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Changes on the heathlands in Dorset, England, between 1987 and 1996
Biological Conservation 93 (2000) 117±125
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Changes on the heathlands in Dorset, England, between 1987 and 1996 R.J. Rose, N.R. Webb*, R.T. Clarke, C.H. Traynor Furzebrook Research Station, NERC Institute of Terrestrial Ecology, Wareham, Dorset BH20 5AS, UK Received 22 April 1998; accepted 4 March 1999
Abstract In 1996 all of the heathland in south-east Dorset, southern England, was surveyed using the same recording protocols as those used in surveys in 1978 and 1987. This approach enabled the extent of the heathlands, the degree of fragmentation, and the composition of the vegetation to be compared at a landscape scale over a period of 18 years. Between 1987 and 1996 the number of heathland patches increased from 142 to 151 and the total area of heathland decreased by 552 ha from 7925 ha in 1987. The principal cause of this loss is succession to scrub and woody vegetation, which continues at a rate of 1.7% per year despite conservation management. The most signi®cant vegetation change was a decline in the area of the vegetation types, wet heath and peatland. These had remained constant between 1978 and 1987, but declined by 376 ha (45%) and 150 ha (25%) respectively between 1987 and 1996. Although there is no direct evidence, it is speculated that changing weather patterns or pollution might be the cause. These heaths, like many in Western Europe, are subject to a wide range of national and international legislative instruments designed to ensure their conservation. These measures call for the monitoring of the status of the conserved areas. This study, which has extended over almost 20 years, provides an insight in to the problems of monitoring change in a biotope of high conservation status. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Conservation; Landscape; Monitoring; Succession; Survey
1. Introduction Heathland is one of the cultural landscapes of western Europe in which, over several millennia, human activities have created and maintained extensive areas of dwarf shrub (subshrub) vegetation. Formerly, heathland extended over several million hectares but today its has been reduced to <10% of its former extent by aorestation, conversion to agricultural land and by urban and industrial development. The best estimates suggest that some 350,000 ha of heathland remains in western Europe (Diemont et al., 1996; Webb, 1998). This remaining heathland is subject to a wide range of international and national conservation designations. Rebane et al. (1997) summarise the international, environmental legislation which aects European heathlands and note that ®ve international, two pan-European and seven European Union legislative instruments are of
* Corresponding author. Fax: +44-1929-551087. E-mail address: [email protected] (N.R. Webb).
importance. The most signi®cant are the EU Directive on the conservation of wild birds (79/409/EEC) and the Directive on the conservation of habitats of wild fauna and ¯ora (92/43/EEC). The habitats directive lists four habitat types that fall within the de®nition of lowland Atlantic heathlands, two of which are priority habitats. Conservation designations such as these call for the favourable conservation status of a habitat to be de®ned and for movement away from this status to be monitored. Where movement away from the favourable conservation status is identi®ed management must be implemented to restore the former status. The heathlands formed on the sandy Tertiary soils of south-east Dorset in southern England have been the subject of regular survey over the last 40 years. These surveys document the rates of loss, the eects of fragmentation and isolation on their biota, and patterns of change in the vegetation (Moore, 1962; Webb and Haskins, 1980; Webb, 1990). The detailed Surveys (strictly censuses) in 1978 and 1987 (Webb and Haskins, 1980; Webb, 1990) make up a database called the Dorset Heathland Survey, which is one of the few studies in
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R.J. Rose et al. / Biological Conservation 93 (2000) 117±125
which vegetation change has been measured at the landscape scale in the United Kingdom. These data have been used to estimate the loss of heathland, to describe changes in the composition of the vegetation within the heathland landscape following ®res (Bullock and Webb, 1995), and to make species-based analyses on habitat availability, occupancy and metapopulation dynamics (Webb and Thomas, 1994; Thomas et al., in press). Between the 1978 and 1987 Surveys two factors were identi®ed as the cause of change on the Dorset Heathlands. First, there were direct losses of heathland caused by conversion to farmland, forestry, urban and industrial development. Secondly, there was succession to scrub and woodland, which showed a 15% increase over this period (Webb, 1990). Compared with losses to succession the direct losses were small. Conversion to farmland ceased when subsidies were withdrawn and urban and industrial development is now controlled through planning and environmental legislation. Over this period losses to successional change exceeded that from direct loss to other land uses. This was because traditional management, principally burning and grazing, had almost ceased and had not been replaced by other forms of vegetation management. Because of these changes programmes of conservation management were implemented (Auld et al., 1992; Woodrow et al., 1996). In 1996 a new census was made using the same protocols. This paper summarises the quantitative changes in both the total stock of heathland and the relative areas of the dierent heathland vegetation types and associated vegetation for the period 1987±1996, and reviews them in the wider context of the changes over the last 18 years. 2. Methods The methods used for the Dorset Heathland Survey have been described previously (Webb and Haskins, 1980; Chapman et al., 1989a). Records were collected between May and October 1996 from all 200200 m grid squares derived from the Ordnance Survey National Grid containing heathland. Within each grid square, the cover of heathland and associated vegetation types, the age and structure of the heathland vegetation, the presence of selected rare species, topography and land use were recorded as attributes on a 3-point cover-abundance scale (1=1±10% cover; 2=>10±50% cover; 3=>50% cover). All squares surveyed in 1978 (3110) and 1987 (3360) were re-surveyed even if they no longer contained heathland. New heathland squares were included in 1987 and 1996 if they had within their boundaries a proportion of dry heath, humid heath, wet heath, peatland or acid grassland. In addition, areas
