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Eutrophication of limestone heath soil by limestone quarrying dust and implications for conservation
EUTROPHICATION OF LIMESTONE HEATH SOIL BY LIMESTONE QUARRYING DUST AND IMPLICATIONS FOR CONSERVATION
JOHN R. ETHERINGTON
Botany Department, University College, Cardiff CFI 1XL, Wales
ABSTRACT
Soil analysis has established that limestone quarrying dust has entered an adjacent limestone heath of high conservation value. The soil profiles of calcium distribution, established by many years of leaching, have been seriously altered by surface enrichment. It is suggested that the calcicole-calcifuge vegetation of the shallowest soils may be threatened by these changes and it is considered very desirable that further pollution of the site should if possible be prevented.
INTRODUCTION
The ecological effects of wind-blown limestone (CaCOa) and other alkaline dusts have been investigated by few workers. Grime (1970) recorded increases in pH of acid soils, close to dust sources , of up to pH 1.5 units. To produce this effect in the plough layer of a medium textured soil the agricultural lime dressing would be about 12 t ha-1 (Etherington, 1975) compared with an estimated leaching loss from soils with average British rainfall Of c. 0"4tha-I y - ~ CaCO 3 (Russel, 1973). Manning (1971) records an annual dust income adjacent to a quarry and cement plant which is more than ten times greater than this estimate. A shallow layer of soil might rapidly be eutrophicated and in these circumstances the whole profile could become alkaline within a few decades. Unless the limestone abuts on acid rocks, for example the Carboniferous Limestone/Millstone Grit boundary in western and northern Britain, limestone quarrying dust will fall on normally calcareous soils and it might be predicted that no great change of soil chemistry would occur though the plants themselves may be damaged by dust deposits (Brandt & Rhoads, 1973). The ecological consequences of alkaline dust input to acid soils are of only local conservation importance as Britain 309 Biol. Conserv. (13) (1978)---© Applied Science Publishers Ltd, England, 1978 Printed in Great Britain
310
JOHNR. ETHERINGTON
has very large areas of upland heath, hill-grassland and other acidophilous speciespoor vegetation, most of which are far from such dust sources. Gilbert (1976) has noted that bark-dwelling lichens are an exception to this generalisation, being replaced by saxieolous species or an eutrophic Xanthorion alliance if exposed to even small amounts of limestone dust. A further exception is likely to be found in limestone heath, a rather rare vegetation type comprising an intimate mixture of ealcicole and caleifuge species on shallow limestone soils. These heaths and the similar chalk heaths of SE England have attracted ecologists' interest for many years (Moss, 1907, 1913; Tansley & Rankin, 1911; Grubb et al., 1969). Calluna vulgaris, which is generally recognised as a calcifuge in southern Britain, occurs commonly in these heaths, associated with plants of limestone grassland, many of which are ealcicolous and a few, obligate ealcieoles. A wide range of other calcifuges including Erica cinerea and Galium saxatile may be encountered but with lesser constancy than C. vulgaris. Extreme calcifuges such as Descharnpsiaflexuosa are very rare or absent. Much speculation and experiment has been devoted to the calcicole--calcifuge problem and the limestone heath vegetation may hold the key to its as yet elusive solution. For this very reason it is of high conservation value but may be unduly sensitive to limestone dust input as the vegetation probably depends on a fairly critical balance of soil chemical conditions related to calcium carbonate content, cation saturation and pH. Shimwell (1971) attributes most limestone heaths to transitions between the alliance Mesobromion and the Agrosto-Ulicetum association of the class NardoCallunetea and he cites Ivimey-Cook's (1955) suggestion that the calcifuges occupy a surface-rooting zone in the limestone heath soil which has been acidified by surface leaching. This explanation of the unusual mixture of plants has been widely accepted in the past (Moss, 1907, 1913) and more recent work on chalk heath has certainly established that surface leaching can occur and is promoted by the calcifuges (Grubb et al., 1969; Grubb & Suter, 1971). Jarvis (~974) found some evidence for root stratification in a calcicole--calcifuge vegetation on a basic dolerite soil in Wales. Grubb et al. (1969), however, questioned the surface-rooting concept as it does not fit the facts of root behaviour in chalk heath and also gives no explanation of seedling regeneration. They suggested that, below pH 5-0, calcicoles would fail to establish and noted that C. vulgaris and E. cinerea flourish in soils of pH 5 to 6 but might need a lower pH for seed germination in the surface few millimetres of soil. Grubb & Suter (1971) suggested that under heavy grazing the calcicole-calcifuge mixture is stable at pH 5 to 6 but if grazing is withdrawn the growth of calcifuges increases, causes soil acidification and outcompetes the caleifuges. Current work in these laboratories suggests that strong surface acidification of Carboniferous Limestone-heath soils is unlikely to be the full explanation for the existence of the vegetation. C. vulgarisand E. cinerea may be found on very steep
E U T R O P H I C A T I O N OF L I M E S T O N E H E A T H SOILS
311
limestone slopes in South Wales and Somerset where extreme soil instability has been promoted by heavy sheep and/or rabbit grazing. Such soils often contain free calcium carbonate though the percentages are much lower than those associated with soft-limestone and chalk soils. Data to be published elsewhere show that they are rather prone to leaching, however, the surface few millimetres fluctuating in calcium content and pH according to rainfall and evaporation. It therefore seems that although it is near neutral or alkaline, much of the soil will be unusually sensitive to input of calcium carbonate dust. The work described here was undertaken when it was discovered that the soil chemistry of a heavily quarried limestone heath site in the Vale of Glamorgan differed significantly from that of other sites in South Wales and Somerset.
