Valley on the north of the Dingle Peninsula, in southwest Ireland, has been occupied by humans, perhaps continuously, since Neolithic times. Since about 5000 B.P. the valley has been essentially deforested, apart from local reinvasions of Betula, and much of the landscape has been covered by blanket bog and heath. Time series analysis of pollen and charcoal influx from two sections of peat sampled at contiguous I cm intervals was carried out to explore the vegetation shifts associated with Betula decline and charcoal dominated phases. Cereal cropping and fire were important vegetationalcontrols resulting in an interplay between heath, wetland, grassland and woodland but forest decline was not a result of fire. The number of pollen taxa interrelationships differed between the phases with a greater number of interrelationships in the Betula decline phase than when fire was more prominent in the environment.
Ireland. Throughout the text Webb (1977) is used as the authority for species names.
Introduction Most pollen analytical investigation in Ireland has been directed toward documenting the processes of vegetation and environmental change since the late-glacial. The broad nature o f this is well-understood but much detail awaits to be filled in. Only recently has the power of pollen analysis been extended to investigate important questions associated with forest dynamics at the stand level or with the nature of change such as the Ulmus decline (Molloy and O'Connell, 1987). Investigations at all scales have identified an important role for h u m a n activity in forest dynamics and clearance for agriculture during the Holocene. Indeed the spread of blanket-bog in m a n y areas has been ascribed, at least in part, to h u m a n activity (Mitchell, 1976; Lynch, 1981). The aim o f the present investigation is to examine the detail of vegetation change associated with h u m a n activity, especially fire, in a small valley in southwest 0034-6667/90/$03.50
Site selection The Dingle Peninsula (Fig.l) has recently been the subject of a detailed archaeological inventory (Cuppage et al., 1986) and has a h u m a n occupational history, perhaps continuous, extending back at least 6000 radiocarbon years ( W o o d m a n et al., 1984). The Lough A d o o n Valley on the northern side of the Peninsula was chosen for study because of the abundant archaeological evidence within its confines. These include a collection of settlement and ritual/funery sites including several hut sites, enclosures, standing stones, rock art, several burning-pit sites (fulachta fiadh), a fortified island, several cist graves and a wedge t o m b of possible Bronze Age. It is probable that people have occupied the valley since Bronze Age time. There is possibly no area on the Dingle Peninsula with such
© 1990 - - Elsevier Science Publishers B.V.
236
J.R. DODSON
N
D
Standing stones
\
Kerry
Vlegalithic~ tomb
XS t ~
-/
) "i
':
•,LO
doon~
,
)
X Core Site
,
" :, )
488rn Slievenagower
F--~---', "~'~ A
,'';;'"
;/
~
Scree
o
30o
I
I metres
Fig.1. Location of Lough Adoon valley, western Ireland and pollen sampling site. a concentration and variety of archaeological material.
Physical and biological setting Lough Adoon and Lough Camclaun are corries in Devonian Group purple-red and grey-green conglomerates, sandstones and mudstones (Horne, 1976). The corries sit hard against valley headwalls. The soils are largely blanket-bog peats along the valley floor or shallow peaty-gleys amongst boul-
der strewn fan deposits around the steep corrie walls. The vegetation is mainly heath and rough pasture. There are a few scattered low trees which are mainly Crataegus monogyna in the upper basin and llex aquifolium on the blanket-bog.
Sampling and analysis strategy Since the aim of the study was to unravel the history of the vegetation in relation to human activity, especially fire, it was essential to choose sediments likely to contain pollen and charcoal
237
FINE RESOLUTION POLLEN ANALYSIS
collected from the same source area. Thus a core from a small raised bog in a relatively confined setting on the northeast flank of Lough Adoon was sampled. The bog is surrounded by moraine and lake on its western edge, scree slope on the east and down valley, outside the corrie walls, by extensive blanket bog and heath. Adjacent to the site are several hut and prebog field systems and enclosures.