identi®ed as being of high potential for the re-creation of heathland (Rose and Webb, 1995) to meet national biodiversity targets (Anon, 1995) were surveyed. The 1996 Survey extended over 3993 grid squares representing a total land area of 15,972 ha. In the previous surveys 184 attributes were recorded (Chapman et al., 1989a), but for the 1996 Survey 40 attributes were added (Appendix 1). These attributes were used to improve the estimates of the area of nonheathland land within the squares surveyed (3 attributes), to describe the potential heathland areas (12 attributes), to provide a census of the distribution of key bird and other species found on heathland and associated habitats (12 attributes), to record recent changes in heathland management practices (7 attributes) and to improve the description of reverting land (3 attributes) and invasive species (3 attributes). This will enable future surveys to quantify the success or failure of heathland re-creation schemes and current management plans. In all three Surveys, cover scores were converted to estimates of area using an algorithm (Chapman et al., 1989a). In 1978 and 1987, the area of the main vegetation types within each square was estimated by applying this algorithm to the four dwarf-shrub heathland types (dry heath, humid heath, wet heath and peatland), and nine associated main cover-types (brackish marsh, carr, scrub, hedges and boundaries, woodland, grassland, bare ground, sand dunes and open water). The bare ground cover-type corresponds to the recently burnt categories of Chapman et al., (1989a) and Webb (1990) but in this paper the term `recently burnt' refers to the area of bare soil and pioneer heathland vegetation that is in the very early stages of re-growth where charred remains indicate that the area was burnt within the 2 years preceding the Survey. In 1996 the algorithm was modi®ed to incorporate three new attributes; arable, urban/industrial and `other' land. The area of each sub-category of covertype within a main cover-type was then estimated using the modi®ed algorithm. These ®gures were used to calculate the total area of heathland as de®ned in previous Surveys. Traditionally, four growth phases of Calluna vulgaris have been recognised (Watt, 1955; Gimingham, 1972), but in the 1996 Survey we recognised a phase during which vegetation establishes from seed or after burning (the pioneer phase of Gimingham (1972) and the postburn phase of Chapman and Webb (1978)) which lasts between 2 and 5 years: a building phase similar to that of Gimingham (1972) and a mature phase incorporating the mature and degenerate phases of Gimingham (1972). The combination of the mature and degenerate phases was necessary because the typical structure of degenerate Calluna bushes is not common on the Dorset Heaths even after 35 years and the other dwarf shrubs,
R.J. Rose et al. / Biological Conservation 93 (2000) 117±125
Erica cinerea, E. tetralix and E. ciliaris, do not produce stems of this longevity (Bannister, 1965, 1966; Rose et al., 1996). The number of fragments was calculated for each Survey using the procedure described by Chapman et al. (1989a). These calculations exclude four outlying heaths to the west of the main block of heathland in south-east Dorset. 3. Results There was little dierence in the estimates of heathland area derived from the old and modi®ed algorithms. Therefore, throughout this paper only the modi®ed algorithm was used. Although these ®gures dier slightly from those published previously, they re¯ect more accurately the area of each land-type in the Survey area (Table 1). Between 1978 and 1987 the total area of heathland showed little change from 7913 to 7925 ha. In 1987 heathland was found in 77 more squares indicating a slight reduction in the average proportional cover of heathland in each square (2.54 ha/square in 1978 and 2.49 ha/square in 1987). Between 1987 and 1996 there was a decrease in the total area of heathland by 552 ha (7%) to 7373 ha, a reduction in the number of squares in which heathland occurred by 54 to 3133, and a reduction in the average amount of heathland in each square (2.35 ha/square). The 3110 squares that were ®rst surveyed in 1978 show a steady decline in the area of total heathland (Table 2). Between 1978 and 1987 the loss was 400 ha Table 1 The total area of heathland and associated vegetation types in Dorset 1978, 1987 and 1996a 1978 Area (ha)
1987 Area (ha)
1996 Area (ha)
Dry heath Humid heath Wet heath Peatland Brackish marsh Carr Scrub Acid grassland Sand dunes Tracks and ®rebreaks Ditches and pools Bare soil
2550 1470 844 589 52 196 1010 43 8 303 236 612
2210 1760 842 615 43 211 1170 109 8 351 247 359
2310 1870 466 465 39 134 1370 296 8 195 114 106
Total heathland (Number of squares)
7913 (3110)
7925 (3187)
7373 (3133)
a The estimates for 1978 and 1987 have been recalculated and therefore dier slightly from those published in Webb and Haskins (1980) and Webb (1990).
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(5.1%) out of a total of 7913 ha, and between 1987 and 1996 a further loss of 629 ha (8.4%) from a total of 7513 ha. In 1978, heathland occupied 63.6% of the total land area surveyed but only 55.3% of that same area in 1996. A striking feature of the 1996 Survey is the decline of wet heath and peatland vegetation since 1987 (Table 1). Over the period 1978±1987 there was almost no change in the extent of wet heath and peatland vegetation; however, between 1987 and 1996 wet heath vegetation declined by 376 ha (45%) and peatland vegetation by 150 ha (25%). The greatest gains in area of vegetation were the continued increase of scrub and woodland (scrub increased by 165 ha between 1978 and 1987 and 190 ha between 1987 and 1996; woodland increased by 119 ha between 1978 and 1987 and 472 ha between 1987 and 1996). There was a reduction in the area of (nonagricultural) bare soil from 612 ha in 1978 to 328 ha in 1987 and only 97 ha in 1996. The area of bare soil is closely related to the intensity of disturbance such as grazing and burning (see below). The areas of acid grassland recorded have also increased with each Survey. Using historical information collected by Moore (1962) and our own Surveys, the pattern of the fragmentation of the Dorset heaths can be built up (Fig. 1). Moore (1962) distinguished between fragments greater or less than 4 ha (10 acres). Fig. 1 shows that as the total area of heathland declined the number of heathland fragments >4 ha increased until 1960. After 1960 the number of fragments above this size also began to Table 2 The area of heathland and associated vegetation types in the 3110 squares originally surveyed in 1978 and re-surveyed in 1987 and 1996 1978 Area (ha)
1987 Area (ha)
1996 Area (ha)
Dry heath Humid heath Wet heath Peatland Brackish marsh Carr Scrub Acid grassland Sand dunes Tracks and ®rebreaks Ditches and pools Bare soil
2550 1470 844 589 52 196 1010 43 8 303 236 612
2013 1628 824 600 44 214 1175 103 7 334 243 328
2057 1766 447 450 37 132 1365 227 8 172 126 97
Total heathland (Number of squares)a
7913 (3110)
7513 (3076)
6884 (2947)
Pasture Hedges Woodland
1406 19 1820
1767 35 1939
1599 42 2411
a This ®gure includes the area of some associated vegetation in squares that have no dwarf shrub or acid grassland vegetation (i.e. they are no-longer classi®ed as heathland).
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R.J. Rose et al. / Biological Conservation 93 (2000) 117±125 Table 3 Number, area and isolation of heathland fragments Total number Mean Median Maximum Mean distance of heath area (ha) area (ha) area (ha) to nearest fragments heath (km) 1978 137 1987 142 1996 151
Fig. 1. A bar chart of the extent of heathland recorded in Surveys between 1750 and 1996. The ®gures above the bars indicate the number of fragments of heathland >4 ha that existed (data from the three ITE Dorset Heathland Surveys, 1978, 1987 and 1996 and historical data from Moore, 1962).