SITES, MATERIALS A N D M E T H O D S
The great majority of the limestone heaths of south-western Britain are formed on the Carboniferous Limestone to the south of the Welsh coalfield and in Somerset. There are a few scattered areas on Lower Lias limestone in South and MidGlamorgan. Circumstantial evidence suggests that the vegetation type may once have been more widespread but that agriculture has destroyed all but a few remnants and limestone heath can now be considered a rare and fragmentary vegetation.
Sites During a current investigation of the iron nutrition of calcifuges in limestone heath, soil samples have been collected from sites on the South Dyfed coast (SR995942-882957), the Gower Peninsula (SS 510850--415872), the Vale of Glamorgan (SS 896760) and from the Somerset limestone (ST 395559). The site in the Vale of Glamorgan is near the village of Ewenny and occupies rough-grazing land known as Old Castle Down, Ogmore Down and Ewenny Down. Part of the site is a Glamorgan Naturalists' Trust reserve and a Nature Conservancy Council SSS1 (Fig. 1). It is particularly interesting and valuable since, in addition to the ubiquitous limestone heath calcifuges such as C. vulgaris and E. cinerea, it has damp, but not waterlogged, areas containing Erica tetralix, Pedicularis sylvatica and Molinia caerulea, none of which occur frequently in all of the other limestone heaths. This calcifugous flora is intimately associated with plants such as Helianthemum nummularium and Asperula cynanchica, both of which behave as calcicoles in Wales and southern England. This site is the most diverse and largest remaining limestone heath in western Britain but has for a number of years been threatened by the extension of two adjacent quarries: Ivimey-Cook (1955), who described the area in detail, commented that future prospects were not bright. During the preliminary analyses of soils from these sites it was noticed that the Ewenny samples were more alkaline than those from other heaths and also more
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EWENNY
Fig. 1. Location of the limestone heath at Ewenny, South Glamorgan. The boundary of the Glamorgan Naturalists" Trust reserve is shown by the heavy line. The dotted lines represent the sharp break of slope between the plateau and the slopes bearing Lulsgate soils. The 100 m squares of the National Grid are shown in the sampled area. The two quarries marked A and B are the dust sources.
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EUTROPHICATION OF LIMESTONE HEATH SOILS
313
alkaline than Ivimey-Cook (1955) had recorded for the same site (Etherington, 1977). A further systematic sampling was undertaken to establish whether blown limestone dust was the cause of this increased alkalinity. Crook Peak in Somerset (ST 395559) was sampled as a control site which is well removed from present-day quarrying dust sources. Soils The soils of the limestone heath association in these areas are sols bruns of the Major Soil Group, Brown Earths attributable to the Lulsgate Series (Findlay, 1965; Crampton, 1972). The Lulsgate soils are generally shallow, unstable and are characteristic of steep slopes overlying Carboniferous Limestone. They are of high cation saturation and sometimes contain free calcium carbonate. Regardless of provenance they contain some material which is not derived from the weathering of the parent limestone and which heavy mineral and particle-size analysis suggest to be wind-blown loess of periglacial origin (Findlay, 1965; Crampton, 1972). In Gower and Dyfed the soils sometimes contain a great deal of wind-blown coastal sand but the Ewenny and Somerset soils are less sand-enriched. The steep slope Lulsgate soils all abut on much deeper, more acid loessic plateau soils with a calcifuge flora which belong to the Nordrach Series of the Brown Earth Group. These more acid soils were also sampled in the present investigation as they form a characteristic part of the limestone heath complex. It will be suggested in a future publication that the steep slopes were formerly mantled in this same loessic material and that its recent erosion has established the present-day catena of soils on which the calcifuges are a relict vegetation surviving near their edaphic limits. Materials and methods The Ewenny site was systematically soil sampled at the intersection points of the 100 m National Grid (Fig. 1). Core samples of 7 cm diameter, 8 cm depth were taken and, where soil depth permitted it, another 8-12 cm sample. The cores were returned to the laboratory in polythene bags and sliced into 2-cm horizontal layers. All of the samples were air-dried before storage and sieved to 2 mm before analysis. Soil depth, slope and vegetation type were recorded at each sampling point. Soil pH was measured electrometrically in a 1:1 v/v slurry. During the first stages of the project calcium carbonate was gasometrically assayed using a Collin's Calcimeter (Wright, 1939) but this was laborious and insensitive to low concentrations. All of the analyses presented here were performed by extracting soil samples overnight with excess M HC1, filtering and measuring calcium by atomic absorption (Etherington, 1967). This technique gives a good correlation with the Collins Calcimeter values and can be considered to measure the pedogenetically significant distribution of calcium and calcium carbonate (Fig. 2). The samples from Crook Peak, Somerset were taken and treated in exactly the same manner but were located randomly rather than grid-sampled.
