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Scrub encroachment and its effects on soil fertility on Newborough Warren, Anglesey, Wales
Biological Conservation 29 (1984) 99-119
Scrub Encroachment and Its Effects on Soil Fertility on Newborough Warren, Anglesey, Wales S. E. Hodgkin Department of Botany and Microbiology,UniversityCollege London, Gower Street, London, WC1E 6BT, Great Britain
A BS TRA C T Scrub species have colonised the dune system of Newborough Warren, apparently since myxomatosis wiped out its rabbit population in 1954. The colonisation by hawthorn, the most abundant scrub species, and its effects on soil fertility were studied. Measurements made of the scrub along four transects showed that 95 % of the hawthorn has colonised since myxomatosis, and there is a decline in the average age and density of bushes towards the sea that is not accounted for by unfavourable growing conditions but rather by a slow rate of colonisation. There is evidence that enrichment of the topsoil in nitrogen and phosphorus is occurring beneath hawthorn bushes, and indications are that certain nutrient-demanding weedy species, not typical dune species, would replace dead bushes. Sparse but widespread individuals of Pinus species have colonised from the adjacent forest. Without appropriate management, hawthorn and pine could become abundant.
INTRODUCTION Myxomatosis first appeared in Great Britain in 1953. Diseased rabbits were noticed at Newborough Warren in August 1954 and soon all but a very few rabbits were wiped out (Ranwell, 1959, 1960b). Previously the Warren was very densely populated with rabbits and the absence of rabbit grazing resulted in a decrease in the number of plant species in fixed dune areas, a great increase in the growth and flowering of most grasses and sedges, and a reduction in low growing dicotyledonous herbs (Ranwell, 99 Biol. Conserv. 0006-3207/84/$03-00 © ElsevierAppliedSciencePublishersLtd, England, 1984. Printed in Great Britain
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1959, 1960b). Similar effects were noticed in many other semi-natural habitats previously overrun by rabbits, for example Blakeney Point, Norfolk (White, 1961). Another consequence of the sudden release of grazing pressure was that scrub species were able to grow up in many sites. D. S. Ranwell (pers. comm.) recalled that there were only 'scattered and very sparse', heavily rabbit-chewed, hawthorn stumps present on the Warren before 1954. Scrub is now thinly spread over most of Newborough Warren and in places is abundant. Rabbits are still scarce. Scrub species which had colonised chalk grassland at Devil's Dyke, Cambridgeshire, were shown to have caused a massive enrichment of the topsoil in inorganic phosphate and to a far lesser extent in inorganic nitrate such that, when cleared, some 'weedy' species, known to need a rich supply of N and P, rather than the typical chalk grassland flora, grew up in their place (Grubb & Key, 1975). There is concern about scrub invasion because, first, each bush represents the displacement of an equal area of the more interesting dune vegetation; second, if the scrub is enriching the topsoil this displacement may not be a temporary, reversible one. The aim of this study was therefore to examine the colonisation, growth and effects on soil fertility of scrub at Newborough Warren following myxomatosis. The form of the dune system and vegetation before 1954 was described by Ranwell (1958, 1959, 1960a); some of the changes subsequent to myxomatosis have been noted by Ranwell (1960b), Hope Jones (1965) and Morton (1974). Plant nomenclature follows Tutin et ai. (1964-1980).
METHODS Fieldwork was carried out between 15 June and 29 July 1981. The site
Newborough Warren lies on the southwest coast of Anglesey (National Grid reference SH 425635) and consists of 2000 ha of dunes, marsh, flats, shingle and cliff. Six hundred and thirty three ha of the site were declared a National Nature Reserve by the then Nature Conservancy in 1955. In their annual report for that year the site was described as one of the largest and biologically richest expanses of dunes in western Britain. In 1948 the
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Forestry Commission began planting pines on about 800 ha o f the dunes. Nonetheless, Newborough Warren is the sixth largest dune system in Britain (Ratcliffe, 1977). Transects across the Warren
A survey was made o f the scrub within a 10 m belt along four transects across the Warren, totalling 7420 m (Fig. 1). The transects were arranged n o n - r a n d o m l y to radiate out from the region o f most abundant, oldestlooking scrub. The species, distance along the transect, m a x i m u m height, ..-
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maximum width of canopy and stem circumference at ground level (later converted to stem diameter) were recorded. Notes were made of the habitats along the length of each transect. A short length of Transect 1 ran through a series of grazing plots but the number and ages of bushes in these plots will not have been significantly affected as the plots had been grazed only for the previous two and a half years. Stem samples were taken at ground level from every bush in T 1 and T3 and from the oldest-looking hawthorn Crataegus monogyna and birch Betula spp. in 30m sections along T4, using an increment borer manipulated with a toggle bar. In the case of very small individuals, sections were cut with secateurs. Towards the end of T3, where the scrub was sparse, bushes in a second, adjacent 10 m belt were included, to give a larger sample for aging. Measurements were made, and samples taken, from an additional 13 bushes in two areas of relatively dense dune Scrub near the Braint estuary (Fig. 1). Stem core samples were subsequently aged and their annual increments recorded using a travelling stage microscope and data punch unit. Soil samples for pH measurements were collected on the same day every 100m along transects 1, 3 and 4 and from the Braint estuary (6 random samples). Where wet slack occurred along a transect an additional sample was collected.
Scrub tagged in 1964 In 1964 a permanent plot of tagged scrub was set up by Hope Jones (1964). Numbered metal tags were attached to every hawthorn bush and the single birch tree in a 0.5 ha corner of the Warren (Fig. 1), and records were made of the height, maximum width and stem diameter of each bush. In 1970 the measurements were repeated by Richards (1970) and new bushes were tagged. The plot was resurveyed in 1981 by the author; coring was not permitted.
Partial correlation analysis Partial correlation analysis was used to identify correlations between the age, dimensions, shape and vigour of a bush, its position along a transect and soil pH. Vigour was defined as the width of the outer five annual rings of a bush at ground level; it was confirmed in this study as a valid measure
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of true vigour by the finding that it is better correlated with stem diameter, canopy width and height than is the age of the bush. The calculation of partial correlation coefficients removes the effect of a third variable (3) by holding it constant to allow the true correlation between two variables (1 and 2) to be sought (Snedecor, 1956). The coefficient is given by the equation: r 1 2 - - r 1 3 X r23 r12x3
-----x/(l
-- r23)(1 -- r223)
In all analyses involving age, only those hawthorn bushes were used from which a clear, complete core had been obtained. Multiple-stemmed bushes were not included in any analysis of stem diameter. Stem diameter was identified by partial correlation analysis, as the parameter of growth best correlated with age, and was used to help interpret data from T2, T4 and the tagged area. The relationships between age, dimensions, vigour of hawthorn and distance from the sea were examined using partial correlation analysis in order to distinguish parameters with a gradient across the Warren caused solely by a cline in age from those parameters with a gradient which might be due to increasing proximity to the sea.