decline although the total number of fragments continued to increase (see below). In addition, the number of heaths >10 ha (the minimum qualifying size suggested by the Nature Conservancy Council (1989), for the designation of heathland Sites of Special Scienti®c Interest) decreased from 61 and 62 in 1978 and 1987 to 51 in 1996. The number of heathland fragments increased from 137 in 1978 to 142 in 1987 and 151 in 1996 (Table 3). The mean fragment size decreased steadily with the increasing number of fragments and the median size declined markedly. Much of this change is due to the loss of heathland but also, where heathland has increased, perhaps by management, there has been some amalgamation of heaths. Hence, the largest heathland fragment has increased in area in each Survey. The large sites (>10 ha) represent over 96% of the total heathland area. The mean distance between heaths has decreased because fragmentation of continuous heath results in several, small, closely adjacent heaths. One way of looking at heathland fragmentation and change at a landscape level is to use the subsets of the data that include those squares that were not recorded as heath in 1978 (`new' squares) and those squares that were heathland in 1978 but have subsequently become other land use types (`lost' squares) see Tables 4 and 5. The `new' heathland squares recorded in 1987 added 506 ha of heathland to the total area for Dorset. This additional area of heath more than compensated for the reduction of 400 ha in the original heathland squares recorded in 1978. In 1996 this same set of squares contained 375 ha of heathland (a 26% decrease) indicating that marginal and small patches of isolated heathland vegetation are more susceptible to change. The 294 'new' heathland squares added to the database in 1996 increased the total area of heathland by 391 ha,
57.2 55.4 48.6
6.6 6.1 3.5
686.2 733.5 990.0
0.25 0.23 0.21
Table 4 The area of heathland and associated vegetation types in the new heathland squares surveyed in 1987 and re-surveyed 1996 and the new heathland squares surveyed in 1996 New squares 1987 (1987 data) Area (ha)
New squares 1987 (1996 data) Area (ha)
New squares 1996 Area (ha)
Dry heath Humid heath Wet heath Peatland Brackish marsh Carr Scrub Acid grassland Sand dunes Tracks and ®rebreaks Ditches and pools Bare soil
193 128 17 15 0.2 6.7 53 5.3 0.9 31 13 43
132 55 8.8 5.3 0.8 7.5 109 27 0.1 14 3.6 12
122 52 10 11 8 10 93 42 0 27 6 10
Total heathland (Number of squares)
506 (250)
375 (243)
391 (294)
although the overall total area of heathland decreased by 552 ha; therefore, the 1987 heathland squares have lost 943 ha during the 9 year period between Surveys. These `new' squares add a signi®cant proportion of heathland to the overall total but their quality, in terms of heathland per square and dwarf shrub and peatland content, is lower than that of the 1978 Heathland Survey squares. Table 5(a) details the squares from which heathland was lost between 1978 and 1987; the loss of 173 squares that contained 381 ha of heathland can be mostly attributed to an increase of pasture, (140 ha) and `other land' (urban and industrial development, roads etc.) (100 ha). The remainder of the loss is due to the associated heathland vegetation types (carr, scrub, bare soil and open water) no longer being classi®ed in the Survey as part of the total heathland area. In 1996, on 48 of these squares (28.2%) some heathland vegetation had re-established (59 ha); however, the amount of heath in the square and the proportion of dwarf shrub vegetation
R.J. Rose et al. / Biological Conservation 93 (2000) 117±125
present in 1996 was on average much lower than the 1978 values. Between 1987 and 1996 there was a further loss of 373 squares [Table 5(b)] including 525 ha of heathland of which 137 ha was to other land types and 149 ha to Table 5 (a) Vegetation types in the 173 squares surveyed as heathland in 1978 that were not heathland in 1987 (48 of which contained a proportion of heathland in 1996); (b) Vegetation types in the 373 squares surveyed as heathland in 1978 and 1987 that were not heathland in 1996a 1978 Area (ha)
1987 Area (ha)
1996 Area (ha)
Dry heath Humid heath Wet heath Peatland Brackish marsh Carr Scrub Sand dunes Acid grassland Bare soil Open water
95 49 33 14 ± 10 57 ± 1.8 106 15
± ± ± ± ± 8.7 50 ± ± 70 13
19 8 0.2 0.2 ± 0.4 44 ± 3.1 15 11
Total heathland (Number of squares)
381 (173)
± (0)
59b (48)
Pasture Hedges Woodland Other land
136 3.5 89 71
276 3.9 88 171
232 2.9 110 233
Dry heath Humid heath Wet heath Peatland Brackish marsh Carr Scrub Sand dunes Acid grassland Bare soil Open water
126 82 45 28 11 27 133 ± 9.3 104 56
82 56 26 20 7.3 30 150 ± 5.9 96 52
± ± ± ± ± 11 155 ± ± 48 48
Total heathland (Number of squares)
621 (373)
525b (373)
0 (0)
Hedges Woodland Pasture Other land
2.0 31 360 209
4.2 335 419 206
4.7 484 399 343
(a)
(b)
a Note: In addition seven squares recorded as heathland in 1987 for the ®rst time were recorded as non-heathland in 1996. Five of these squares were re-planted forestry where heather had re-established by 1987 after felling but were understorey vegetation by 1996. The other two squares were minimum areas (0.2 ha) of heath in 1987 on the edge of larger heathland blocks. b The area of the following vegetation types (brackish marsh, carr, scrub, sand dunes, bare soil and open water) is only included in this ®gure where dry heath, humid heath, wet heath, peatland or acid grassland also occur within the same square.
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woodland. The associated heathland vegetation types remained relatively constant in area and contributed 262 ha to the overall loss. In addition seven squares recorded as heathland in 1987 for the ®rst time were recorded as non-heathland in 1996. Five of these squares were re-planted forestry where heather had reestablished by 1987 after felling but were under-storey vegetation and therefore no longer classi®ed as heathland by 1996. The other two squares were minimum areas (0.2 ha) of heath in 1987 on the edge of larger heathland blocks that were probably recorded as being within one of the adjacent squares in 1996. In 1960 about 600 ha (5.1% of the total area of heathland) of the heaths were grazed (Moore, 1962) and the area declined to 290 ha (3.7%) by 1987 (Webb, 1990); however, we recorded 561 ha (7.6%) being grazed in 1996, which reverses this trend. In 1996 a square was recorded as being grazed only when an animal was present or if there was evidence that animals had been present recently (i.e. grazed or trampled vegetation, dung or poached ground). Therefore, despite a number of recent initiatives to reintroduce grazing to some of the heaths, the intensity of grazing in 1996 was very low. The ®gure of 7.6% is an underestimate of the total area of heathland fenced for grazing animals, but is an accurate estimate of the area where there are visible signs that the vegetation is being grazed. There has been a dramatic decline in the area of recently burnt heathland between the three Surveys. The following ®gures are derived from the area of bare soil and the proportion of pioneer heath that was burnt within the 2 years preceding the Survey. Therefore, they are twice the annual rates of burning. The ®gure calculated for 1978 was 945 ha covering 656 squares and represents 12% of the total heathland area at that time giving an average annual burning rate of 6%. The overall ®gure is high, not only because it includes the extensive areas burnt in the very dry summer of 1976 but also a large area of Godlingston Heath (burnt in 1974) was also recorded as being recently burnt. If this area is excluded from the analysis a ®gure of 775 ha (9.8% of the total area of heath) is obtained. This is still considerably higher than the 382 ha (293 squares, 4.8% of the total area of heath) in 1987 and only 85 ha (121 squares, 1.2% of the total area of heath) in 1996. The percentage cover of the three main vegetation phases for the three Surveys is given in Table 6. The ®gures show that in each survey most of the heathland is in the mature (>16 years) phase with smaller amounts of vegetation in the building (6±15 years old) and pioneer (0±5 year old) phases. The higher proportion of building phase heather and correspondingly lower proportion of mature heath in 1987 re¯ects the re-growth of heather after the 1976 ®res. This eect is not seen in the 1978 pioneer phase because much of the area burnt in 1976 was classi®ed as recently burnt bare ground.