Sampling, preparation, stratigraphy and dating The bog was cored in its deepest section using Russian (Jowsey, 1966) and Dachnowsky corers (West, 1968), the latter to extract the basal few centimetres of sediment. Samples were extruded on-site, measured and wrapped in plastic film and aluminium foil, labelled and stored under refrigeration after return from the field and before sampling. Samples for pollen and charcoal analysis were 1 cm thicknesses of sediment. Throughout most of the core samples were collected at 5 cm depth intervals; avoiding the ends of core sections where possible, as it was noted these sometimes included possible contaminants. For two sections of sediment later analysis involved finer sampling with contiguous 1 cm samples. Each subsample of core was collected by peeling back the core surface and removing 0.5 cm 3 with a fixed volume plunger into a 15 ml centrifuge tube. One or more Eucalyptus pollen stock tablets (Batch No. 106720) containing 13,550_210 grains was added to obtain an estimate of pollen concentration for later influx calculations. Each sample was dispersed in 10% HC1 then diluted with distilled H 2 0 and centrifuged and decanted. The preparation procedure otherwise followed that described by Moore and Webb (1976) with a standard NaOH washing, Erdtman's acetolysis and dehydration and mounting a further subsample in silicone oil. The stratigraphy of the core was as follows: 0-2.5 cm: living plant mat of Calluna vulgaris, Ulex gallii, Sphagnum spp. and bryophytes 2.5-14 cm:dark-brown turfa with abundant rootlets
14-22 cm:
dark-brown turfa, less fibrous than above 22-46 cm: dark-brown turfa, lighter than above 46-247 cm: dark-brown turfa, darker than above, some wood flecks at 156-157 cm and below 210 cm. The Dachnowsky corer hit rock in the deepest section but as no soil was recovered it is assumed that the basal peat is flush with the rock. The core therefore records a history of peat growth initially amongst some woodland, but later through to the heathland and bog of today. Four samples were dated using radiocarbon analysis (Table I). These show that peat growth began before 3700 B.P. and that the sedimentation rate has increased toward the present day. Pollen was counted until a minimum of 300 grains of terrestrial taxa had been identified. This sum included Cyperaceae but not aquatic taxa or fern spores. Charcoal influx was estimated by the point-count method of Clark (1982). The pollen diagram (Fig.2) shows percentages for selected taxa based on total pollen excluding aquatics and spores. The diagram also shows charcoal plotted as cm2/cm 3 of sediment.
General vegetation change The diagram is generally dominated by Poaceae, Cyperaceae and Calluna pollen but has a distinct basal peak dominated by Betula and accompanied by other woody taxa. In the middle of the diagram there is a pronounced charcoal peak. The occurrence of single peak values for Sphagnum, Plantago lanceolata and Rumex, among others, is taken to reflect that local pollen dominates the spectra. TABLE I Radiocarbon age determinationsfrom LoughAdoon Hillside Bog Sample depth ( c m )
Laboratory number
Age (B.P.)
78-85 105-115 215-220 240-245
SUA 2732 SUA 2733 SUA 2734 SUA 2735
770_+ 80 1180_+80 2960_+ 70 3650_+ 70
238
J.R. DODSOb
LOUGH ADOON HILL BOG Pollen Sum: Age {yr B.P)
fofol ferresfriol pollen
Depth (m)
OC. . . . . . . . . . . . . . . . . . . . . . . . . . . .
770 ± 80
i<
1180 ± 80
i i
r
I
~
D
F--
g
_
2 C,2 9 6 0 + 70 •
--~-__-_--_-
3650 ± 70.
2.5. - Betula
Age ( y r E~.P.)
Pinus Ulmus Quercus Fraxinus AInus
Depth r ,
LA_H 1
[lex CaIluna saliz Corylus Hedera Rosaceae (undiff.) Filipendula
C CI q - -
7 7 0 4- 8 0
'
a--
~.
.=,--
-
1180±80"
LAH 2 ~-
:541
:,,._._
L ;
-I
2960 + 70 " 3650 ± 70 -
Age
(yr B.p.)
2 5
--I
Depth
~. . . . .
.
.
.
.
.
,55
~5o+7o
Poaeeae
(re)
:.
.
.
.