Bioassay of soil fertility As in most soils, including dune soils, nitrogen and phosphorus are likely to be the two nutrients most limiting plant growth on the Warren and therefore controlling the composition of the vegetation. At the nearby Aberffraw dune system for example, Pemadasa & Lovell (1974) found that nitrogen and phosphorus most limited the growth and performance of dune annuals. In order to determine whether hawthorn, the commonest scrub species on Newborough Warren, is enriching the topsoil beneath it, Festuca rubra, a common dune species, was grown as a bioassay of soil fertility, in paired pots of soil taken from beneath and adjacent to seven large bushes, aged between 18 and 27 years. Such a comparison between the growth of experimental plants in paired samples probably gives a better idea of the availability to plants of limiting nutrients than does any given test involving a chemical extractant. Soil was collected from a depth of 0-5 cm (excluding litter), near the stem of each bush, and from a point 50 cm to the south of the edge of each
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bush. Measurements were made, and stem samples taken, as for the transects. Dominant species in the adjacent vegetation were recorded. Paired soil samples were taken to measure pH, soil moisture content and loss-on-ignition. It has been shown that the rooting density in a pot does not affect the availability to an individual plant of immobile ions but in the case of mobile ions less is available to each plant the more densely they are sown (Cornforth, 1968; Peace & Grubb, 1982). A single plant may use virtually all the nitrate that becomes available in the pot because the nitrate ion is very mobile, but it can absorb phosphate only from a thin layer around its roots because the orthophosphate ion is so immobile. When, say, six plants are grown in one pot then each plant will again be able to take up only that phosphate supplied from the thin layer around its roots but its nitrate supply will be substantially reduced because each plant can use only one-sixth of the soil volume for nitrate. Thus, Peace & Grubb (1982) showed that lmpatiensparviflora grown one per small pot responded only to additional phosphate in the short term, but plants grown 10 per small pot responded only to additional nitrogen. Each of the 14 soil samples was divided between two 9 cm pots, holding about 270cm 3. The pots were sown with Festuca rubra on 18 June 1981, and the seedlings were gradually thinned to one per pot to test for phosphate enrichment, and to six per pot to test for nitrate. One week after sowing the pots were randomly arranged outside under a broad mesh wire cage. The 28 pots consisted of: 2 densities x 2 sample positions per bush x 7 bushes. The plants were harvested after 61 days, oven-dried and the shoot and root dry weights recorded.
RESULTS
Scrub species Nineteen species of tree and shrub were recorded on the Warren (Table 1), of which 15 were found along the transects. Hawthorn is the most abundant scrub species on the Warren, with an estimated total of 7400 individuals. Prunus spinosa and Ulex europaeus grow in small, dense patches at the north end of the Warren. Birch occurs scattered across the Warren. Most samples of birch wood were impossible to age with certainty and,
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TABLE 1 List of the Tree and Shrub Species Recorded on Newborough Warren during this Survey
Acer pseudoplatanus Betula pendula Betula pubescens Calluna vulgaris Carpinus betulus Cotoneaster sp. Crataegus monogyna
Cytisus scoparius Fraxinus excelsior Hippopha~ rhamnoides
Malus sylvestris Pinus contorta Pinus spp. Prunus spinosa Salix atrocinerea Sambucus nigra Sorbus aucuparia Taxus baccata Ulex europaeus
except for hawthorn, no other species was recorded in large enough numbers for analysis to be possible. Most of the discussion is therefore confined to a consideration of hawthorn. Age-class distribution Figure 2 illustrates the age-class distribution of hawthorn along T 1 and T3. Only three out of 109 bushes pre-date myxomatosis and there has been a steady increase in the number of bushes since then (Fig. 3). There appears to have been a levelling offin recruitment in the last ten years but this is very likely to be due to the difficulty in finding small seedlings and saplings among taller vegetation. No correlation was apparent between climatic conditions (annual rainfall, number of days with more than 1 mm of rain, mean minimum and maximum July temperatures) and recruitment in any one year, but the samples might have been too small to reveal any correlation that did exist. The oldest birch dated back to 1955, the year after myxomatosis wiped out the Newborough Warren rabbit population. The other 20 stem cores that could be aged indicate a more or less even rate of recruitment since that year. Direction and rate of colonisation by hawthorn The oldest hawthorn bushes o c c u r in the northern part of the Warren and the average age declines towards the sea. In the cases of T1 and T3 the
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negative correlation between hawthorn age and distance from the north edge of the Warren is significant (p < 0.OOl). Correlation between the stem diameters of unaged bushes in T2 and their distance along the transect indicated (p < 0.05) that here too the average age of bushes also declined towards the sea. No such correlation existed in T4 where there is a second source of colonisation with several old bushes halfway along it, by the Braint estuary. The average annual rate of encroachment seawards, calculated from the inverse of the slope of the regression line for age versus distance along a transect, is 270m per annum along TI and 170m per annum along T3. Distribution of hawthorn across the Warren
Along all four transects there is a more or less steady decline in the density of hawthorn seawards (Fig. 4). The density and vigour of hawthorn in different habitats, and the relation between soil pH and vigour, were examined in order to determine whether this decline could be attributed to unfavourable environmental conditions or simply a slow rate of encroachment. Hawthorn was found in yellow dune, grey dune, dry slack and fixed dune grassland; none was seen on fore-dunes and only one individual was recorded along the transects in wet slack (and that a relatively dry one). Figure 4 illustrates that the decline in numbers towards the sea, before the fore-dunes are reached, is not entirely due to the increasing area of wet slack: there is a decline in the density in other habitats. In the figure, the width of the transect belts represents the density of hawthorn in suitable habitats (wet slack excluded) per 100m sections; the breaks in the line represent the length of wet slack habitat. The soil samples taken along T 1 and T3 showed a significant increase in pH (p < 0.001 andp < 0.01, respectively) towards the sea but there was no correlation between the vigour of hawthorn and the pH of the corresponding soil sample. Indeed, using partial correlation analysis to exclude the significant negative correlation between age and vigour (p < 0.001; positive correlations with stem diameter excluded), hawthorn along TI and T3 is shown to grow more vigorously closer to the sea. The correlation is significant in the case of T3 (p < 0.05). The absence of any trend in p'H along T4 is perhaps due to the fact that it runs entirely through an area of old, fixed dunes.
S. E. Hodgkin
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Changes in growth rate since myxomatosis In order to determine whether conditions have continued to be equally favourable for the growth of hawthorn over the years since myxomatosis, despite the vegetation becoming generally taller and thicker, the width of five annual rings over the same age was compared, as a measure ofvigour, between one old class and two younger classes of hawthorn. There was no difference in the radial increment over the 10th to 14th years of growth between one class aged 23-27 years (that is, growing up in the five years following myxomatosis) and a second aged 15-19 years (Table 2). Neither was there any difference in the radial increment over the 5th to 9th years of growth between two classes aged 23-27 years and 10-14 years, respectively.
Area by the Braint estuary Scrub was very abundant in the areas A 1 and A2 by the Braint estuary (Fig. 4). Several hawthorn bushes pre-dated myxomatosis; the oldest aged bush was 33 years old. The hawthorn here looked healthier and appeared to be growing more vigorously than bushes elsewhere on the Warren; the foliage was a darker green and all the bushes sampled were fruiting copiously. Indeed, the mean vigour of 13 bushes in A1 and A2 aged between 18 and 32 years was greater (p < 0.001) by two to three times than that of a sample from T1, T2 and T4 comprising all those bushes within the same age range.
Scrub tagged in 1964 A comparison between the frequency distributions in 1964, 1970 and 1981 of the stem diameters of the tagged bushes, used as the best available TABLE 2 Comparison of Vigour: Radial Increment over Five-Year Period (Mean + SE) between Two Age Classes of Crataegus over the Same Age
Vigour (mm) at 10-14 years
Vigour (mm) at 5-9 years
Age class
23-27 years
15-19 years
23-27 years
10-14 years
n
2.66 + 0'51 19
3-05 + 0.52 34
2"11 _ 0.64 19
1.69 + 0-26 25
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Frequency distribution of stem diameter of hawthorn in the 0-5 ha area of tagged bushes: 1964 (diagonal lines), 1970 (solid) and 1981 (vertical lines).
indicator of age, suggests that there is a more or less even-aged stand of hawthorn in this area (Fig. 5). The mean annual recruitment rate between 1964 and 1970 was 1.8 (total of I l bushes), and 0.5 between 1970 and 198 l (6 bushes). The single birch tree recorded in 1964 still survives; no further recruitment has occurred. Blackthorn and gorse have spread vegetatively and surrounded more bushes since 1970.