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R.J. Rose et al. / Biological Conservation 93 (2000) 117±125
4. Discussion The gross area of heathland in Dorset remained constant from 1978 until 1987 but during the 9 years preceding 1996 there was a 7% decline in the area of some 552 ha (Table 1). Planning legislation now controls direct losses of heathland. The losses we have observed have been caused by vegetation succession with many areas of open heathland changing to scrub and woodland. However, the picture is complex, as at some locations heathland has been lost as a result of succession to scrub and trees while at others heathland has regenerated although there is a downward trend in the area of heathland within each heathland square. Analysis of the 3110 squares ®rst surveyed in 1978 show a steady decline in the total area of heathland and a steady increase in the amount of scrub. The rate of encroachment of scrub and trees on to open heathland is about 1.7% per year; this is equivalent to 15% over the 9 years from 1987 to 1996, a rate similar to that between 1978 and 1987. Conservation management has done no more than contain the rate of succession that prevailed between the two earlier Surveys. The decline in controlled burning has undoubtedly been the principal factor leading to this growth of scrub. In his 1962 paper Moore said `Probably the most important ecological eect of the decline in rough grazing has been the virtual extinction of controlled burning that used to accompany it'. The decline identi®ed by Moore (1962) has continued and the 1978 Survey showed that both grazing and burning had been largely abandoned as the principal methods of vegetation management (Webb, 1990). Over the same period the control and management of many of these heaths has passed out of the hands of farmers in to those of conservation agencies. Recently, agric-environment schemes have been introduced as a conservation measure to compensate for the decline in traditional management. Free-range grazing (mostly cattle and ponies) has also been introduced on a number of nature reserves. This type of grazing will maintain existing open heathland, but will not recover areas invaded by scrub. Ponies may be eective in curtailing the development of scrub as they graze the young shoots of gorse Ulex europaeus and birch Betula spp., but invasion by pine Pinus spp. is unlikely to be checked by the animals. During the very dry summers of 1975 and 1976, 775 ha (9.8%) of the Dorset heathlands were burnt. The occurrence of these large ®res led to the introduction of stringent protection measures and an almost complete cessation of controlled burning. Surprisingly, no adequate alternative form of vegetation management was substituted. As a consequence, at the present time, these heaths carry extensive areas of mature heather and increasing amounts of scrub. This vegetation provides a greater quantity of fuel and when ®res do occur they are
hotter and more intense (Allchin, 1997). The increased temperatures attained during these ®res may impede the recovery of heathland vegetation and result in the establishment of an increased number of invasive species. The situation is analogous to that in many ®re-dominated communities in other parts of the world, especially in North America (e.g. Keiter and Boyce, 1991). Here also, ®re protection measures have disrupted natural cycles of ®re and regeneration to which the vegetation was adapted. Ultimately excessive ®re protection has proved harmful to the regeneration of the vegetation. Controlled burning aims to maintain a mosaic of different aged stands of heather and a range of habitats. To maintain a 25-year cycle by ®re management 4% of the total area of heathland (295 ha in 1996) should be burnt annually. This rotation would result in a number of stands of dierent ages of which 20% was either bare soil or in the post-burn or pioneer phase, 40% building phase and 40% mature heathland. In each of the survey years the proportion of mature heath has been considerably higher than the theoretical ideal (Table 6). In 1978 the combined area of bare soil and pioneer heath (17.8%) was less than the prescribed ®gure and in 1987, when this same area of vegetation was in the building phase, it fell short of the target of 40%. In 1996 the proportions of bare soil and the three main vegetation phases were close to matching a 60-year burning cycle where 8.3% was either bare soil or in the pioneer phase, 16.7% in the building phase and 75% was mature heathland. This cycle assumes that each area is burnt in strict rotation. However, most of the ®res that do occur are not managed (wild-®res) and tend to be most frequent on the heaths in the urban fringes, whilst many heaths may not be burnt for considerably longer than the average rotation of 60 years. There is now a clear case for the reinstatement of programmes of controlled burning because other methods of management have failed to check succession to scrub and woodland. The total number of individual heaths has continued to rise and their size has decreased (Table 3). The median heath size in 1996 is now below the 4 ha minimum size unit used by Moore (1962) in his analysis of the
Table 6 The total areas of bare soil and dwarf shrub heathland (dry heath, humid heath, wet heath and peatland) in each of the three main vegetation phases Year Bare soil Area % (ha) 1978 1987 1996
612 359 106
Pioneer (0±5 years)
Building (6±15 years)
Mature (over 16 years)
Area (ha)
%
Area (ha)
%
Area (ha)
%
7.8 5.7 5.2
940 1510 922
15.5 26.1 17.7
4039 3585 3919
66.6 62.0 75.1
10.1 474 6.2 332 2.0 270
R.J. Rose et al. / Biological Conservation 93 (2000) 117±125
fragmentation of the Dorset heathlands (see Fig. 1.). Using the 10 ha minimum size of site suggested for quali®cation as a Site of Special Scienti®c Interest (Nature Conservancy Council, 1989), 11 of 62 sites fell below this threshold between 1987 and 1996. These sites are still noti®ed because in Dorset most Sites of Special Scienti®c Interest are noti®ed for reasons other than habitat area such as the presence of nationally rare or nationally scarce species. The degree of isolation of these small heaths is not great; indeed, the spatial separation of the fragments shows a small decrease in each of the surveys. However, the proportion of heathland in the vicinity of each individual fragment is declining, and likewise the opportunities for dispersal by heathland species is reduced. The transitory nature of heathland is demonstrated in Tables 4 and 5 which give details of those squares from which heathland has been lost and those where it has been established or re-established. These squares are often heathland edge squares that are important not only for their own heathland content but as buer zones for the other squares within the heathland block. They are often the closest points to adjacent heaths and form natural links with other blocks of squares. Therefore, in terms of fragmentation and encroachment of other vegetation types they are an important part of the heathland landscape. The loss of 100 ha of heathland between 1987 and 1996 to urban and industrial development, roads etc. is an irreversible change. However, losses to mineral extraction, agricultural improvement and aorestation (140 ha between 1987 and 1996) may be reversed using existing re-establishment procedures (Putwain and Rae, 1988). A further 140 ha that has been lost to scrub and semi-natural woodland could be re-claimed as heathland using good heathland management practice (Gimingham, 1992). The gains are squares where heathland has either naturally regenerated or has been restored. These areas include abandoned open-cast mineral workings, felled woodland, cleared scrub and abandoned agricultural land. Some of these areas are being managed as heathland for restoration, others are reverting naturally to heathland, and some in the felled woodland category are a short-term transition between tree crops. In general these squares have a relatively small proportion of dry and humid heath and rarely any wet heath or peatland. There is often a high proportion of other vegetation or bare ground and, as can be seen from the 1987 survey `new squares' they are particularly prone to the re-invasion by other species. The 1996 Survey revealed dramatic changes in the composition of the heathland vegetation with a reduction in the area of both peatland and wet heath vegetation types. These changes may be attributed to a number of environmental factors. On Dutch heathlands ericaceous communities have been replaced by grassdominated communities as a result of inputs of nitrogen
123
in excess of the critical loads of 15±20 kgN haÿ1 yearÿ1 for dry heath and 17±22 kgN haÿ1 yearÿ1 for wet heath (Bobbink et al., 1992). The estimated current deposition for the Dorset heaths is 16 kg N haÿ1 yearÿ1 (Pitcairn et al., 1995) which is just below the critical load for these heaths. However, the response of vegetation to increased inputs of nitrogen on the Dorset heaths is limited by the low availability of phosphorus in these soils (Chapman et al., 1989b). Changes in hydrological conditions are a more likely explanation than increases of inputs of atmospheric nitrogen for the observed vegetation changes. Three factors may be responsible. First, after 20 years of unmanaged growth the heathlands carry a larger standing crop of vegetation with a greater structural diversity. As a consequence, both interception of precipitation and evapo-transpiration may have increased and less water reaches the wet heath and peatland communities. Secondly, rainfall in Dorset has shown a downward trend, particularly in the summer months, during the last decade (Paxman, 1992). Thirdly, land use changes, such as the growth of coniferous plantations, outside of the heathlands yet within the heathland catchments may aect ground water levels and lateral ¯ows that are essential to the maintenance of wet and valley mire communities. The most remarkable feature of these changes is the relatively rapid response, detectable over the entire Dorset heathland landscape, of the heathland vegetation. The expectation might have been that such a response would be slow in view of the low growth rates of the plants. It also remains an interesting point for speculation whether or not a series of wet years would result in a simple reversal of these vegetation changes or whether the recovery of wet heath and peatland vegetation would show a hysteresis eect or for the vegetation to develop in a new direction. The rate at which wet heath and peatland vegetation would recover or change is also a matter for speculation. Throughout the 20 years of the Dorset Heathland Survey the value of using a comparable recording protocol and a repeatable methodology for each Survey has been demonstrated. It is by using this approach that change can be detected successfully and accurately. Nevertheless, the analysis of change becomes increasingly complex as new data are added to the Survey. The dynamic nature of the biotope becomes increasingly evident and it is more dicult to distinguish between changes arising from succession, those due to management and those induced by external factors such as climate and pollution. The relatively rapid response, equivalent to the rate of successional change and detectable at the landscape scale, which the Dorset Heathlands have shown to changes in the weather is, perhaps, the most interesting feature of the new Survey. If the trend for less rainfall and drier summers continues then the conservation of the wet heath and peatland
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R.J. Rose et al. / Biological Conservation 93 (2000) 117±125
communities may not only depend on the management vegetation but on controlling the hydrological conditions that are required. This provides a basis for further experimental study.