.
.
.
.
•
770 + 8 0
2960 + 70
Sphagnum
Potentil!a Narthecium Cyperaceae .
I;60_+50
LAH 1
'. . . . . . . . . . .
Ericales (undiff.) Myr/ca
.
.
.
.
~L _
.
.
.
.
.
.
.
.
.
.
.
.
:
-
ii
.
.
,_.,
.
.
.
.
.
.
.
.
.
L
.I
_L .L
~
i.,,- I
--:
LAH2
i
~"
-
'
"
I
.
: "
'
i . . . . . . . . . . .
~- _
_ _
~
~.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
ii1~: Asteraceae (Tub) Caltha
Po~amogeton Plan~ago major,/media Mw'ica Pter'idium Asteraeeae (gig) Monolete spores Cerealia Pediastrum coenobia Urticaceae Botryococcus PoIygonum Polypodiaceae
0.54 770 ± 80
2 9 6 0 _+ 7 0 3650
_+ 7 0
:,04
i ,
.
I
Plantago lanceolata Ranunculaeeae Apiaceae Succisa Age ~eD:~ Rumez (yr B.P.) (m',
118o±6o
.
LAH
_
2
_ -.
.
.
.
.
.
.
.
.
.
.
L
A
~
§Charcoal (em2/'cm3)
Fig.2. Percentage pollen diagram for selected pollen taxa from the whole peat section from Lough Adoon Hill Bog.
LAH 1
FINE RESOLUTION POLLEN ANALYSIS
The record clearly suggests that a bog- heathgrassland complex has dominated the site for much of the record following the decline in woodland. The early woodland phase was dominated by Betula and Alnus but some Quercus, Ulmus and possibly Pinus was present. The sediments of the early phase contained wood, mainly Betula, attesting to the local occurrence of woodland. This was replaced by heath, grassland and bog around 3080 B.P. The charcoal input was low. The high charcoal influx phase began around 1100 B.P. and occurred during a period dominated by heath grassland and bog. Toward the end of this phase there was an increase in bogland. A likely scenario is that early bog growth began as woodland clearance was taking place. During the charcoal phase conditions became wetter and probably boggier since Cyperaceae, Potamogeton, Narthecium and Pediastrum are present and increase. A more detailed description of the record will be given elsewhere (Dodson, in press) and the following analyses concentrate on vegetation dynamics within the Betula woodland and charcoal dominated phases. Figures 3 and 4 show pollen influx estimates for major taxa during sections of the two phases. These phases are identified as Betula and a younger charcoal series. The charcoal phase is here interpreted as being a period of more intensive and sustained use of fire by people.
Numerical analysis of the pollen influx data The following section analyses the influx data to detect significant trends in vegetation cover and interactions between taxa and fire within each series. It should be borne in mind that analyses of the whole diagram would result in a different time scale of changes being examined. The contiguous samples of 1 cm thickness have mean pollen accumulation rates of 27.6 and 13.7 years in the Betula and charcoal series respectively. The time separation of samples therefore also has a mean of these values. The method of binary splits is designed to measure effective presences and absences of taxa within a series (Walker and Wilson, 1978). The method identifies sections with relatively constant
239
occurrence or absence of pollen taxa. The analysis was performed on major taxa (Figs.3 and 4). Figure 5 shows positions of significant splits for taxa in each series. The Betula series had 25 binary splits for the major taxa with Ulmus (4), Pinus (2), Fraxinus (1) and Ericales (2) represented amongst woody taxa; the other splits were mostly herbaceous taxa, with Plantago and Asteraceae species and cereal pollen, plus one for charcoal. When these are analysed in conjunction with the influx diagram (Fig.3) the record shows a decline in tree taxa and replacement by Myrica, Calluna and Poaceae, then a small increase in tree taxa toward the end of the phase. The split analysis suggests part of the trend was due to cereal production with the main period between c. 2990 and 3540 B.P. Probably loss of trees was accompanied by increasing cereal production which later decreased due to increasing bog as shown by an increase in Cyperaceae, Narthecium and possibly Sphagnum around 2990 B.P. For the charcoal series 17 splits were found with those for Pinus (3) probably not significant ecologically due to the low influx values. The main trend of the influx diagram (Fig.4) shows a decrease in Poaceae and Plantago accompanied by a rise in Betula, Quercus and Alnus which later decline and are replaced by Cyperaceae, Potamogeton and some Poaceae. Cereal pollen was present throughout and presumably increasing wetness was a factor in tree decline. Cross-correlograms (Figs.6, 7) compare two series by calculating correlation coefficients at zero and all possible lags between the series. The vertical scale represents the correlation coefficient from + 1.0, through zero to - 1.0. The value at the origin gives the correlation coefficient between the series at all data levels. Values to the right give the correlation coefficient for the first named series against the second successively lagging in time whilst values to the left are for the second named series against the first. On the diagrams are given the 0.01 significance level of the correlation coefficients and also a time scale. The 0.01 level of significance was chosen since the series were about 30 samples long and spurious peaks were more likely to arise by chance if the 0.05 level was used.