314
JOHNR, ETHERINGTON RESULTS
Figure 3 shows variation o f p H and calcium with depth in 60 soil profiles at Ewenny, sorted into three categories on the basis of soil pH in the deepest layer ( > 8 cm). The arbitrary pH boundaries have been set at pH < 5.5 for the acid plateau soils and pH > 6.5 for slope soils which may contain free calcium carbonate. The third group is the overlap range o f p H 5.5-6-5 and contains mainly those profiles from the break of slope between plateau and Lulsgate soils.
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Relationship between Collins' Calcimeter calcium carbonate determinations and hydrochloric acid extractable calcium from a range of air-dry soil samples.
Figure 4 is a similar presentation of data from twelve profiles at Crook Peak. The plateau deposits here are not so deep or widespread as at Ewenny and consequently no soils fall into the pH < 5.5 category. Comparison of the two sets of data shows a marked difference: the soils at Ewenny increase in pH toward the surface, a trend which is particularly obvious in the less alkaline plateau soils. By contrast the Crook Peak soils have lower pH values in their surface layers. These two opposed distributions are reflected even more strongly in the calcium profiles, all of the Ewenny soils being strongly enriched at the surface but the Crook Peak soils are high in calcium only where they are in contact with limestone at the base of profiles or where the soils are very mobile. During the summer months of the past seven years almost all of the E. tetralix on the lower slopes of Old Castle Down to the east of the quarries has shown strong
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Fig. 3. Soil pH and acid-extractable calcium of Ewenny soils. Analyses are presented in three groups separated on the basis of soil pH in the > 8 cm layer. The upper group is the acid plateau soil with pH < 5.5, the lower group is the Lulsgate soil with pH > 6.5 and the middle group is intermediate. Each histogram block represents the mean value for a 2 c m depth increment.
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EUTROPHICATION OF LIMESTONE HEATH SOILS
317
chlorosis which is manifested as whitish-yellow coloration of all young growth. This contrasts sharply with its normal deep green coloration when it grows on the acid, uncontaminated plateau soils above. Occasionally the chlorosis develops in plants on the plateau soils adjacent to quarry A (Fig. 1). E. cinerea growing on the Lulsgate soils of the steep slopes close to the two quarries develops yellow chlorosis of shoot tips coupled with red or purple discoloration of older leaves which persists during the whole of the year. The E. tetralix chlorosis is much less marked during the winter months. These abnormal colours closely resemble lime-induced chlorosis (Brown, 1961 ; Grime & Hutchinson, 1967) and, though very common at the Ewenny site, have rarely been observed elsewhere with the exception of some Gower cliff populations which are exposed to blown sea-spray. C. vulgaris does not appear to be so susceptible to chlorosis in these conditions. Many of the Ewenny limestone heath ericaceous plants are quite old as shown by their thick buried stem bases, suggesting that the vegetational status of the site has been unchanged for some time. During the past ten years the abundance of the ericaceous plants has decreased on the Lulsgate soils of the Naturalists' Trust reserve to the extent that they are now absent from some areas. This may probably be attributed to the blown-dust effects as grazing and burning management have not significantly altered and the sward is very shallow and open. The rabbit population has recently grown but as the heath survived heavy rabbit grazing prior to the advent of myxomatosis it is unlikely that the rabbits would have caused such a large change in the vegetation cover, as they preferentially graze the much more abundant dominant grasses.
DISCUSSION
Most soils which have developed under high precipitation:evaporation regimes show leaching of soluble materials from the surface layers. Etherington (1967) sampled old calcareous sand dunes at Kenfig Burrows about 12 km north-west of Ewenny and found that marked leaching of calcium carbonate had occurred in an annual precipitation regime of c. 110cm balanced against evapotranspiration of c. 64 cm. The soils at Crook Peak show the expected surface leaching in a similar evaporation climate and with slightly less precipitation ( c . 9 0 c m y - l : by interpolation in data of Findlay, 1965). The surface enrichment with calcium at Ewenny is unexpected and the most obvious explanation is that it must derive from atmospheric input. The two adjacent quarries (Fig. 1) produce considerable amounts of dust during drilling, blasting and crushing operations and, in dry weather, this may be seen as surface coatings on plants, sometimes in sufficient quantity to be identified as a carbonate by reaction with dilute hydrochloric acid. The measured enrichment of
318
JOHNR. ETHERINGTON
the surface layer (0-2 cm) of most Ewenny soils is between 2.5 and 5 mg g- 1 CaCO 3 which, with a soil bulk density of 1.5, represents 0-8-1.6 tha-1 CaCO3, a figure which is comparable with the possible annual incomes discussed in the introduction. The observed enrichment of the surface soils at this site is thus consistent with dust pollution from the adjacent limestone quarries. The degree of enrichment with calcium and the position of plateau soils provides further evidence that wind-blown dust is the source. The prevailing wind in coastal Glamorganshire is south-west: few of the plateau soils to the west of the quarries show a high ratio of surface to deep calcium content whereas many of the soils to the eastern, downwind side have very high ratios (Table 1). TABLE 1 ACID-SOLUBLE CALCIUM OF EWENNY PLATEAUSOILSTOWlNDWARD(WEST) AND LEEWARD (EAST) OF QUARRIES. CALCIUM IN THE 0 - 2 c m LAYER IS SHOWN BY RATIO TO THE > 8 c m LAYER --I-STANDARD ERROR Sample position Number of samples 0-2 cm Ca - >8 cm
West 15
East 6
6.7 + 1.1
10.9 + 2.1
Comparison of the Ewenny and Crook Peak pH profiles suggests that the normal surface pH of the Lulsgate soils would be in the region of pH 6.0-6.6 and that this has been raised to about pH 7.2 by quarrying dust which has approximately doubled the calcium content at Ewenny. The combined evidence of the extensive chlorosis and the observed decline in the ericaceous population suggests that limestone dust is having a harmful effect. This is one of the finest remaining limestone heaths in western Britain and a part of the site is already reduced in research value by the soil damage which has occurred. The calcicole--calcifuge mixtures which are found on the shallowest soils are probably of relict status associated with the past few centuries of overgrazing and soil erosion. They are most unlikely to be recreatable and are consequently of considerable conservation importance. It is not known whether the affected part of the site would recover if dust emission ceased, as a decade might elapse before the profile is leached to its former calcium content. It is essential that quarrying dust is prevented from damaging the remaining areas of the Ewenny limestone heath.