Acknowledgements We are especially grateful to the Royal Society for the Protection of Birds for their interest in and generous support for this project both in the provision of funds and for deploying members of their Dorset Heathland Project Team for the 1996 ®eld Survey. In particular, we thank John Waldon, RSPB Regional Ocer for south west England and Nigel Symes, Manager of the RSPB Dorset Heathland Project. We are also grateful for ®nancial support and advice from English Nature and the Dorset County Council. In particular we thank Jim White and Dr Andrew Nicholson of English Nature, and Dr Phil Stirling of Dorset County Council. Appendix A Non-heathland land These three attributes were added as main cover land types, Arable, Urban/Industrial and Other were added to record the areas of arable, urban/industrial and other land within the square. The previous surveys did not record these categories directly and they were added to improve the accuracy of all estimates of area and thereby those of the heathland land types. Potential heathland Codes for the area and most common species within the existing scrub, woodland, grassland and bare ground categories were added and the most common understorey species in woodland noted. Notable species Twelve species were added to the list of species recorded, these were; petty whin (Genista anglica), silver-studded blue butter¯y (Plebejus argus), sand lizard (Lacerta agilis), smooth snake, (Coronella austriaca), Dartford warbler (Sylvia undata), woodlark (Lullula arborea), stonechat (Saxicola torquata), skylark (Alauda arvensis), linnet (Carduelis cannabina), lapwing (Vanellus vanellus), reed bunting (Emberiza schoeniculus) and snipe (Galinago galinago). These species were recorded if seen or heard within the square but no systematic search was made for them.
Heathland and potential heathland management The original heathland grazing attribute was retained but to cover recent changes in heathland management practices sub-categories for sheep, cattle and horses were added. These attributes were repeated for the grazing of potential heathland for which an additional code for fencing was also added. Reverting land Codes for scattered heather and felled+scattered heather were added to the grassland and woodland vegetation types and litter was added to the bare ground category. Invasive species The introduced shrub, shallon (Gaultheria shallon) was added to the dry heath, scrub and woodland categories. References Allchin, E.A., 1997. Vegetation dynamics following management burning of lowland heathland. PhD Thesis, University of Liverpool, 208 pp. Anon, 1995. Biodiversity: The UK Steering Group Report, Vol. 2 Action Plans, HMSO, London. Auld, M., Davis, S., Pickess, B., 1992. Restoration of lowland heaths in Dorset. RSPB Conservation Review No. 6 1992. Royal Society for the Protection of Birds, Sandy, pp. 68±73. Bannister, P., 1965. Biological Flora of the British Isles. Erica cinerea L.. Journal of Ecology 53, 527±542. Bannister, P., 1966. Biological Flora of the British Isles. Erica tetralix L.. Journal of Ecology 54, 795±813. Bobbink, R., Boxman, D., Fremstad, E., Heil, G., Houdijk, A., Roelofs, J., 1992. Critical loads for nitrogen eutrophication of terrestrial and wetland ecosystems based upon changes in vegetation and fauna. In: Greenfelt, P., ThoÈrneloÈf, E. (Eds.), Critical Loads for NitrogenÐA Workshop Report. ThemaNord 1992/42. Nordic Council of Ministers, Copenhagen, pp. 113±159. Bullock, J.M., Webb, N.R., 1995. Response to severe ®res in heathland mosaics in southern England. Biological Conservation 73, 207±214. Chapman, S.B., Webb, N.R., 1978. The productivity of Callunaheathland in southern England. In: Heal, O.W., Perkins, D.F. (Eds.), The Ecology of some British Moors and Montane Grasslands. Springer-Verlag, Berlin, pp. 246±262. Chapman, S.B., Clarke, R.T., Webb, N.R., 1989a. The survey and assessment of heathland in Dorset for conservation. Biological Conservation 47, 137±152. Chapman, S.B., Rose, R.J., Bassanta, M., 1989b. Phosphorus adsorption by soils from heathlands in southern England in relation to successional change. Journal of Applied Ecology 26, 673±680. Diemont, W.H., Degn, H.J., Webb, N.R., 1996. A pan-European view on heathland conservation. Proceedings National Heathland Conference 1996. English Nature, Peterborough, pp. 21±32. Gimingham, C.H., 1972. Ecology of heathlands. Chapman and Hall, London. Gimingham, C.H., 1992. The lowland heathland management handbook. English Nature, Peterborough. Keiter, R.B., Boyce, M.S. (Eds.), 1991. The Greater Yellowstone Ecosystem. Yale University Press, New Haven. Moore, N.W., 1962. The heaths of Dorset and their conservation. Journal of Ecology 50, 369±391.
R.J. Rose et al. / Biological Conservation 93 (2000) 117±125 Nature Conservancy Council, 1989. Guidelines for Selection of Biological SSSIs. Nature Conservancy Council, Peterborough. Paxman, D.J., 1992. A downward trend in rainfall. Proceedings of the Dorset Natural History and Archaeological Society 113, 118±119. Pitcairn, C.E.R., Fowler, D., Grace, J., 1995. Deposition of ®xed atmospheric nitrogen and foliar nitrogen content of bryophytes and Calluna vulgaris (L.) Hull. Environmental Pollution 88, 193± 205. Putwain, P.D., Rae, P.A.S., 1988. Heathland Restoration: A Handbook of Techniques. British Gas (Southern), Southampton. Rebane, M., Wynde, R., Diemont, W.H., Jensen, F.P., Pahlsson, L., Webb, N.R., 1997. Lowland atlantic heathland. In: Tucker, G.M., Evans, M.I. (Eds.), Habitats for Birds in Europe: A Conservation Strategy for the Wider Environment. (Birdlife Conservation series No. 6). Birdlife, Cambridge, pp. 187±202. Rose, R.J., Bannister, P., Chapman, S.B., 1996. Biological Flora of the British Isles. Erica ciliaris L.. Journal of Ecology 84, 617±628. Rose, R. J., Webb, N.R., 1995. Heathland re-creation plan for Dorset. English Nature/Natural Environment Research Council. Thomas, J.A., Rose, R.J., Clarke, R.T., Thomas, C.D., Webb, N.R., in
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press. Intraspeci®c variation in habitat availability among ectothermic animals near their climatic limits and the centres of their ranges. Functional Ecology. Watt, A.S., 1955. Bracken versus heather, a study in plant sociology. Journal of Ecology 43, 281±296. Webb, N.R., 1990. Changes on the heathlands of Dorset, England between 1978 and 1987. Biological Conservation 51, 272±286. Webb, N.R., 1998. History and ecology of European heathlands. Transactions of the Suolk Naturalists' Society 34, 1±8. Webb, N.R., Haskins, L.E., 1980. An ecological survey of heathlands in the Poole Basin, Dorset, England in 1978. Biological Conservation 17, 281±296. Webb, N.R., Thomas, J.A., 1994. Conserving insects habitats in heathland biotopes: a question of scale. In: Edwards, P.J., May, R.M., Webb, N.R. (Eds.), Large Scale Ecology and Conservation Biology. Blackwell Scienti®c Publications, Oxford, pp. 129±151. Woodrow, W., Symes, N., Auld, M., Cadbury, J., 1996. Restoring Dorset's heathland: the RSPB Dorset heathland project. RSPB Conservation Review No.10 1996. Royal Society for the Protection of Birds, Sandy, pp. 69±81.