240
JR.
DODSON
LOUGH ADOONHILL BOG: birch series
Influx diagram Depth
(cm) 0 1000 gr/cm'2/'yr
1' = Scale x 10.00
§ = Scole x 0.10
bL
225
250
Pinus
§Betula
Ulmus
Depth
(~m)
215 225 i
250 ]
Quercus
Alnus
Depth I
£raxi~us
Depth
~yr~ca
Calluna
Covylus Salix Hedeva Ilex
(~m)
25o ~' Sphagnum
II
•
Ericales (undiif.)
Potentilla Cyperaceae
R o s a c e a e (undift.)
Fili~e~dula §Poaceae
Pla~tago lanceolata Ptevidium Plantago majo%/media Rumex Narfhe cium Cerealia t C h a r c o a l (cm3,/cm ~) Asteraceae (Tub) . Asteraceae (Lig)
Fig.3. Pollen influx diagram for the Betulaseries.
Box and Jenkins (1970) describe the theory and here a program described by Green (1981) was used to perform the analyses. Many permutations of analyses amongst pollen taxa can be performed and a massive data set
arises. What follows is an analysis for specific questions concerning the relationships amongst tree, shrub and heath taxa and of fire. A selection of correlograms (Figs.6 and 7) illustrate the results described below.
241
FINE RESOLUTION POLLEN ANALYSIS
LOUGHADOONHILL BOG: fire series
Influx diogrom Depth (cm) 0_~1000 ~r/cm2/yr
t = Scale x 10.00
100 Betula
Pivt~s Saliz Calluvta ElTn~s Hedera Q~erc~s llez Al~s Rcsaceae (undiff.) Frazivt~s M!frica
Depth
Co~l~s
(cm)
,::1 imt . r
~ P
Ericales (undiff.) Pote~tilla
Depth
(cm)
so]
I00~ Sphag~u~n
Cyperaceae
Depth
(cm)
801 t
1001 Filipe~td~ta voaceae Depth (cm)
100
'° l Plavttago la'ttceolata
Pota~ogeto~ Plavttago ~ajo'rf ~edia Nm'thec~gm Cerealia ~Charcoal (cm'/cm =) Asteraceae (Tub) Rumez Ptev'idi~
Fig.4. Pollen influxdiagramfor the charcoalseries.
242
.I.R. DODSON
kough Adoon Hill Bog Presence / absence changes
80 cm
90
100
110cm
Charcoal series Corylus SalJx
215 cm
Ptnus
Compositae Tubulillorae Potam~2eton Ulmus Pinus Fraxmus Compositae Tubuliflorae Sa#x Pinus Compositae Tubuhflorae Potamogeton Potamogeton Ulmus Corylus Compositae Tubuliflorae
220
230
240
1
247cm
Betula Series Ulmos Ulmus Ericales Compositae Tubuliflorae Ulmus Cereal Composllae Tubulfflorae Compositae Liguhflorae Plantago lanceolata Ulmus Plantago major Composilae Liguliflorae Compositae Tubuliflorae Fraxmus Plantago lanceolata Ptnus Plnus Composilae Tubuliflorae Cereal Ericales Cereal Narthecmm Plantago major Pteodium Charcoai
Fig.5. Binary splits for the two influx series.