ACKNOWLEDGEMENTS
I am grateful to Mr J. S. Stockdale who undertook most of the sample collection and analysis and to Mrs P. C. Wyville whose preliminary analytical work first detected the problem. The study was supported by a Natural Environment Research Council Research Grant. Permission to sample in the Naturalists' Trust Reserve was granted by the Surveyor to the Duchy of Lancashire.
EUTROPHICATION OF LIMESTONE HEATH SOILS
319
REFERENCES BRANDT, C. J. & RHOADS, R. W. (1973). Effects of limestone dust on lateral growth of forest trees. Environ. Pollut., 4,.207-13. BROWN, J. C. (1961). Iron chlorosis in plants. Adv. Agron., 13, 329-69. CRAMPTON, C. B. (1972). Soils of the Vale of Glamorgan. Mere. Soil Surv. Gt. Br. Harpenden, Agricultural Research Council. ETHERINGTON,J. R. (1967). Studies of nutrient cycling and productivity in oligotrophic ecosystems I. Soil potassium and wind-blown sea-spray in a South Wales dune grassland. J. Ecol., 55, 743-52. ETHEPaNGTON, J. R. (1975). Environment and plant ecology. London, Wiley. ETHERINGTON,J, R. (1977). The effects of limestone quarrying dust on a limestone heath in South Wales. Nature in Wales, 15, 218-23. FINDLAY,D. C. (1965). The soils of the Mendip district of Somerset, Mere. Soil Surv. Gt. Br., Harpenden, Agricultural Research Council. GILBERT, O. L. (1976). An alkaline dust effect on epiphytic lichens. Lichenologist, 8, 173-8. GRIME, J. P. (1970). People andplants in Derbyshire. Matlock, Derbyshire Naturalists' Trust. GRIME, J. P. & HUTCHINSON,T. C. (1967). The incidence of lime chlorosis in the natural vegetation of England. J. EcoL, 55, 557~6. GRUBB, P. J., GREEN, H. E. & MERRIFIELD,R. C. J. (1969). The ecology of chalk heath, its relevance to the calcicole-calcifuge and acidification problem. J. Ecol., 57, 175-210. GRUBB, P. J. & SUTER,M. B. (1971). The mechanism of the acidification°ofsoil by Calluna and Ulex and the significance for conservation. In The scientific management of animal and plant eommunities Jbr conservatfon, ed. by E. Duffey and A. S. Watt, 115-33. Oxford, Blackwell. IVlMEY-CooK, R. B. (1955). The ecology of a limestone heath at Ewenny, Glamorganshire. Ph.D. thesis, University of Wales, Cardiff. !ASVlS, S. C. (1974). Soil factors affecting the distribution of plant communities on the cliffs of Craig Breidden, Montgomeryshire. J. Ecol., 62, 721-33. MANNING, W. J. (1971). Effects of limestone dust on leaf condition foliar disease incidence and leaf surface microflora of mature plants. Environ. Pollut., 2, 69-76. Moss, C. E. (1907). Geographical~distribution o f vegetation in Somerset; Bath and Bridgewater district. London, Royal Geographical Society. Moss, C. E. (1913). Vegetation of the Peak District. Cambridge, Cambridge University Press. RUSSEL, E. W. (1973). Soil conditions andplant growth, 10th edn. London, Longman. SHIMWELL, O. W. (1971). Festuco-Brometea Br.-B1. & R. Tx. 1943 in the British Isles: The phytogeography and phytoso~iology of limestone grasslands Part I. (a) General introduction; (b) Xerobromion in England. Part II. Eu-Mesobromion in the British Isles. Vegetatio, 23, 1-28 and 29-60. TANSLEY,A. G. ( 1939). The British Islands and their vegetation. Cambridge, Cambridge University Press. TANSLEY,A. G. & RANKIN,W. M. (1911). The plant formations of calcareous soils. B. The sub-formation of the chalk. In Types of British vegetation, ed. by A. G. Tansley, 161-86. Cambridge, Cambridge University Press. WRIGHT,C. (1934). Soil analysis: a handbook ofphysical and chemical methods. London, Murby.
JOHN R. ETHERINGTON
Botany Department, University College, Cardiff CFI 1XL, Wales
ABSTRACT
Soil analysis has established that limestone quarrying dust has entered an adjacent limestone heath of high conservation value. The soil profiles of calcium distribution, established by many years of leaching, have been seriously altered by surface enrichment. It is suggested that the calcicole-calcifuge vegetation of the shallowest soils may be threatened by these changes and it is considered very desirable that further pollution of the site should if possible be prevented.