Bioassay of soil fertility The pH value, soil moisture content and loss-on-ignition of the samples are given in Table 3. There is a considerable variation in the pH of the samples of soil under grass (pH 5.15-7-65) but less in the case of soil under scrub (pH 5.65-7.15). The mean pH values did not differ significantly but the scrub soil was more alkaline than soil from the adjacent grassland in 6 out of 7 cases. This is likely to result from the deposition of sodium and magnesium ions on the crowns of the bushes from salt-laden winds (Malloch, 1972) later washed off into the soil below. Loss-on-ignition was generally greater in the samples of scrub soil than grass soil; this is probably related to a higher rate of litter fall from the canopy. Every sample of scrub soil had a higher moisture content than the corresponding sample of grass soil; the higher levels of organic matter would enable more water to be retained in the soil, possibly helped by a reduced evaporation rate in the shelter beneath the bush.
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TABLE 3 pH, Moisture Content and Loss-on-lgnition of Paired Soil Samples Collected Beneath Crataegus and from the Adjacent Grassland Soil
Trial
1 2 3 4 5 6 7 Mean +SE
pH
Moisture content % oven-dry wt
Loss-on-ignition % oven-dry wt
Scrub soil
Grass soil
Scrub soil
Grass soil
Scrub soil
Grass soil
6'15 6-05 5"95 7-15 6"95 5"70 5"65 6-23 +0.2
5"45 5"55 5"25 6-25 7"65 5"50 5'15 5.83 +0.3
13 12 13 7 !8 9 17 13 +1.5
11 8 6 4 3 4 15 7 +i.7
7 17 8 3 8 3 6 7 +1.8
4 6 4 4 1 5 6 4 +0.6
It was not possible to harvest the roots of the Festuca rubra bioassay plants completely; a lot of small particles of organic matter and humus was bound up in the mats of fine roots. For this reason only the dry weight yield of the shoots is considered. In the one-plant pots the scrub soil yielded, in total, 41% more shoot dry weight than the grass soil, indicating that the scrub soil is relatively enriched in phosphate, and perhaps nitrate. In the case of the six-plant pots, shoot dry weight was 36 % greater in scrub soil than in grass soil, indicating that there has been nitrate enrichment of the scrub soil. The total shoot yield in each six-plant pot was only slightly greater than the yield of the single individual in the corresponding one-plant pot; in no case does the yield approach six times greater, as would be expected if there was an ample supply of the major limiting mobile nutrient ion, nitrate. However, there was a considerable variation in yield between individual trials even in the 'control' grass soils (Table 4). Relative enrichment of the scrub soil is indicated in bushes 2, 5, 6 and 7. Trials 1 and 4 show the opposite result, with a higher than average yield from the grass soil. Trial 3 produces conflicting results, indicating the possibility of a slight scrub soil enrichment with six plants per pot but a relative deficiency with one plant per pot. Trials 1, 3 and 4 were distinguished from the others by the
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TABLE 4 Yield of Festuca rubra after 61 days' Growth in Soil collected Beneath or Beside Crataegus, Expressed as Oven-dry Weight of Shoots (nag) and Yield from Soil Under Scrub as per cent of Yield from Soil Under Grass
Trial
I 2 3 4 5 6 7 Mean +SE
One plant per pot
Six plants per pot (mean +_SE)
Scrub soil
Grass soil
~
401-8 378-7 208"8 231.9 557-3 254"0 698"0 390-! +68-1
410"9 219-6 292"2 461"5 283"0 150'8 125"0 277-6 +47.5
98 172 71 50 197 168 558
Scrub soil 93"5 + 13-3 68-8+5"5 70"7 + 12"5 6 8 '6 + 1 0 - 3 88"8 + 7'7 124.2 + 27-0 181"6 + 24"8 99-5 +8.3
Grass soil
%
115.1 + 16-4 50-5-t-9.1 64.7 + 12-0 98-7+ 11"9 64.7 + 4"9 59.9 + 5-2 58"8 + 4-5 73.2 +8-9
84 136 109 70 137 207 309
dominance of Ammophila arenaria in the vegetation surrounding the bushes by which the soil was collected.
DISCUSSION Hawthorn is the most abundant scrub species on the Warren; it is widespread, growing to within 330 m of the sea. The density and age of hawthorn bushes across Newborough Warren declines away from the northern, landward end, suggesting that the initial source of seed was in the bushes and hedgerows near and bordering the north side of the Warren. Another source of colonisation is apparent in the area of relatively dense scrub near the Braint estuary where several bushes date from before myxomatosis. The results from the transects confirm the opinion that heavy grazing pressure from rabbits was responsible for the lack of scrub on the Warren before 1954. Ninety five per cent of the hawthorn present in 1981, probably an underestimate because of the position of the transects (see Methods), and 20 out of 21 birch trees, became established after myxomatosis. The few individuals of other species which were aged postdated 1954. The rabbit population is still at a low level so that rabbit
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grazing is unlikely to interfere significantly with scrub recruitment. No more than 20 rabbits were seen during the course of the survey and the only active burrows observed were some near the sea by Transect 3. At the present time hawthorn is less frequent closer to the sea (even discounting unsuitable habitats). However, it appears to grow more vigorously nearer the sea and it is evident that the current distribution is due to a slow rate of encroachment limited at present by the number of mature bushes. There is no reason why hawthorn, at least, could not with time become much more abundant across the Warren. Comparisons of vigour at the same age in classes of old and young hawthorn show that conditions are not becoming less favourable for growth. Its growth is most vigorous in a small area of marshy grassland near the Braint estuary; it is likely that an input of nutrients from the estuary, perhaps by periodic flooding, is responsible. Elsewhere on the Warren its growth is equally less vigorous in all other habitats (apart from fore dunes and wet slack in which it does not occur) throughout the Warren. A progressive leaching of carbonates from sand occurs with time, causing a steady fall in pH from embryo dunes inland to older, stable dunes (Salisbury, 1925); this effect is not likely to be responsible for the seawards decline in hawthorn numbers because their vigour was not correlated with soil pH. In the most landward region of the Warren around Pen-Ion, there is some very tall dense grassland. In 1964, studying the plot of tagged scrub in that area, Hope Jones (1965)was of the opinion that grass species had grown so vigorously since myxomatosis that further scrub colonisation was precluded. Prunus spinosa and Ulex europaeus are still spreading vegetatively in the area near Pen-Ion but there has probably been little recent recruitment to the hawthorn population. Data from the tagged plot, which reveals very little recruitment since 1964, support this view. Over the Warren as a whole, recruitment to the hawthorn population is occurring steadily. There has been an apparent fall-off in the overall rate of recruitment in the last ten years. However, it is unlikely that there has been any significant increase in the density and height of the dune grassland over that period that might have caused a real fall-off. All the evidence points to a very rapid change in the vegetation of the Warren soon after myxomatosis (Ranwell, 1960b; Hope Jones, 1965; Morton, 1974). It is suggested that young seedlings and saplings shorter than the surrounding vegetation were under-recorded. In the aged sample no hawthorn younger than five years was found in the open grassland; four