www.elsevier.com/locate/biocon
Changes on the heathlands in Dorset, England, between 1987 and 1996 R.J. Rose, N.R. Webb*, R.T. Clarke, C.H. Traynor Furzebrook Research Station, NERC Institute of Terrestrial Ecology, Wareham, Dorset BH20 5AS, UK Received 22 April 1998; accepted 4 March 1999
Abstract In 1996 all of the heathland in south-east Dorset, southern England, was surveyed using the same recording protocols as those used in surveys in 1978 and 1987. This approach enabled the extent of the heathlands, the degree of fragmentation, and the composition of the vegetation to be compared at a landscape scale over a period of 18 years. Between 1987 and 1996 the number of heathland patches increased from 142 to 151 and the total area of heathland decreased by 552 ha from 7925 ha in 1987. The principal cause of this loss is succession to scrub and woody vegetation, which continues at a rate of 1.7% per year despite conservation management. The most signi®cant vegetation change was a decline in the area of the vegetation types, wet heath and peatland. These had remained constant between 1978 and 1987, but declined by 376 ha (45%) and 150 ha (25%) respectively between 1987 and 1996. Although there is no direct evidence, it is speculated that changing weather patterns or pollution might be the cause. These heaths, like many in Western Europe, are subject to a wide range of national and international legislative instruments designed to ensure their conservation. These measures call for the monitoring of the status of the conserved areas. This study, which has extended over almost 20 years, provides an insight in to the problems of monitoring change in a biotope of high conservation status. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Conservation; Landscape; Monitoring; Succession; Survey
1. Introduction Heathland is one of the cultural landscapes of western Europe in which, over several millennia, human activities have created and maintained extensive areas of dwarf shrub (subshrub) vegetation. Formerly, heathland extended over several million hectares but today its has been reduced to <10% of its former extent by aorestation, conversion to agricultural land and by urban and industrial development. The best estimates suggest that some 350,000 ha of heathland remains in western Europe (Diemont et al., 1996; Webb, 1998). This remaining heathland is subject to a wide range of international and national conservation designations. Rebane et al. (1997) summarise the international, environmental legislation which aects European heathlands and note that ®ve international, two pan-European and seven European Union legislative instruments are of
* Corresponding author. Fax: +44-1929-551087. E-mail address: [email protected] (N.R. Webb).
importance. The most signi®cant are the EU Directive on the conservation of wild birds (79/409/EEC) and the Directive on the conservation of habitats of wild fauna and ¯ora (92/43/EEC). The habitats directive lists four habitat types that fall within the de®nition of lowland Atlantic heathlands, two of which are priority habitats. Conservation designations such as these call for the favourable conservation status of a habitat to be de®ned and for movement away from this status to be monitored. Where movement away from the favourable conservation status is identi®ed management must be implemented to restore the former status. The heathlands formed on the sandy Tertiary soils of south-east Dorset in southern England have been the subject of regular survey over the last 40 years. These surveys document the rates of loss, the eects of fragmentation and isolation on their biota, and patterns of change in the vegetation (Moore, 1962; Webb and Haskins, 1980; Webb, 1990). The detailed Surveys (strictly censuses) in 1978 and 1987 (Webb and Haskins, 1980; Webb, 1990) make up a database called the Dorset Heathland Survey, which is one of the few studies in
0006-3207/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0006-3207(99)00047-6
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R.J. Rose et al. / Biological Conservation 93 (2000) 117±125
which vegetation change has been measured at the landscape scale in the United Kingdom. These data have been used to estimate the loss of heathland, to describe changes in the composition of the vegetation within the heathland landscape following ®res (Bullock and Webb, 1995), and to make species-based analyses on habitat availability, occupancy and metapopulation dynamics (Webb and Thomas, 1994; Thomas et al., in press). Between the 1978 and 1987 Surveys two factors were identi®ed as the cause of change on the Dorset Heathlands. First, there were direct losses of heathland caused by conversion to farmland, forestry, urban and industrial development. Secondly, there was succession to scrub and woodland, which showed a 15% increase over this period (Webb, 1990). Compared with losses to succession the direct losses were small. Conversion to farmland ceased when subsidies were withdrawn and urban and industrial development is now controlled through planning and environmental legislation. Over this period losses to successional change exceeded that from direct loss to other land uses. This was because traditional management, principally burning and grazing, had almost ceased and had not been replaced by other forms of vegetation management. Because of these changes programmes of conservation management were implemented (Auld et al., 1992; Woodrow et al., 1996). In 1996 a new census was made using the same protocols. This paper summarises the quantitative changes in both the total stock of heathland and the relative areas of the dierent heathland vegetation types and associated vegetation for the period 1987±1996, and reviews them in the wider context of the changes over the last 18 years. 2. Methods The methods used for the Dorset Heathland Survey have been described previously (Webb and Haskins, 1980; Chapman et al., 1989a). Records were collected between May and October 1996 from all 200200 m grid squares derived from the Ordnance Survey National Grid containing heathland. Within each grid square, the cover of heathland and associated vegetation types, the age and structure of the heathland vegetation, the presence of selected rare species, topography and land use were recorded as attributes on a 3-point cover-abundance scale (1=1±10% cover; 2=>10±50% cover; 3=>50% cover). All squares surveyed in 1978 (3110) and 1987 (3360) were re-surveyed even if they no longer contained heathland. New heathland squares were included in 1987 and 1996 if they had within their boundaries a proportion of dry heath, humid heath, wet heath, peatland or acid grassland. In addition, areas