INTRODUCTION
The ecological effects of wind-blown limestone (CaCOa) and other alkaline dusts have been investigated by few workers. Grime (1970) recorded increases in pH of acid soils, close to dust sources , of up to pH 1.5 units. To produce this effect in the plough layer of a medium textured soil the agricultural lime dressing would be about 12 t ha-1 (Etherington, 1975) compared with an estimated leaching loss from soils with average British rainfall Of c. 0"4tha-I y - ~ CaCO 3 (Russel, 1973). Manning (1971) records an annual dust income adjacent to a quarry and cement plant which is more than ten times greater than this estimate. A shallow layer of soil might rapidly be eutrophicated and in these circumstances the whole profile could become alkaline within a few decades. Unless the limestone abuts on acid rocks, for example the Carboniferous Limestone/Millstone Grit boundary in western and northern Britain, limestone quarrying dust will fall on normally calcareous soils and it might be predicted that no great change of soil chemistry would occur though the plants themselves may be damaged by dust deposits (Brandt & Rhoads, 1973). The ecological consequences of alkaline dust input to acid soils are of only local conservation importance as Britain 309 Biol. Conserv. (13) (1978)---© Applied Science Publishers Ltd, England, 1978 Printed in Great Britain
310
JOHNR. ETHERINGTON
has very large areas of upland heath, hill-grassland and other acidophilous speciespoor vegetation, most of which are far from such dust sources. Gilbert (1976) has noted that bark-dwelling lichens are an exception to this generalisation, being replaced by saxieolous species or an eutrophic Xanthorion alliance if exposed to even small amounts of limestone dust. A further exception is likely to be found in limestone heath, a rather rare vegetation type comprising an intimate mixture of ealcicole and caleifuge species on shallow limestone soils. These heaths and the similar chalk heaths of SE England have attracted ecologists' interest for many years (Moss, 1907, 1913; Tansley & Rankin, 1911; Grubb et al., 1969). Calluna vulgaris, which is generally recognised as a calcifuge in southern Britain, occurs commonly in these heaths, associated with plants of limestone grassland, many of which are ealcicolous and a few, obligate ealcieoles. A wide range of other calcifuges including Erica cinerea and Galium saxatile may be encountered but with lesser constancy than C. vulgaris. Extreme calcifuges such as Descharnpsiaflexuosa are very rare or absent. Much speculation and experiment has been devoted to the calcicole--calcifuge problem and the limestone heath vegetation may hold the key to its as yet elusive solution. For this very reason it is of high conservation value but may be unduly sensitive to limestone dust input as the vegetation probably depends on a fairly critical balance of soil chemical conditions related to calcium carbonate content, cation saturation and pH. Shimwell (1971) attributes most limestone heaths to transitions between the alliance Mesobromion and the Agrosto-Ulicetum association of the class NardoCallunetea and he cites Ivimey-Cook's (1955) suggestion that the calcifuges occupy a surface-rooting zone in the limestone heath soil which has been acidified by surface leaching. This explanation of the unusual mixture of plants has been widely accepted in the past (Moss, 1907, 1913) and more recent work on chalk heath has certainly established that surface leaching can occur and is promoted by the calcifuges (Grubb et al., 1969; Grubb & Suter, 1971). Jarvis (~974) found some evidence for root stratification in a calcicole--calcifuge vegetation on a basic dolerite soil in Wales. Grubb et al. (1969), however, questioned the surface-rooting concept as it does not fit the facts of root behaviour in chalk heath and also gives no explanation of seedling regeneration. They suggested that, below pH 5-0, calcicoles would fail to establish and noted that C. vulgaris and E. cinerea flourish in soils of pH 5 to 6 but might need a lower pH for seed germination in the surface few millimetres of soil. Grubb & Suter (1971) suggested that under heavy grazing the calcicole-calcifuge mixture is stable at pH 5 to 6 but if grazing is withdrawn the growth of calcifuges increases, causes soil acidification and outcompetes the caleifuges. Current work in these laboratories suggests that strong surface acidification of Carboniferous Limestone-heath soils is unlikely to be the full explanation for the existence of the vegetation. C. vulgarisand E. cinerea may be found on very steep
E U T R O P H I C A T I O N OF L I M E S T O N E H E A T H SOILS
311
limestone slopes in South Wales and Somerset where extreme soil instability has been promoted by heavy sheep and/or rabbit grazing. Such soils often contain free calcium carbonate though the percentages are much lower than those associated with soft-limestone and chalk soils. Data to be published elsewhere show that they are rather prone to leaching, however, the surface few millimetres fluctuating in calcium content and pH according to rainfall and evaporation. It therefore seems that although it is near neutral or alkaline, much of the soil will be unusually sensitive to input of calcium carbonate dust. The work described here was undertaken when it was discovered that the soil chemistry of a heavily quarried limestone heath site in the Vale of Glamorgan differed significantly from that of other sites in South Wales and Somerset.
SITES, MATERIALS A N D M E T H O D S
The great majority of the limestone heaths of south-western Britain are formed on the Carboniferous Limestone to the south of the Welsh coalfield and in Somerset. There are a few scattered areas on Lower Lias limestone in South and MidGlamorgan. Circumstantial evidence suggests that the vegetation type may once have been more widespread but that agriculture has destroyed all but a few remnants and limestone heath can now be considered a rare and fragmentary vegetation.