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two-year,old individuals and one five-year-old were found beneath the canopy of larger bushes which were being cored; a further three hawthorn aged 15-17 years, as well as three, unaged but judged to be at least ten years old, were found growing suppressed beneath the canopy of much larger bushes. Establishment in such conditions must be at least as difficult as in the dune grassland. Only nine individuals from the aged sample, aged 5-10 years, were found in the grassland and it was felt many others could have been missed. It has been shown that, in the continued absence of rabbits and larger herbivores, nothing is hindering the growth of hawthorn on the Warren, and regeneration will probably continue in all but the area immediately around the reserve boundary near Pen-Ion. Further seawards, the relative sparsity of hawthorn seems to be due only to a lack of mature bushes to provide seed. As the younger bushes growing up across the Warren reach maturity the rate of recruitment, without management, will probably accelerate. There is no evident reason why there should not eventually be an extensive cover of hawthorn. The abundance of scrub near the Braint estuary, where there is no lack of reproducing bushes, perhaps indicates the possibilities. The growth of hawthorn on Newborough Warren does affect the top soil, causing an increase in the pH, organic matter and soil moisture content. The bioassay provided evidence that it has also increased the fertility of the soil, enriching it in nitrate and phosphate. The aboveaverage yields in the grassland soil for trials 1, 3 and 4 are very likely to be connected with the presence of Ammophila arenaria as the sole or dominant species in the grassland adjacent to these sampled bushes. Abdel Wahab & Wareing (1980) have shown that nitrogen-fixing bacteria in the rhizosphere of Arnrnophila seem to be responsible for significant rates of nitrogen fixation associated with its presence. This process may have resulted in the relatively greater fertility of the grassland soil in trials 1, 3 and 4. The different pH, organic matter content and soil moisture content might favour the growth of other species; the higher levels of nitrogen and phosphorus in the topsoil would almost certainly do so. Dune systems are generally very deficient in nitrogen and phosphorus, and their addition may produce considerable changes in the vegetation. At Braunton Burrows, Devon, additions of N and P in the field resulted in the complete elimination of many small plants and the emergence of Festuca rubra and Poa pratensis as strong dominants (Willis, 1963). Boorman & Fuller
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(1982) found that the species diversity of nutrient-poor, rabbit-grazed dune swards at Holkham, Norfolk, decreased with additions of nitrogen fertiliser and that the diversity of species in the dunes was maintained by both low nutrient status and rabbit grazing. In a gap in the middle of a blackthorn thicket near the area of tagged scrub at Newborough, nettles were observed growing up above the level of the surrounding vegetation. It is not possible to be certain which species might grow in the place of cut hawthorn on Newborough Warren but all the indications are that there would be fewer of the characteristic sand dune vegetation communities for which Newborough Warren is especially noted. Young pines originating in the adjacent Forestry Commission plantation occur sporadically across Newborough Warren. Seven lodgepole pines Pinus contorta were recorded on the transects. Newborough Forest is composed largely of Corsican pine Pinus nigra ssp. laricio but there is a small amount of lodgepole pine. The Corsican pine was planted mainly between 1953 and 1956. The species does not flower until it is between 20 and 25 years of age, producing its first heavy cone crop between 25 and 30 years, with subsequent good crops at three- to five-year intervals. The small areas of lodgepole pine, located mainly on the Warren side of Newborough Forest, were mostly planted in 1953. Lodgepole pine produces its first flowers from the fifth to tenth year and good crops of cones from the fifteenth to twentieth year (Aldhous, 1957). The presence of lodgepole pine and absence of Corsican pine along the transects on the Warren in 1981 reflect their different maturation times. Corsican pine is a strong light demander. Seed production in Britain is good and in nurseries approximately 60 9/0 of viable seeds produce one year seedlings (Aldhous, 1957). In a Forestry Commission experiment in Thetford Forest, seedlings were not often found on the ground because of predation by voles and mice (Forestry Commission, pers. comm.). Nonetheless, in open conditions regeneration may occur readily. At Holkham National Nature Reserve, Norfolk, Corsican pine was the most widely planted species in a plantation established on the dunes between 1870 and 189 I. Prolific natural regeneration has occurred there, despite heavy predation of young seedlings by voles and rabbits, resulting in the formation of a belt of young uneven-aged Corsican pine seawards of the plantation. The species seems likely to colonise all north-facing (moister) slopes with the exception of the most seaward ones (Johnson, 1976).
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Very few individuals of Corsican pine in Newborough Forest were mature by 1981 (T. A. Carter, pers. comm.) so it seems very likely, in view of the example at Holkham, that pines will, without appropriate management, gradually become abundant on the Warren after the Corsican pines begin fruiting.
CONCLUSION Scattered young pine trees have grown up and hawthorn has spread over the dune system at Newborough Warren since 1954 and is enriching the top soil in nitrogen and phosphorus. This is likely to reduce the ecological interest of the reserve, and so it is suggested that appropriate management is undertaken to prevent further scrub and pine colonisation. Areas of dense scrub can provide important food and shelter for migrant birds but the scrub cover at Newborough Warren does not, at least as yet, do so (R. A. Bennet, pers. comm.). Initial scrub clearance, followed by the introduction of grazing in the landward parts of the Warren, would both prevent scrub growth and reverse the trend towards rank, species-poor grassland. If widespread grazing were not practicable, initial scrub clearance should be followed up by periodic mowing or swiping. A small, already well-colonised area, for example the plot of tagged bushes, could perhaps be left to develop further.
ACKNOWLEDGEMENTS I am very grateful to Dr F. B. Goldsmith for supervising the work, D. G. Hewett for his helpful comments during the fieldwork and Dr P. J. Grubb for his ready advice on setting up the bioassay and comments on the results; Dr F. B. Goldsmith, D. G. Hewett and T. H. Blackstock made useful criticisms of the first draft of the paper, and Dr P. J. Grubb of the bioassay sections. I would like to thank the Nature Conservancy Council for permission to work at Newborough Warren and for access to the unpublished tagged plot data; and staff of the Institute of Terrestrial Ecology, Bangor, for technical assistance and the use of their facilities. The work was carried out while in receipt of a NERC studentship.
118
S. E. Hodgkin REFERENCES
Abdel Wahab, A. M. & Wareing, P. F. (1980). Nitrogenase activity associated with the rhizosphere of Ammophila arenaria L. and effect of inoculation of seedlings with Azotobacter. New Phytol., 84, 711-21. Aldhous, J. R. (1957). Exotic forest trees in Great Britain. Bull. For. Commn, No. 30. Boorman, L. A. & Fuller, R. M. (1982). Effects of added nutrients on dune swards grazed by rabbits. J. Ecol., 70, 345-57. Cornforth, I. S. (1968). Relationships between soil volume used by roots and nutrient accessibility. J. Soil Sci., 19, 291-301. Grubb, P. J. & Key, B. A. (1975). Clearance of scrub and re-establishment of chalk grassland on the Devil's Dyke. Nature in Cambs., 18, 18-22. Hope Jones, P. (1964). Measurements of woody perennials found and tagged on 1"3 acres of fixed dune grassland at Newborough Warren, 1964. Bangor, Nature Conservancy (Unpublished report). Hope Jones, P. (1965). Notes on some recent changes in the vegetation at Newborough Warren, Anglesey. Nature in Wales, 9, 165-9. Johnson, C. L. (1976). Factors affecting the establishment and distribution of Corsican pine natural regeneration at Holkham National Nature Reserve. J. For., 74, 95-102. Malloch, A. J. C. (1972). Salt-spray deposition on the maritime cliffs of the Lizard Peninsula. J. Ecol., 60, 103-12. Moore, P. D. (1971). Computer analysis of sand dune vegetation in Norfolk, England, and its implications for conservation. Vegetatio, 23, 323-38. Morton, A. J. (1974). Ecological studies of a fixed dune grassland system at Newborough Warren, Anglesey. I. The structure of the grassland. J. Ecol., 62, 253-60. Nature Conservancy (1953). Report of the Nature Conservancy for the year ended 30 September 1953. London, HMSO. Peace, W. J. H. & Grubb, P. J. (1982). Interaction of light and mineral nutrient supply in the growth of Impatiens parviflora. New .Phytol., 90, 127-50. Pemadasa, M. A. & Lovell, P. H. (1974). The mineral nutrition of some dune annuals. J. Ecol., 62, 647-57. Ranwell, D. S. (1958). Movement of vegetated sand dunes at Newborough Warren, Anglesey. J. Ecol., 46, 83-100. Ranwell, D. S. (1959). Newborough Warren, Anglesey, I. The dune system and dune slack habitat. J. Ecol., 47, 571-601. Ranwell, D. S. (1960a). Newborough Warren, Anglesey, II. Plant associes and succession cycles of the sand dune and dune slack vegetation. J. Ecol., 48, 117-41.