identi®ed as being of high potential for the re-creation of heathland (Rose and Webb, 1995) to meet national biodiversity targets (Anon, 1995) were surveyed. The 1996 Survey extended over 3993 grid squares representing a total land area of 15,972 ha. In the previous surveys 184 attributes were recorded (Chapman et al., 1989a), but for the 1996 Survey 40 attributes were added (Appendix 1). These attributes were used to improve the estimates of the area of nonheathland land within the squares surveyed (3 attributes), to describe the potential heathland areas (12 attributes), to provide a census of the distribution of key bird and other species found on heathland and associated habitats (12 attributes), to record recent changes in heathland management practices (7 attributes) and to improve the description of reverting land (3 attributes) and invasive species (3 attributes). This will enable future surveys to quantify the success or failure of heathland re-creation schemes and current management plans. In all three Surveys, cover scores were converted to estimates of area using an algorithm (Chapman et al., 1989a). In 1978 and 1987, the area of the main vegetation types within each square was estimated by applying this algorithm to the four dwarf-shrub heathland types (dry heath, humid heath, wet heath and peatland), and nine associated main cover-types (brackish marsh, carr, scrub, hedges and boundaries, woodland, grassland, bare ground, sand dunes and open water). The bare ground cover-type corresponds to the recently burnt categories of Chapman et al., (1989a) and Webb (1990) but in this paper the term `recently burnt' refers to the area of bare soil and pioneer heathland vegetation that is in the very early stages of re-growth where charred remains indicate that the area was burnt within the 2 years preceding the Survey. In 1996 the algorithm was modi®ed to incorporate three new attributes; arable, urban/industrial and `other' land. The area of each sub-category of covertype within a main cover-type was then estimated using the modi®ed algorithm. These ®gures were used to calculate the total area of heathland as de®ned in previous Surveys. Traditionally, four growth phases of Calluna vulgaris have been recognised (Watt, 1955; Gimingham, 1972), but in the 1996 Survey we recognised a phase during which vegetation establishes from seed or after burning (the pioneer phase of Gimingham (1972) and the postburn phase of Chapman and Webb (1978)) which lasts between 2 and 5 years: a building phase similar to that of Gimingham (1972) and a mature phase incorporating the mature and degenerate phases of Gimingham (1972). The combination of the mature and degenerate phases was necessary because the typical structure of degenerate Calluna bushes is not common on the Dorset Heaths even after 35 years and the other dwarf shrubs,
R.J. Rose et al. / Biological Conservation 93 (2000) 117±125
Erica cinerea, E. tetralix and E. ciliaris, do not produce stems of this longevity (Bannister, 1965, 1966; Rose et al., 1996). The number of fragments was calculated for each Survey using the procedure described by Chapman et al. (1989a). These calculations exclude four outlying heaths to the west of the main block of heathland in south-east Dorset. 3. Results There was little dierence in the estimates of heathland area derived from the old and modi®ed algorithms. Therefore, throughout this paper only the modi®ed algorithm was used. Although these ®gures dier slightly from those published previously, they re¯ect more accurately the area of each land-type in the Survey area (Table 1). Between 1978 and 1987 the total area of heathland showed little change from 7913 to 7925 ha. In 1987 heathland was found in 77 more squares indicating a slight reduction in the average proportional cover of heathland in each square (2.54 ha/square in 1978 and 2.49 ha/square in 1987). Between 1987 and 1996 there was a decrease in the total area of heathland by 552 ha (7%) to 7373 ha, a reduction in the number of squares in which heathland occurred by 54 to 3133, and a reduction in the average amount of heathland in each square (2.35 ha/square). The 3110 squares that were ®rst surveyed in 1978 show a steady decline in the area of total heathland (Table 2). Between 1978 and 1987 the loss was 400 ha Table 1 The total area of heathland and associated vegetation types in Dorset 1978, 1987 and 1996a 1978 Area (ha)
1987 Area (ha)
1996 Area (ha)
Dry heath Humid heath Wet heath Peatland Brackish marsh Carr Scrub Acid grassland Sand dunes Tracks and ®rebreaks Ditches and pools Bare soil
2550 1470 844 589 52 196 1010 43 8 303 236 612
2210 1760 842 615 43 211 1170 109 8 351 247 359
2310 1870 466 465 39 134 1370 296 8 195 114 106
Total heathland (Number of squares)
7913 (3110)
7925 (3187)
7373 (3133)
a The estimates for 1978 and 1987 have been recalculated and therefore dier slightly from those published in Webb and Haskins (1980) and Webb (1990).
119
(5.1%) out of a total of 7913 ha, and between 1987 and 1996 a further loss of 629 ha (8.4%) from a total of 7513 ha. In 1978, heathland occupied 63.6% of the total land area surveyed but only 55.3% of that same area in 1996. A striking feature of the 1996 Survey is the decline of wet heath and peatland vegetation since 1987 (Table 1). Over the period 1978±1987 there was almost no change in the extent of wet heath and peatland vegetation; however, between 1987 and 1996 wet heath vegetation declined by 376 ha (45%) and peatland vegetation by 150 ha (25%). The greatest gains in area of vegetation were the continued increase of scrub and woodland (scrub increased by 165 ha between 1978 and 1987 and 190 ha between 1987 and 1996; woodland increased by 119 ha between 1978 and 1987 and 472 ha between 1987 and 1996). There was a reduction in the area of (nonagricultural) bare soil from 612 ha in 1978 to 328 ha in 1987 and only 97 ha in 1996. The area of bare soil is closely related to the intensity of disturbance such as grazing and burning (see below). The areas of acid grassland recorded have also increased with each Survey. Using historical information collected by Moore (1962) and our own Surveys, the pattern of the fragmentation of the Dorset heaths can be built up (Fig. 1). Moore (1962) distinguished between fragments greater or less than 4 ha (10 acres). Fig. 1 shows that as the total area of heathland declined the number of heathland fragments >4 ha increased until 1960. After 1960 the number of fragments above this size also began to Table 2 The area of heathland and associated vegetation types in the 3110 squares originally surveyed in 1978 and re-surveyed in 1987 and 1996 1978 Area (ha)
1987 Area (ha)
1996 Area (ha)
Dry heath Humid heath Wet heath Peatland Brackish marsh Carr Scrub Acid grassland Sand dunes Tracks and ®rebreaks Ditches and pools Bare soil
2550 1470 844 589 52 196 1010 43 8 303 236 612
2013 1628 824 600 44 214 1175 103 7 334 243 328
2057 1766 447 450 37 132 1365 227 8 172 126 97
Total heathland (Number of squares)a
7913 (3110)
7513 (3076)
6884 (2947)
Pasture Hedges Woodland
1406 19 1820
1767 35 1939
1599 42 2411
a This ®gure includes the area of some associated vegetation in squares that have no dwarf shrub or acid grassland vegetation (i.e. they are no-longer classi®ed as heathland).
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R.J. Rose et al. / Biological Conservation 93 (2000) 117±125 Table 3 Number, area and isolation of heathland fragments Total number Mean Median Maximum Mean distance of heath area (ha) area (ha) area (ha) to nearest fragments heath (km) 1978 137 1987 142 1996 151
Fig. 1. A bar chart of the extent of heathland recorded in Surveys between 1750 and 1996. The ®gures above the bars indicate the number of fragments of heathland >4 ha that existed (data from the three ITE Dorset Heathland Surveys, 1978, 1987 and 1996 and historical data from Moore, 1962).