Sites During a current investigation of the iron nutrition of calcifuges in limestone heath, soil samples have been collected from sites on the South Dyfed coast (SR995942-882957), the Gower Peninsula (SS 510850--415872), the Vale of Glamorgan (SS 896760) and from the Somerset limestone (ST 395559). The site in the Vale of Glamorgan is near the village of Ewenny and occupies rough-grazing land known as Old Castle Down, Ogmore Down and Ewenny Down. Part of the site is a Glamorgan Naturalists' Trust reserve and a Nature Conservancy Council SSS1 (Fig. 1). It is particularly interesting and valuable since, in addition to the ubiquitous limestone heath calcifuges such as C. vulgaris and E. cinerea, it has damp, but not waterlogged, areas containing Erica tetralix, Pedicularis sylvatica and Molinia caerulea, none of which occur frequently in all of the other limestone heaths. This calcifugous flora is intimately associated with plants such as Helianthemum nummularium and Asperula cynanchica, both of which behave as calcicoles in Wales and southern England. This site is the most diverse and largest remaining limestone heath in western Britain but has for a number of years been threatened by the extension of two adjacent quarries: Ivimey-Cook (1955), who described the area in detail, commented that future prospects were not bright. During the preliminary analyses of soils from these sites it was noticed that the Ewenny samples were more alkaline than those from other heaths and also more
Ogmore ~
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~""
"
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77
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Afon Alun
7°
Prevailing S.W. Wind Direction
N
Ewenny Park
EWENNY
Fig. 1. Location of the limestone heath at Ewenny, South Glamorgan. The boundary of the Glamorgan Naturalists" Trust reserve is shown by the heavy line. The dotted lines represent the sharp break of slope between the plateau and the slopes bearing Lulsgate soils. The 100 m squares of the National Grid are shown in the sampled area. The two quarries marked A and B are the dust sources.
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EUTROPHICATION OF LIMESTONE HEATH SOILS
313
alkaline than Ivimey-Cook (1955) had recorded for the same site (Etherington, 1977). A further systematic sampling was undertaken to establish whether blown limestone dust was the cause of this increased alkalinity. Crook Peak in Somerset (ST 395559) was sampled as a control site which is well removed from present-day quarrying dust sources. Soils The soils of the limestone heath association in these areas are sols bruns of the Major Soil Group, Brown Earths attributable to the Lulsgate Series (Findlay, 1965; Crampton, 1972). The Lulsgate soils are generally shallow, unstable and are characteristic of steep slopes overlying Carboniferous Limestone. They are of high cation saturation and sometimes contain free calcium carbonate. Regardless of provenance they contain some material which is not derived from the weathering of the parent limestone and which heavy mineral and particle-size analysis suggest to be wind-blown loess of periglacial origin (Findlay, 1965; Crampton, 1972). In Gower and Dyfed the soils sometimes contain a great deal of wind-blown coastal sand but the Ewenny and Somerset soils are less sand-enriched. The steep slope Lulsgate soils all abut on much deeper, more acid loessic plateau soils with a calcifuge flora which belong to the Nordrach Series of the Brown Earth Group. These more acid soils were also sampled in the present investigation as they form a characteristic part of the limestone heath complex. It will be suggested in a future publication that the steep slopes were formerly mantled in this same loessic material and that its recent erosion has established the present-day catena of soils on which the calcifuges are a relict vegetation surviving near their edaphic limits. Materials and methods The Ewenny site was systematically soil sampled at the intersection points of the 100 m National Grid (Fig. 1). Core samples of 7 cm diameter, 8 cm depth were taken and, where soil depth permitted it, another 8-12 cm sample. The cores were returned to the laboratory in polythene bags and sliced into 2-cm horizontal layers. All of the samples were air-dried before storage and sieved to 2 mm before analysis. Soil depth, slope and vegetation type were recorded at each sampling point. Soil pH was measured electrometrically in a 1:1 v/v slurry. During the first stages of the project calcium carbonate was gasometrically assayed using a Collin's Calcimeter (Wright, 1939) but this was laborious and insensitive to low concentrations. All of the analyses presented here were performed by extracting soil samples overnight with excess M HC1, filtering and measuring calcium by atomic absorption (Etherington, 1967). This technique gives a good correlation with the Collins Calcimeter values and can be considered to measure the pedogenetically significant distribution of calcium and calcium carbonate (Fig. 2). The samples from Crook Peak, Somerset were taken and treated in exactly the same manner but were located randomly rather than grid-sampled.
314
JOHNR, ETHERINGTON RESULTS
Figure 3 shows variation o f p H and calcium with depth in 60 soil profiles at Ewenny, sorted into three categories on the basis of soil pH in the deepest layer ( > 8 cm). The arbitrary pH boundaries have been set at pH < 5.5 for the acid plateau soils and pH > 6.5 for slope soils which may contain free calcium carbonate. The third group is the overlap range o f p H 5.5-6-5 and contains mainly those profiles from the break of slope between plateau and Lulsgate soils.