Ranwell, D. S. (1960b). Newborough Warren, Anglesey, Ill. Changes in the vegetation on parts of the dune system after the loss of rabbits by myxomatosis. J. Ecol., 48, 385-95. Ratcliffe, D. A. (1977). A nature conservation review. Cambridge, Cambridge University Press.
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119
Richards, N. D. (1970). Repeated measurements of Crataegus monogyna and Betula spp. in an area of 1.3 acres of fixed dune grassland at Newborough Warren, 1970. Bangor, Nature Conservancy (Unpublished report). Salisbury, E. J. (1925). Note on the edaphic succession in some dune soils with special reference to the time factor. J. Ecol., 13, 322-8. Snedecor, G. W. (1956). Statistical methods. Iowa, Iowa State College Press. Tutin, T. G., Heywood, V. H., Burges, N. A., Moore, D. M., Valentine, D. H., Waiters, S. M. & Webb, D. A. (1964-1980). Flora Europaea, Vols. 1-5. Cambridge, Cambridge University Press. White, D. J. B. (1961). Some observations on the vegetation of Blakeney Point, Norfolk, following the disappearance of the rabbits in 1954. J. Ecol., 49, !13-18. Willis, A. J. (1963). Braunton Burrows: the effects on the vegetation of the addition of mineral nutrients to the dune soils. J. Ecol., 51, 353-74.
Scrub Encroachment and Its Effects on Soil Fertility on Newborough Warren, Anglesey, Wales S. E. Hodgkin Department of Botany and Microbiology,UniversityCollege London, Gower Street, London, WC1E 6BT, Great Britain
A BS TRA C T Scrub species have colonised the dune system of Newborough Warren, apparently since myxomatosis wiped out its rabbit population in 1954. The colonisation by hawthorn, the most abundant scrub species, and its effects on soil fertility were studied. Measurements made of the scrub along four transects showed that 95 % of the hawthorn has colonised since myxomatosis, and there is a decline in the average age and density of bushes towards the sea that is not accounted for by unfavourable growing conditions but rather by a slow rate of colonisation. There is evidence that enrichment of the topsoil in nitrogen and phosphorus is occurring beneath hawthorn bushes, and indications are that certain nutrient-demanding weedy species, not typical dune species, would replace dead bushes. Sparse but widespread individuals of Pinus species have colonised from the adjacent forest. Without appropriate management, hawthorn and pine could become abundant.
INTRODUCTION Myxomatosis first appeared in Great Britain in 1953. Diseased rabbits were noticed at Newborough Warren in August 1954 and soon all but a very few rabbits were wiped out (Ranwell, 1959, 1960b). Previously the Warren was very densely populated with rabbits and the absence of rabbit grazing resulted in a decrease in the number of plant species in fixed dune areas, a great increase in the growth and flowering of most grasses and sedges, and a reduction in low growing dicotyledonous herbs (Ranwell, 99 Biol. Conserv. 0006-3207/84/$03-00 © ElsevierAppliedSciencePublishersLtd, England, 1984. Printed in Great Britain
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S. E. Hodgkin
1959, 1960b). Similar effects were noticed in many other semi-natural habitats previously overrun by rabbits, for example Blakeney Point, Norfolk (White, 1961). Another consequence of the sudden release of grazing pressure was that scrub species were able to grow up in many sites. D. S. Ranwell (pers. comm.) recalled that there were only 'scattered and very sparse', heavily rabbit-chewed, hawthorn stumps present on the Warren before 1954. Scrub is now thinly spread over most of Newborough Warren and in places is abundant. Rabbits are still scarce. Scrub species which had colonised chalk grassland at Devil's Dyke, Cambridgeshire, were shown to have caused a massive enrichment of the topsoil in inorganic phosphate and to a far lesser extent in inorganic nitrate such that, when cleared, some 'weedy' species, known to need a rich supply of N and P, rather than the typical chalk grassland flora, grew up in their place (Grubb & Key, 1975). There is concern about scrub invasion because, first, each bush represents the displacement of an equal area of the more interesting dune vegetation; second, if the scrub is enriching the topsoil this displacement may not be a temporary, reversible one. The aim of this study was therefore to examine the colonisation, growth and effects on soil fertility of scrub at Newborough Warren following myxomatosis. The form of the dune system and vegetation before 1954 was described by Ranwell (1958, 1959, 1960a); some of the changes subsequent to myxomatosis have been noted by Ranwell (1960b), Hope Jones (1965) and Morton (1974). Plant nomenclature follows Tutin et ai. (1964-1980).
METHODS Fieldwork was carried out between 15 June and 29 July 1981. The site
Newborough Warren lies on the southwest coast of Anglesey (National Grid reference SH 425635) and consists of 2000 ha of dunes, marsh, flats, shingle and cliff. Six hundred and thirty three ha of the site were declared a National Nature Reserve by the then Nature Conservancy in 1955. In their annual report for that year the site was described as one of the largest and biologically richest expanses of dunes in western Britain. In 1948 the
Scrub encroachment on dunes
101
Forestry Commission began planting pines on about 800 ha o f the dunes. Nonetheless, Newborough Warren is the sixth largest dune system in Britain (Ratcliffe, 1977). Transects across the Warren
A survey was made o f the scrub within a 10 m belt along four transects across the Warren, totalling 7420 m (Fig. 1). The transects were arranged n o n - r a n d o m l y to radiate out from the region o f most abundant, oldestlooking scrub. The species, distance along the transect, m a x i m u m height, ..-
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102
S. E. Hodgkin
maximum width of canopy and stem circumference at ground level (later converted to stem diameter) were recorded. Notes were made of the habitats along the length of each transect. A short length of Transect 1 ran through a series of grazing plots but the number and ages of bushes in these plots will not have been significantly affected as the plots had been grazed only for the previous two and a half years. Stem samples were taken at ground level from every bush in T 1 and T3 and from the oldest-looking hawthorn Crataegus monogyna and birch Betula spp. in 30m sections along T4, using an increment borer manipulated with a toggle bar. In the case of very small individuals, sections were cut with secateurs. Towards the end of T3, where the scrub was sparse, bushes in a second, adjacent 10 m belt were included, to give a larger sample for aging. Measurements were made, and samples taken, from an additional 13 bushes in two areas of relatively dense dune Scrub near the Braint estuary (Fig. 1). Stem core samples were subsequently aged and their annual increments recorded using a travelling stage microscope and data punch unit. Soil samples for pH measurements were collected on the same day every 100m along transects 1, 3 and 4 and from the Braint estuary (6 random samples). Where wet slack occurred along a transect an additional sample was collected.
Scrub tagged in 1964 In 1964 a permanent plot of tagged scrub was set up by Hope Jones (1964). Numbered metal tags were attached to every hawthorn bush and the single birch tree in a 0.5 ha corner of the Warren (Fig. 1), and records were made of the height, maximum width and stem diameter of each bush. In 1970 the measurements were repeated by Richards (1970) and new bushes were tagged. The plot was resurveyed in 1981 by the author; coring was not permitted.
Partial correlation analysis Partial correlation analysis was used to identify correlations between the age, dimensions, shape and vigour of a bush, its position along a transect and soil pH. Vigour was defined as the width of the outer five annual rings of a bush at ground level; it was confirmed in this study as a valid measure
Scrub encroachment on dunes
103
of true vigour by the finding that it is better correlated with stem diameter, canopy width and height than is the age of the bush. The calculation of partial correlation coefficients removes the effect of a third variable (3) by holding it constant to allow the true correlation between two variables (1 and 2) to be sought (Snedecor, 1956). The coefficient is given by the equation: r 1 2 - - r 1 3 X r23 r12x3
-----x/(l
-- r23)(1 -- r223)
In all analyses involving age, only those hawthorn bushes were used from which a clear, complete core had been obtained. Multiple-stemmed bushes were not included in any analysis of stem diameter. Stem diameter was identified by partial correlation analysis, as the parameter of growth best correlated with age, and was used to help interpret data from T2, T4 and the tagged area. The relationships between age, dimensions, vigour of hawthorn and distance from the sea were examined using partial correlation analysis in order to distinguish parameters with a gradient across the Warren caused solely by a cline in age from those parameters with a gradient which might be due to increasing proximity to the sea.