Tree taxa dynamics (a) Betula series There were few interrelationships evident showing that while Betula and Pinus declined there were no impacts as a result o f this on other tree taxa. Ulmus and Quercus were positively correlated at zero lag, showing they increased and decreased together while Alnus positively lagged Quercus by about 30 years and Ulmus lagged Fraxinus by 85 years and Fraxinus also lagged Ulmus by 30 years. The most complex interrelationship therefore appears to be replacement between Ulmus and
Fraxinus.
Tree and shrub dynamics (a) Betula series The most common relationship observed was positive correlation at zero tag between Corylus, Salix, Myrica and Ericales and between Betula/Calluna, Ulmus/ Corylus, Quercus/ Myrica and Fraxinus/ Salix. These data suggest that the abundance of major shrub taxa and for some tree/shrub pairs changes occur in concert. In addition there were 30-60 year positive lags for Fraxinus and Ulmus on Ericales and Myrica on Fraxinus suggesting a replacement series of Ericales to Fraxinus/Ulmus followed by Myrica invasion. The only negative correlation and hence competition or other replacement interaction observed was that between Corylus and Betula.
(b) Charcoal series The pairs Betula/Alnus, Ulmus/Fraxinus, Fraxinus/Quereus and Quercus/Alnus showed positive correlation at zero lag, suggesting most trees increased or decreased together within a single sample interval of 14 years. In addition there was a positive lag of 70 years for Ulmus on Alnus.
(b) Charcoal series Positive correlation at zero lag was found for Corvlus with Salix, Calluna and Ericales and for Salix with Alnus, Fraxinus and Quercus. Thus many shrub and tree taxa increased or decreased together. In addition there were positive lags at
243
FINE RESOLUTION POLLEN ANALYSIS
U/mus against
Quercus against
Ulmus against Fraxinus
Corylus against ;afix
Myrica against Corylus
Fraxinus against
--,,,X,Wv-v~
•
U/mus against
~a~
Fraxinus against
Sa/ix against
h a r c o a ~
~
Cereal against
'V"V
'try" v•
V
Poaceae against
Salix against
Cereal
l IVI" "~,~"
-W' "V
Fig.6. Selected cross-correlograms for the Betula series. The curve shows the position of the 0.01 significance level of the correlation coefficient and the vertical lines either side of the origin give a 200 year lag scale.
about 25 years for Corylus on Calluna, 55 years for Alnus on Corylus and 95 years for Ulmus on Corylus. These latter show longer term tree replacements of shrubs by trees. The only negative relationship observed was Betula on Calluna suggesting a suppression of Betula by Calluna.
Fire relationships (a) Betula series Only Salix, cereal and Plantago major pollen were found to have any relationship with charcoal influx with positive correlations at 55 year lags.
244
JR. DODSON
BetUllnaugains'
against
/
iii
iJtv ,v,v - -
Atnus against Salix
Ulnus against
Quercusagainst
/
v V ~
,qV-~IV-~ vv
V
Corylusagainst /
Calluna
" U V,A/~AaAA V v
\
Alnus against
Betula against
/
/~V Betula against
"It,Jv''v
il Cereal against
Fig.7. Selected cross-correlograms for the charcoal series. The curve s h o w s the position o f the 0.01 significance level o f the correlation coefficient and the vertical lines either side o f the origin give a 100 year lag scale.
Thus fire seems to have had little role in tree and shrub dynamics. It may have been used as a tool by people on a rotation of about 55 years to enhance conditions for cereal cropping or have been the mean natural fire return period. Since Salix and
cereal pollen were also positively correlated at zero lag as were cereal with Poaceae, Corylus and Myrica it would appear that fire probably had the effect of promoting conditions for Salix-Corylus shrubland, grassland and cereal cropping.