To~
"17
I
I
I
3
4
5
Calcimeter CaCQ3 Log jjg g-I Fig. 2,
Relationship between Collins' Calcimeter calcium carbonate determinations and hydrochloric acid extractable calcium from a range of air-dry soil samples.
Figure 4 is a similar presentation of data from twelve profiles at Crook Peak. The plateau deposits here are not so deep or widespread as at Ewenny and consequently no soils fall into the pH < 5.5 category. Comparison of the two sets of data shows a marked difference: the soils at Ewenny increase in pH toward the surface, a trend which is particularly obvious in the less alkaline plateau soils. By contrast the Crook Peak soils have lower pH values in their surface layers. These two opposed distributions are reflected even more strongly in the calcium profiles, all of the Ewenny soils being strongly enriched at the surface but the Crook Peak soils are high in calcium only where they are in contact with limestone at the base of profiles or where the soils are very mobile. During the summer months of the past seven years almost all of the E. tetralix on the lower slopes of Old Castle Down to the east of the quarries has shown strong
8
6
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8
6
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8
6
4
2
,
÷
pH +- Std Error
t
,÷ ~p + ,+
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i
6
½
I
7
+
+
,÷
++
"-
pH> 6"5
pH
pH<5.5
,
I
10 x 103
t
i
Fig. 3. Soil pH and acid-extractable calcium of Ewenny soils. Analyses are presented in three groups separated on the basis of soil pH in the > 8 cm layer. The upper group is the acid plateau soil with pH < 5.5, the lower group is the Lulsgate soil with pH > 6.5 and the middle group is intermediate. Each histogram block represents the mean value for a 2 c m depth increment.
{Cm.}
SOIL DEPTH 0
o
rll
=
z
o
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Fig. 4.
(Cm)
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pH >6"5
6
5 pH -+Std. Error
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÷
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!
Soil pH and acid extractable calcium of Crook Peak soils presented as in Fig. 3. No soils of pH < 5 . 5 were found.
SOIL DEPTH
O Z
re Z
EUTROPHICATION OF LIMESTONE HEATH SOILS
317
chlorosis which is manifested as whitish-yellow coloration of all young growth. This contrasts sharply with its normal deep green coloration when it grows on the acid, uncontaminated plateau soils above. Occasionally the chlorosis develops in plants on the plateau soils adjacent to quarry A (Fig. 1). E. cinerea growing on the Lulsgate soils of the steep slopes close to the two quarries develops yellow chlorosis of shoot tips coupled with red or purple discoloration of older leaves which persists during the whole of the year. The E. tetralix chlorosis is much less marked during the winter months. These abnormal colours closely resemble lime-induced chlorosis (Brown, 1961 ; Grime & Hutchinson, 1967) and, though very common at the Ewenny site, have rarely been observed elsewhere with the exception of some Gower cliff populations which are exposed to blown sea-spray. C. vulgaris does not appear to be so susceptible to chlorosis in these conditions. Many of the Ewenny limestone heath ericaceous plants are quite old as shown by their thick buried stem bases, suggesting that the vegetational status of the site has been unchanged for some time. During the past ten years the abundance of the ericaceous plants has decreased on the Lulsgate soils of the Naturalists' Trust reserve to the extent that they are now absent from some areas. This may probably be attributed to the blown-dust effects as grazing and burning management have not significantly altered and the sward is very shallow and open. The rabbit population has recently grown but as the heath survived heavy rabbit grazing prior to the advent of myxomatosis it is unlikely that the rabbits would have caused such a large change in the vegetation cover, as they preferentially graze the much more abundant dominant grasses.
DISCUSSION
Most soils which have developed under high precipitation:evaporation regimes show leaching of soluble materials from the surface layers. Etherington (1967) sampled old calcareous sand dunes at Kenfig Burrows about 12 km north-west of Ewenny and found that marked leaching of calcium carbonate had occurred in an annual precipitation regime of c. 110cm balanced against evapotranspiration of c. 64 cm. The soils at Crook Peak show the expected surface leaching in a similar evaporation climate and with slightly less precipitation ( c . 9 0 c m y - l : by interpolation in data of Findlay, 1965). The surface enrichment with calcium at Ewenny is unexpected and the most obvious explanation is that it must derive from atmospheric input. The two adjacent quarries (Fig. 1) produce considerable amounts of dust during drilling, blasting and crushing operations and, in dry weather, this may be seen as surface coatings on plants, sometimes in sufficient quantity to be identified as a carbonate by reaction with dilute hydrochloric acid. The measured enrichment of
318
JOHNR. ETHERINGTON
the surface layer (0-2 cm) of most Ewenny soils is between 2.5 and 5 mg g- 1 CaCO 3 which, with a soil bulk density of 1.5, represents 0-8-1.6 tha-1 CaCO3, a figure which is comparable with the possible annual incomes discussed in the introduction. The observed enrichment of the surface soils at this site is thus consistent with dust pollution from the adjacent limestone quarries. The degree of enrichment with calcium and the position of plateau soils provides further evidence that wind-blown dust is the source. The prevailing wind in coastal Glamorganshire is south-west: few of the plateau soils to the west of the quarries show a high ratio of surface to deep calcium content whereas many of the soils to the eastern, downwind side have very high ratios (Table 1). TABLE 1 ACID-SOLUBLE CALCIUM OF EWENNY PLATEAUSOILSTOWlNDWARD(WEST) AND LEEWARD (EAST) OF QUARRIES. CALCIUM IN THE 0 - 2 c m LAYER IS SHOWN BY RATIO TO THE > 8 c m LAYER --I-STANDARD ERROR Sample position Number of samples 0-2 cm Ca - >8 cm
West 15
East 6
6.7 + 1.1
10.9 + 2.1
Comparison of the Ewenny and Crook Peak pH profiles suggests that the normal surface pH of the Lulsgate soils would be in the region of pH 6.0-6.6 and that this has been raised to about pH 7.2 by quarrying dust which has approximately doubled the calcium content at Ewenny. The combined evidence of the extensive chlorosis and the observed decline in the ericaceous population suggests that limestone dust is having a harmful effect. This is one of the finest remaining limestone heaths in western Britain and a part of the site is already reduced in research value by the soil damage which has occurred. The calcicole--calcifuge mixtures which are found on the shallowest soils are probably of relict status associated with the past few centuries of overgrazing and soil erosion. They are most unlikely to be recreatable and are consequently of considerable conservation importance. It is not known whether the affected part of the site would recover if dust emission ceased, as a decade might elapse before the profile is leached to its former calcium content. It is essential that quarrying dust is prevented from damaging the remaining areas of the Ewenny limestone heath.