Bioassay of soil fertility As in most soils, including dune soils, nitrogen and phosphorus are likely to be the two nutrients most limiting plant growth on the Warren and therefore controlling the composition of the vegetation. At the nearby Aberffraw dune system for example, Pemadasa & Lovell (1974) found that nitrogen and phosphorus most limited the growth and performance of dune annuals. In order to determine whether hawthorn, the commonest scrub species on Newborough Warren, is enriching the topsoil beneath it, Festuca rubra, a common dune species, was grown as a bioassay of soil fertility, in paired pots of soil taken from beneath and adjacent to seven large bushes, aged between 18 and 27 years. Such a comparison between the growth of experimental plants in paired samples probably gives a better idea of the availability to plants of limiting nutrients than does any given test involving a chemical extractant. Soil was collected from a depth of 0-5 cm (excluding litter), near the stem of each bush, and from a point 50 cm to the south of the edge of each
104
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bush. Measurements were made, and stem samples taken, as for the transects. Dominant species in the adjacent vegetation were recorded. Paired soil samples were taken to measure pH, soil moisture content and loss-on-ignition. It has been shown that the rooting density in a pot does not affect the availability to an individual plant of immobile ions but in the case of mobile ions less is available to each plant the more densely they are sown (Cornforth, 1968; Peace & Grubb, 1982). A single plant may use virtually all the nitrate that becomes available in the pot because the nitrate ion is very mobile, but it can absorb phosphate only from a thin layer around its roots because the orthophosphate ion is so immobile. When, say, six plants are grown in one pot then each plant will again be able to take up only that phosphate supplied from the thin layer around its roots but its nitrate supply will be substantially reduced because each plant can use only one-sixth of the soil volume for nitrate. Thus, Peace & Grubb (1982) showed that lmpatiensparviflora grown one per small pot responded only to additional phosphate in the short term, but plants grown 10 per small pot responded only to additional nitrogen. Each of the 14 soil samples was divided between two 9 cm pots, holding about 270cm 3. The pots were sown with Festuca rubra on 18 June 1981, and the seedlings were gradually thinned to one per pot to test for phosphate enrichment, and to six per pot to test for nitrate. One week after sowing the pots were randomly arranged outside under a broad mesh wire cage. The 28 pots consisted of: 2 densities x 2 sample positions per bush x 7 bushes. The plants were harvested after 61 days, oven-dried and the shoot and root dry weights recorded.
RESULTS
Scrub species Nineteen species of tree and shrub were recorded on the Warren (Table 1), of which 15 were found along the transects. Hawthorn is the most abundant scrub species on the Warren, with an estimated total of 7400 individuals. Prunus spinosa and Ulex europaeus grow in small, dense patches at the north end of the Warren. Birch occurs scattered across the Warren. Most samples of birch wood were impossible to age with certainty and,
Scrub encroachment on dunes
105
TABLE 1 List of the Tree and Shrub Species Recorded on Newborough Warren during this Survey
Acer pseudoplatanus Betula pendula Betula pubescens Calluna vulgaris Carpinus betulus Cotoneaster sp. Crataegus monogyna
Cytisus scoparius Fraxinus excelsior Hippopha~ rhamnoides
Malus sylvestris Pinus contorta Pinus spp. Prunus spinosa Salix atrocinerea Sambucus nigra Sorbus aucuparia Taxus baccata Ulex europaeus
except for hawthorn, no other species was recorded in large enough numbers for analysis to be possible. Most of the discussion is therefore confined to a consideration of hawthorn. Age-class distribution Figure 2 illustrates the age-class distribution of hawthorn along T 1 and T3. Only three out of 109 bushes pre-date myxomatosis and there has been a steady increase in the number of bushes since then (Fig. 3). There appears to have been a levelling offin recruitment in the last ten years but this is very likely to be due to the difficulty in finding small seedlings and saplings among taller vegetation. No correlation was apparent between climatic conditions (annual rainfall, number of days with more than 1 mm of rain, mean minimum and maximum July temperatures) and recruitment in any one year, but the samples might have been too small to reveal any correlation that did exist. The oldest birch dated back to 1955, the year after myxomatosis wiped out the Newborough Warren rabbit population. The other 20 stem cores that could be aged indicate a more or less even rate of recruitment since that year. Direction and rate of colonisation by hawthorn The oldest hawthorn bushes o c c u r in the northern part of the Warren and the average age declines towards the sea. In the cases of T1 and T3 the
S. E. Hodgkin
106
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Scrub encroachment on dunes
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negative correlation between hawthorn age and distance from the north edge of the Warren is significant (p < 0.OOl). Correlation between the stem diameters of unaged bushes in T2 and their distance along the transect indicated (p < 0.05) that here too the average age of bushes also declined towards the sea. No such correlation existed in T4 where there is a second source of colonisation with several old bushes halfway along it, by the Braint estuary. The average annual rate of encroachment seawards, calculated from the inverse of the slope of the regression line for age versus distance along a transect, is 270m per annum along TI and 170m per annum along T3. Distribution of hawthorn across the Warren
Along all four transects there is a more or less steady decline in the density of hawthorn seawards (Fig. 4). The density and vigour of hawthorn in different habitats, and the relation between soil pH and vigour, were examined in order to determine whether this decline could be attributed to unfavourable environmental conditions or simply a slow rate of encroachment. Hawthorn was found in yellow dune, grey dune, dry slack and fixed dune grassland; none was seen on fore-dunes and only one individual was recorded along the transects in wet slack (and that a relatively dry one). Figure 4 illustrates that the decline in numbers towards the sea, before the fore-dunes are reached, is not entirely due to the increasing area of wet slack: there is a decline in the density in other habitats. In the figure, the width of the transect belts represents the density of hawthorn in suitable habitats (wet slack excluded) per 100m sections; the breaks in the line represent the length of wet slack habitat. The soil samples taken along T 1 and T3 showed a significant increase in pH (p < 0.001 andp < 0.01, respectively) towards the sea but there was no correlation between the vigour of hawthorn and the pH of the corresponding soil sample. Indeed, using partial correlation analysis to exclude the significant negative correlation between age and vigour (p < 0.001; positive correlations with stem diameter excluded), hawthorn along TI and T3 is shown to grow more vigorously closer to the sea. The correlation is significant in the case of T3 (p < 0.05). The absence of any trend in p'H along T4 is perhaps due to the fact that it runs entirely through an area of old, fixed dunes.
S. E. Hodgkin
110
Changes in growth rate since myxomatosis In order to determine whether conditions have continued to be equally favourable for the growth of hawthorn over the years since myxomatosis, despite the vegetation becoming generally taller and thicker, the width of five annual rings over the same age was compared, as a measure ofvigour, between one old class and two younger classes of hawthorn. There was no difference in the radial increment over the 10th to 14th years of growth between one class aged 23-27 years (that is, growing up in the five years following myxomatosis) and a second aged 15-19 years (Table 2). Neither was there any difference in the radial increment over the 5th to 9th years of growth between two classes aged 23-27 years and 10-14 years, respectively.
Area by the Braint estuary Scrub was very abundant in the areas A 1 and A2 by the Braint estuary (Fig. 4). Several hawthorn bushes pre-dated myxomatosis; the oldest aged bush was 33 years old. The hawthorn here looked healthier and appeared to be growing more vigorously than bushes elsewhere on the Warren; the foliage was a darker green and all the bushes sampled were fruiting copiously. Indeed, the mean vigour of 13 bushes in A1 and A2 aged between 18 and 32 years was greater (p < 0.001) by two to three times than that of a sample from T1, T2 and T4 comprising all those bushes within the same age range.