245
FINE RESOLUTION POLLEN ANALYSIS
(b) Charcoal series Betula and charcoal were positively correlated at zero lag and cereal pollen influx was negatively correlated at a 14 year lag. In addition charcoal positively lagged Corylus at 55 and 95 year periods. Fire may have therefore been used to clear Betula and Corylus thicket but this appears to have possibly had a negative impact on cereal production, perhaps in part a result of increased boggy conditions created by fire as evidenced by increased Narthecium values.
time interval of a decade; thus clearly where the time scale can be reduced statistical analysis of pollen influx data can undoubtedly provide a valuable perspective on vegetation change.
Acknowledgements I wish to thank my wife Mary for her assistance in the field and for completing the laboratory preparations. The Department of Geography, Trinity College Dublin generously provided laboratory facilities.
Conclusions References In general terms decline in woodland taxa resulted from expansion of heathland and bog. This was possibly related in part to reduced evapotranspiration and such change at Lough Adoon seems to have taken place on time scales measurable in centuries. However there is also interplay at scales of a few decades. Where woodland recovers Salix, Calluna, Ericales, Myrica and Corylus develop. It was noted that Ericales recovery precedes Fraxinus and Ulmus increase and Myrica developed as understorey later. The only negative relationship observed was between Betula and Corylus. Fire was found to have few direct relationships with pollen and while it may have been used to clear Betula and Corylus thicket it was otherwise absent from woodland. It played a role in land management as both positive and negative correlations were observed between cereal pollen and charcoal influx. During the Betula phase fire apparently promoted shrub regrowth as well as conditions for cereal cropping but the negative impact during the charcoal phase may have been due to promotion of boggier conditions. Management for cereal cropping was always accompanied by increased grassland and undoubtedly pasture was always part of the vegetation. Fire was largely restricted to heathland, grassland and bog and there is a suggestion that these formations represent a series of ecological drift resulting from continued burning. Most interactions were found to occur within a
Box, G.E.P. and Jenkins, G.M., 1970. Time series analysis: forecasting and control. Holden-Day, San Francisco, 553 pp. Clark, R.L., 1982. Point count estimation of charcoal in pollen preparations and thin sections of sediments. Pollen Spores, 24: 523-535. Cuppage, J., Bennett, I., Cotter, C. and Rahilly, C.O., 1986. Archaeological Survey of the Dingle Peninsula: Surbhe Seandalaiochta Chorca Dhuibhne. Ordhreacht Chorca Dhuibhne, Ballyferriter, 462 pp. Dodson, J.R., in press. The Holocene of a prehistorically inhabited valley, Dingle Peninsula, Co. Kerry, western Ireland. Proc. R. Irish Acad., Dublin. Green, D.G., 1981. Time series and postglacial forest ecology. Quat. Res., 15: 265-277. Home, R.R., 1976. Geological Guide to the Dingle Peninsula. Geol. Surv. Ireland Guide Ser. No. 1. Geol. Surv. Ireland, Dublin, 52 pp. Jowsey, P.C., 1966. An improved peat sampler. New Phytol., 65: 245-248. Lynch, A., 1981. Man and environment in S.W. Ireland. Br. Archaeol. Rep: Br. Ser. 85, 175 pp. Mitchell. F., 1976. The Irish Landscape. Collins, London, 240 pp. Molloy, K. and O'Connell, M., 1987. The nature of the vegetational changes at about 5000 B.P. with particular reference to the elm decline: fresh evidence from Connemara, western Ireland. New Phytol., 106: 203-220. Moore, P.D. and Webb, J.A., 1976. An illustrated guide to pollen analysis. Hodder and Stoughton, London, 131 pp. Walker, D. and Wilson, S.R., 1978. A statistical alternative to the zoning of pollen diagrams. J. Biogeogr., 5: 1-21. Webb, D.A., 1977. An Irish Flora. 6th edition, Dundalk, 277 pp. West, R.G., 1968. Pleistocene geology and biology. Longman, London, 379 pp. Woodman, P., Duggan, M.A. and McCarthy, A., 1984. Excavations at Ferriter's Cove. J. Kerry Archaeol. Hist. Soc., 17: 5-9.