ACKNOWLEDGEMENTS
I am grateful to Mr J. S. Stockdale who undertook most of the sample collection and analysis and to Mrs P. C. Wyville whose preliminary analytical work first detected the problem. The study was supported by a Natural Environment Research Council Research Grant. Permission to sample in the Naturalists' Trust Reserve was granted by the Surveyor to the Duchy of Lancashire.
EUTROPHICATION OF LIMESTONE HEATH SOILS
319
REFERENCES BRANDT, C. J. & RHOADS, R. W. (1973). Effects of limestone dust on lateral growth of forest trees. Environ. Pollut., 4,.207-13. BROWN, J. C. (1961). Iron chlorosis in plants. Adv. Agron., 13, 329-69. CRAMPTON, C. B. (1972). Soils of the Vale of Glamorgan. Mere. Soil Surv. Gt. Br. Harpenden, Agricultural Research Council. ETHERINGTON,J. R. (1967). Studies of nutrient cycling and productivity in oligotrophic ecosystems I. Soil potassium and wind-blown sea-spray in a South Wales dune grassland. J. Ecol., 55, 743-52. ETHEPaNGTON, J. R. (1975). Environment and plant ecology. London, Wiley. ETHERINGTON,J, R. (1977). The effects of limestone quarrying dust on a limestone heath in South Wales. Nature in Wales, 15, 218-23. FINDLAY,D. C. (1965). The soils of the Mendip district of Somerset, Mere. Soil Surv. Gt. Br., Harpenden, Agricultural Research Council. GILBERT, O. L. (1976). An alkaline dust effect on epiphytic lichens. Lichenologist, 8, 173-8. GRIME, J. P. (1970). People andplants in Derbyshire. Matlock, Derbyshire Naturalists' Trust. GRIME, J. P. & HUTCHINSON,T. C. (1967). The incidence of lime chlorosis in the natural vegetation of England. J. EcoL, 55, 557~6. GRUBB, P. J., GREEN, H. E. & MERRIFIELD,R. C. J. (1969). The ecology of chalk heath, its relevance to the calcicole-calcifuge and acidification problem. J. Ecol., 57, 175-210. GRUBB, P. J. & SUTER,M. B. (1971). The mechanism of the acidification°ofsoil by Calluna and Ulex and the significance for conservation. In The scientific management of animal and plant eommunities Jbr conservatfon, ed. by E. Duffey and A. S. Watt, 115-33. Oxford, Blackwell. IVlMEY-CooK, R. B. (1955). The ecology of a limestone heath at Ewenny, Glamorganshire. Ph.D. thesis, University of Wales, Cardiff. !ASVlS, S. C. (1974). Soil factors affecting the distribution of plant communities on the cliffs of Craig Breidden, Montgomeryshire. J. Ecol., 62, 721-33. MANNING, W. J. (1971). Effects of limestone dust on leaf condition foliar disease incidence and leaf surface microflora of mature plants. Environ. Pollut., 2, 69-76. Moss, C. E. (1907). Geographical~distribution o f vegetation in Somerset; Bath and Bridgewater district. London, Royal Geographical Society. Moss, C. E. (1913). Vegetation of the Peak District. Cambridge, Cambridge University Press. RUSSEL, E. W. (1973). Soil conditions andplant growth, 10th edn. London, Longman. SHIMWELL, O. W. (1971). Festuco-Brometea Br.-B1. & R. Tx. 1943 in the British Isles: The phytogeography and phytoso~iology of limestone grasslands Part I. (a) General introduction; (b) Xerobromion in England. Part II. Eu-Mesobromion in the British Isles. Vegetatio, 23, 1-28 and 29-60. TANSLEY,A. G. ( 1939). The British Islands and their vegetation. Cambridge, Cambridge University Press. TANSLEY,A. G. & RANKIN,W. M. (1911). The plant formations of calcareous soils. B. The sub-formation of the chalk. In Types of British vegetation, ed. by A. G. Tansley, 161-86. Cambridge, Cambridge University Press. WRIGHT,C. (1934). Soil analysis: a handbook ofphysical and chemical methods. London, Murby.