Scrub tagged in 1964 A comparison between the frequency distributions in 1964, 1970 and 1981 of the stem diameters of the tagged bushes, used as the best available TABLE 2 Comparison of Vigour: Radial Increment over Five-Year Period (Mean + SE) between Two Age Classes of Crataegus over the Same Age
Vigour (mm) at 10-14 years
Vigour (mm) at 5-9 years
Age class
23-27 years
15-19 years
23-27 years
10-14 years
n
2.66 + 0'51 19
3-05 + 0.52 34
2"11 _ 0.64 19
1.69 + 0-26 25
Scrub encroachment on dunes
111
24
20
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(mm)
Frequency distribution of stem diameter of hawthorn in the 0-5 ha area of tagged bushes: 1964 (diagonal lines), 1970 (solid) and 1981 (vertical lines).
indicator of age, suggests that there is a more or less even-aged stand of hawthorn in this area (Fig. 5). The mean annual recruitment rate between 1964 and 1970 was 1.8 (total of I l bushes), and 0.5 between 1970 and 198 l (6 bushes). The single birch tree recorded in 1964 still survives; no further recruitment has occurred. Blackthorn and gorse have spread vegetatively and surrounded more bushes since 1970.
Bioassay of soil fertility The pH value, soil moisture content and loss-on-ignition of the samples are given in Table 3. There is a considerable variation in the pH of the samples of soil under grass (pH 5.15-7-65) but less in the case of soil under scrub (pH 5.65-7.15). The mean pH values did not differ significantly but the scrub soil was more alkaline than soil from the adjacent grassland in 6 out of 7 cases. This is likely to result from the deposition of sodium and magnesium ions on the crowns of the bushes from salt-laden winds (Malloch, 1972) later washed off into the soil below. Loss-on-ignition was generally greater in the samples of scrub soil than grass soil; this is probably related to a higher rate of litter fall from the canopy. Every sample of scrub soil had a higher moisture content than the corresponding sample of grass soil; the higher levels of organic matter would enable more water to be retained in the soil, possibly helped by a reduced evaporation rate in the shelter beneath the bush.
S. E. Hodgkin
112
TABLE 3 pH, Moisture Content and Loss-on-lgnition of Paired Soil Samples Collected Beneath Crataegus and from the Adjacent Grassland Soil
Trial
1 2 3 4 5 6 7 Mean +SE
pH
Moisture content % oven-dry wt
Loss-on-ignition % oven-dry wt
Scrub soil
Grass soil
Scrub soil
Grass soil
Scrub soil
Grass soil
6'15 6-05 5"95 7-15 6"95 5"70 5"65 6-23 +0.2
5"45 5"55 5"25 6-25 7"65 5"50 5'15 5.83 +0.3
13 12 13 7 !8 9 17 13 +1.5
11 8 6 4 3 4 15 7 +i.7
7 17 8 3 8 3 6 7 +1.8
4 6 4 4 1 5 6 4 +0.6
It was not possible to harvest the roots of the Festuca rubra bioassay plants completely; a lot of small particles of organic matter and humus was bound up in the mats of fine roots. For this reason only the dry weight yield of the shoots is considered. In the one-plant pots the scrub soil yielded, in total, 41% more shoot dry weight than the grass soil, indicating that the scrub soil is relatively enriched in phosphate, and perhaps nitrate. In the case of the six-plant pots, shoot dry weight was 36 % greater in scrub soil than in grass soil, indicating that there has been nitrate enrichment of the scrub soil. The total shoot yield in each six-plant pot was only slightly greater than the yield of the single individual in the corresponding one-plant pot; in no case does the yield approach six times greater, as would be expected if there was an ample supply of the major limiting mobile nutrient ion, nitrate. However, there was a considerable variation in yield between individual trials even in the 'control' grass soils (Table 4). Relative enrichment of the scrub soil is indicated in bushes 2, 5, 6 and 7. Trials 1 and 4 show the opposite result, with a higher than average yield from the grass soil. Trial 3 produces conflicting results, indicating the possibility of a slight scrub soil enrichment with six plants per pot but a relative deficiency with one plant per pot. Trials 1, 3 and 4 were distinguished from the others by the
Scrub encroachment on dunes
113
TABLE 4 Yield of Festuca rubra after 61 days' Growth in Soil collected Beneath or Beside Crataegus, Expressed as Oven-dry Weight of Shoots (nag) and Yield from Soil Under Scrub as per cent of Yield from Soil Under Grass
Trial
I 2 3 4 5 6 7 Mean +SE
One plant per pot
Six plants per pot (mean +_SE)
Scrub soil
Grass soil
~
401-8 378-7 208"8 231.9 557-3 254"0 698"0 390-! +68-1
410"9 219-6 292"2 461"5 283"0 150'8 125"0 277-6 +47.5
98 172 71 50 197 168 558
Scrub soil 93"5 + 13-3 68-8+5"5 70"7 + 12"5 6 8 '6 + 1 0 - 3 88"8 + 7'7 124.2 + 27-0 181"6 + 24"8 99-5 +8.3
Grass soil
%
115.1 + 16-4 50-5-t-9.1 64.7 + 12-0 98-7+ 11"9 64.7 + 4"9 59.9 + 5-2 58"8 + 4-5 73.2 +8-9
84 136 109 70 137 207 309
dominance of Ammophila arenaria in the vegetation surrounding the bushes by which the soil was collected.
DISCUSSION Hawthorn is the most abundant scrub species on the Warren; it is widespread, growing to within 330 m of the sea. The density and age of hawthorn bushes across Newborough Warren declines away from the northern, landward end, suggesting that the initial source of seed was in the bushes and hedgerows near and bordering the north side of the Warren. Another source of colonisation is apparent in the area of relatively dense scrub near the Braint estuary where several bushes date from before myxomatosis. The results from the transects confirm the opinion that heavy grazing pressure from rabbits was responsible for the lack of scrub on the Warren before 1954. Ninety five per cent of the hawthorn present in 1981, probably an underestimate because of the position of the transects (see Methods), and 20 out of 21 birch trees, became established after myxomatosis. The few individuals of other species which were aged postdated 1954. The rabbit population is still at a low level so that rabbit
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grazing is unlikely to interfere significantly with scrub recruitment. No more than 20 rabbits were seen during the course of the survey and the only active burrows observed were some near the sea by Transect 3. At the present time hawthorn is less frequent closer to the sea (even discounting unsuitable habitats). However, it appears to grow more vigorously nearer the sea and it is evident that the current distribution is due to a slow rate of encroachment limited at present by the number of mature bushes. There is no reason why hawthorn, at least, could not with time become much more abundant across the Warren. Comparisons of vigour at the same age in classes of old and young hawthorn show that conditions are not becoming less favourable for growth. Its growth is most vigorous in a small area of marshy grassland near the Braint estuary; it is likely that an input of nutrients from the estuary, perhaps by periodic flooding, is responsible. Elsewhere on the Warren its growth is equally less vigorous in all other habitats (apart from fore dunes and wet slack in which it does not occur) throughout the Warren. A progressive leaching of carbonates from sand occurs with time, causing a steady fall in pH from embryo dunes inland to older, stable dunes (Salisbury, 1925); this effect is not likely to be responsible for the seawards decline in hawthorn numbers because their vigour was not correlated with soil pH. In the most landward region of the Warren around Pen-Ion, there is some very tall dense grassland. In 1964, studying the plot of tagged scrub in that area, Hope Jones (1965)was of the opinion that grass species had grown so vigorously since myxomatosis that further scrub colonisation was precluded. Prunus spinosa and Ulex europaeus are still spreading vegetatively in the area near Pen-Ion but there has probably been little recent recruitment to the hawthorn population. Data from the tagged plot, which reveals very little recruitment since 1964, support this view. Over the Warren as a whole, recruitment to the hawthorn population is occurring steadily. There has been an apparent fall-off in the overall rate of recruitment in the last ten years. However, it is unlikely that there has been any significant increase in the density and height of the dune grassland over that period that might have caused a real fall-off. All the evidence points to a very rapid change in the vegetation of the Warren soon after myxomatosis (Ranwell, 1960b; Hope Jones, 1965; Morton, 1974). It is suggested that young seedlings and saplings shorter than the surrounding vegetation were under-recorded. In the aged sample no hawthorn younger than five years was found in the open grassland; four
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two-year,old individuals and one five-year-old were found beneath the canopy of larger bushes which were being cored; a further three hawthorn aged 15-17 years, as well as three, unaged but judged to be at least ten years old, were found growing suppressed beneath the canopy of much larger bushes. Establishment in such conditions must be at least as difficult as in the dune grassland. Only nine individuals from the aged sample, aged 5-10 years, were found in the grassland and it was felt many others could have been missed. It has been shown that, in the continued absence of rabbits and larger herbivores, nothing is hindering the growth of hawthorn on the Warren, and regeneration will probably continue in all but the area immediately around the reserve boundary near Pen-Ion. Further seawards, the relative sparsity of hawthorn seems to be due only to a lack of mature bushes to provide seed. As the younger bushes growing up across the Warren reach maturity the rate of recruitment, without management, will probably accelerate. There is no evident reason why there should not eventually be an extensive cover of hawthorn. The abundance of scrub near the Braint estuary, where there is no lack of reproducing bushes, perhaps indicates the possibilities. The growth of hawthorn on Newborough Warren does affect the top soil, causing an increase in the pH, organic matter and soil moisture content. The bioassay provided evidence that it has also increased the fertility of the soil, enriching it in nitrate and phosphate. The aboveaverage yields in the grassland soil for trials 1, 3 and 4 are very likely to be connected with the presence of Ammophila arenaria as the sole or dominant species in the grassland adjacent to these sampled bushes. Abdel Wahab & Wareing (1980) have shown that nitrogen-fixing bacteria in the rhizosphere of Arnrnophila seem to be responsible for significant rates of nitrogen fixation associated with its presence. This process may have resulted in the relatively greater fertility of the grassland soil in trials 1, 3 and 4. The different pH, organic matter content and soil moisture content might favour the growth of other species; the higher levels of nitrogen and phosphorus in the topsoil would almost certainly do so. Dune systems are generally very deficient in nitrogen and phosphorus, and their addition may produce considerable changes in the vegetation. At Braunton Burrows, Devon, additions of N and P in the field resulted in the complete elimination of many small plants and the emergence of Festuca rubra and Poa pratensis as strong dominants (Willis, 1963). Boorman & Fuller
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(1982) found that the species diversity of nutrient-poor, rabbit-grazed dune swards at Holkham, Norfolk, decreased with additions of nitrogen fertiliser and that the diversity of species in the dunes was maintained by both low nutrient status and rabbit grazing. In a gap in the middle of a blackthorn thicket near the area of tagged scrub at Newborough, nettles were observed growing up above the level of the surrounding vegetation. It is not possible to be certain which species might grow in the place of cut hawthorn on Newborough Warren but all the indications are that there would be fewer of the characteristic sand dune vegetation communities for which Newborough Warren is especially noted. Young pines originating in the adjacent Forestry Commission plantation occur sporadically across Newborough Warren. Seven lodgepole pines Pinus contorta were recorded on the transects. Newborough Forest is composed largely of Corsican pine Pinus nigra ssp. laricio but there is a small amount of lodgepole pine. The Corsican pine was planted mainly between 1953 and 1956. The species does not flower until it is between 20 and 25 years of age, producing its first heavy cone crop between 25 and 30 years, with subsequent good crops at three- to five-year intervals. The small areas of lodgepole pine, located mainly on the Warren side of Newborough Forest, were mostly planted in 1953. Lodgepole pine produces its first flowers from the fifth to tenth year and good crops of cones from the fifteenth to twentieth year (Aldhous, 1957). The presence of lodgepole pine and absence of Corsican pine along the transects on the Warren in 1981 reflect their different maturation times. Corsican pine is a strong light demander. Seed production in Britain is good and in nurseries approximately 60 9/0 of viable seeds produce one year seedlings (Aldhous, 1957). In a Forestry Commission experiment in Thetford Forest, seedlings were not often found on the ground because of predation by voles and mice (Forestry Commission, pers. comm.). Nonetheless, in open conditions regeneration may occur readily. At Holkham National Nature Reserve, Norfolk, Corsican pine was the most widely planted species in a plantation established on the dunes between 1870 and 189 I. Prolific natural regeneration has occurred there, despite heavy predation of young seedlings by voles and rabbits, resulting in the formation of a belt of young uneven-aged Corsican pine seawards of the plantation. The species seems likely to colonise all north-facing (moister) slopes with the exception of the most seaward ones (Johnson, 1976).
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Very few individuals of Corsican pine in Newborough Forest were mature by 1981 (T. A. Carter, pers. comm.) so it seems very likely, in view of the example at Holkham, that pines will, without appropriate management, gradually become abundant on the Warren after the Corsican pines begin fruiting.
CONCLUSION Scattered young pine trees have grown up and hawthorn has spread over the dune system at Newborough Warren since 1954 and is enriching the top soil in nitrogen and phosphorus. This is likely to reduce the ecological interest of the reserve, and so it is suggested that appropriate management is undertaken to prevent further scrub and pine colonisation. Areas of dense scrub can provide important food and shelter for migrant birds but the scrub cover at Newborough Warren does not, at least as yet, do so (R. A. Bennet, pers. comm.). Initial scrub clearance, followed by the introduction of grazing in the landward parts of the Warren, would both prevent scrub growth and reverse the trend towards rank, species-poor grassland. If widespread grazing were not practicable, initial scrub clearance should be followed up by periodic mowing or swiping. A small, already well-colonised area, for example the plot of tagged bushes, could perhaps be left to develop further.
ACKNOWLEDGEMENTS I am very grateful to Dr F. B. Goldsmith for supervising the work, D. G. Hewett for his helpful comments during the fieldwork and Dr P. J. Grubb for his ready advice on setting up the bioassay and comments on the results; Dr F. B. Goldsmith, D. G. Hewett and T. H. Blackstock made useful criticisms of the first draft of the paper, and Dr P. J. Grubb of the bioassay sections. I would like to thank the Nature Conservancy Council for permission to work at Newborough Warren and for access to the unpublished tagged plot data; and staff of the Institute of Terrestrial Ecology, Bangor, for technical assistance and the use of their facilities. The work was carried out while in receipt of a NERC studentship.
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Richards, N. D. (1970). Repeated measurements of Crataegus monogyna and Betula spp. in an area of 1.3 acres of fixed dune grassland at Newborough Warren, 1970. Bangor, Nature Conservancy (Unpublished report). Salisbury, E. J. (1925). Note on the edaphic succession in some dune soils with special reference to the time factor. J. Ecol., 13, 322-8. Snedecor, G. W. (1956). Statistical methods. Iowa, Iowa State College Press. Tutin, T. G., Heywood, V. H., Burges, N. A., Moore, D. M., Valentine, D. H., Waiters, S. M. & Webb, D. A. (1964-1980). Flora Europaea, Vols. 1-5. Cambridge, Cambridge University Press. White, D. J. B. (1961). Some observations on the vegetation of Blakeney Point, Norfolk, following the disappearance of the rabbits in 1954. J. Ecol., 49, !13-18. Willis, A. J. (1963). Braunton Burrows: the effects on the vegetation of the addition of mineral nutrients to the dune soils. J. Ecol., 51, 353-74.