decline although the total number of fragments continued to increase (see below). In addition, the number of heaths >10 ha (the minimum qualifying size suggested by the Nature Conservancy Council (1989), for the designation of heathland Sites of Special Scienti®c Interest) decreased from 61 and 62 in 1978 and 1987 to 51 in 1996. The number of heathland fragments increased from 137 in 1978 to 142 in 1987 and 151 in 1996 (Table 3). The mean fragment size decreased steadily with the increasing number of fragments and the median size declined markedly. Much of this change is due to the loss of heathland but also, where heathland has increased, perhaps by management, there has been some amalgamation of heaths. Hence, the largest heathland fragment has increased in area in each Survey. The large sites (>10 ha) represent over 96% of the total heathland area. The mean distance between heaths has decreased because fragmentation of continuous heath results in several, small, closely adjacent heaths. One way of looking at heathland fragmentation and change at a landscape level is to use the subsets of the data that include those squares that were not recorded as heath in 1978 (`new' squares) and those squares that were heathland in 1978 but have subsequently become other land use types (`lost' squares) see Tables 4 and 5. The `new' heathland squares recorded in 1987 added 506 ha of heathland to the total area for Dorset. This additional area of heath more than compensated for the reduction of 400 ha in the original heathland squares recorded in 1978. In 1996 this same set of squares contained 375 ha of heathland (a 26% decrease) indicating that marginal and small patches of isolated heathland vegetation are more susceptible to change. The 294 'new' heathland squares added to the database in 1996 increased the total area of heathland by 391 ha,
57.2 55.4 48.6
6.6 6.1 3.5
686.2 733.5 990.0
0.25 0.23 0.21
Table 4 The area of heathland and associated vegetation types in the new heathland squares surveyed in 1987 and re-surveyed 1996 and the new heathland squares surveyed in 1996 New squares 1987 (1987 data) Area (ha)
New squares 1987 (1996 data) Area (ha)
New squares 1996 Area (ha)
Dry heath Humid heath Wet heath Peatland Brackish marsh Carr Scrub Acid grassland Sand dunes Tracks and ®rebreaks Ditches and pools Bare soil
193 128 17 15 0.2 6.7 53 5.3 0.9 31 13 43
132 55 8.8 5.3 0.8 7.5 109 27 0.1 14 3.6 12
122 52 10 11 8 10 93 42 0 27 6 10
Total heathland (Number of squares)
506 (250)
375 (243)
391 (294)
although the overall total area of heathland decreased by 552 ha; therefore, the 1987 heathland squares have lost 943 ha during the 9 year period between Surveys. These `new' squares add a signi®cant proportion of heathland to the overall total but their quality, in terms of heathland per square and dwarf shrub and peatland content, is lower than that of the 1978 Heathland Survey squares. Table 5(a) details the squares from which heathland was lost between 1978 and 1987; the loss of 173 squares that contained 381 ha of heathland can be mostly attributed to an increase of pasture, (140 ha) and `other land' (urban and industrial development, roads etc.) (100 ha). The remainder of the loss is due to the associated heathland vegetation types (carr, scrub, bare soil and open water) no longer being classi®ed in the Survey as part of the total heathland area. In 1996, on 48 of these squares (28.2%) some heathland vegetation had re-established (59 ha); however, the amount of heath in the square and the proportion of dwarf shrub vegetation
R.J. Rose et al. / Biological Conservation 93 (2000) 117±125
present in 1996 was on average much lower than the 1978 values. Between 1987 and 1996 there was a further loss of 373 squares [Table 5(b)] including 525 ha of heathland of which 137 ha was to other land types and 149 ha to Table 5 (a) Vegetation types in the 173 squares surveyed as heathland in 1978 that were not heathland in 1987 (48 of which contained a proportion of heathland in 1996); (b) Vegetation types in the 373 squares surveyed as heathland in 1978 and 1987 that were not heathland in 1996a 1978 Area (ha)
1987 Area (ha)
1996 Area (ha)
Dry heath Humid heath Wet heath Peatland Brackish marsh Carr Scrub Sand dunes Acid grassland Bare soil Open water
95 49 33 14 ± 10 57 ± 1.8 106 15
± ± ± ± ± 8.7 50 ± ± 70 13
19 8 0.2 0.2 ± 0.4 44 ± 3.1 15 11
Total heathland (Number of squares)
381 (173)
± (0)
59b (48)
Pasture Hedges Woodland Other land
136 3.5 89 71
276 3.9 88 171
232 2.9 110 233
Dry heath Humid heath Wet heath Peatland Brackish marsh Carr Scrub Sand dunes Acid grassland Bare soil Open water
126 82 45 28 11 27 133 ± 9.3 104 56
82 56 26 20 7.3 30 150 ± 5.9 96 52
± ± ± ± ± 11 155 ± ± 48 48
Total heathland (Number of squares)
621 (373)
525b (373)
0 (0)
Hedges Woodland Pasture Other land
2.0 31 360 209
4.2 335 419 206
4.7 484 399 343
(a)
(b)
a Note: In addition seven squares recorded as heathland in 1987 for the ®rst time were recorded as non-heathland in 1996. Five of these squares were re-planted forestry where heather had re-established by 1987 after felling but were understorey vegetation by 1996. The other two squares were minimum areas (0.2 ha) of heath in 1987 on the edge of larger heathland blocks. b The area of the following vegetation types (brackish marsh, carr, scrub, sand dunes, bare soil and open water) is only included in this ®gure where dry heath, humid heath, wet heath, peatland or acid grassland also occur within the same square.
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woodland. The associated heathland vegetation types remained relatively constant in area and contributed 262 ha to the overall loss. In addition seven squares recorded as heathland in 1987 for the ®rst time were recorded as non-heathland in 1996. Five of these squares were re-planted forestry where heather had reestablished by 1987 after felling but were under-storey vegetation and therefore no longer classi®ed as heathland by 1996. The other two squares were minimum areas (0.2 ha) of heath in 1987 on the edge of larger heathland blocks that were probably recorded as being within one of the adjacent squares in 1996. In 1960 about 600 ha (5.1% of the total area of heathland) of the heaths were grazed (Moore, 1962) and the area declined to 290 ha (3.7%) by 1987 (Webb, 1990); however, we recorded 561 ha (7.6%) being grazed in 1996, which reverses this trend. In 1996 a square was recorded as being grazed only when an animal was present or if there was evidence that animals had been present recently (i.e. grazed or trampled vegetation, dung or poached ground). Therefore, despite a number of recent initiatives to reintroduce grazing to some of the heaths, the intensity of grazing in 1996 was very low. The ®gure of 7.6% is an underestimate of the total area of heathland fenced for grazing animals, but is an accurate estimate of the area where there are visible signs that the vegetation is being grazed. There has been a dramatic decline in the area of recently burnt heathland between the three Surveys. The following ®gures are derived from the area of bare soil and the proportion of pioneer heath that was burnt within the 2 years preceding the Survey. Therefore, they are twice the annual rates of burning. The ®gure calculated for 1978 was 945 ha covering 656 squares and represents 12% of the total heathland area at that time giving an average annual burning rate of 6%. The overall ®gure is high, not only because it includes the extensive areas burnt in the very dry summer of 1976 but also a large area of Godlingston Heath (burnt in 1974) was also recorded as being recently burnt. If this area is excluded from the analysis a ®gure of 775 ha (9.8% of the total area of heath) is obtained. This is still considerably higher than the 382 ha (293 squares, 4.8% of the total area of heath) in 1987 and only 85 ha (121 squares, 1.2% of the total area of heath) in 1996. The percentage cover of the three main vegetation phases for the three Surveys is given in Table 6. The ®gures show that in each survey most of the heathland is in the mature (>16 years) phase with smaller amounts of vegetation in the building (6±15 years old) and pioneer (0±5 year old) phases. The higher proportion of building phase heather and correspondingly lower proportion of mature heath in 1987 re¯ects the re-growth of heather after the 1976 ®res. This eect is not seen in the 1978 pioneer phase because much of the area burnt in 1976 was classi®ed as recently burnt bare ground.
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4. Discussion The gross area of heathland in Dorset remained constant from 1978 until 1987 but during the 9 years preceding 1996 there was a 7% decline in the area of some 552 ha (Table 1). Planning legislation now controls direct losses of heathland. The losses we have observed have been caused by vegetation succession with many areas of open heathland changing to scrub and woodland. However, the picture is complex, as at some locations heathland has been lost as a result of succession to scrub and trees while at others heathland has regenerated although there is a downward trend in the area of heathland within each heathland square. Analysis of the 3110 squares ®rst surveyed in 1978 show a steady decline in the total area of heathland and a steady increase in the amount of scrub. The rate of encroachment of scrub and trees on to open heathland is about 1.7% per year; this is equivalent to 15% over the 9 years from 1987 to 1996, a rate similar to that between 1978 and 1987. Conservation management has done no more than contain the rate of succession that prevailed between the two earlier Surveys. The decline in controlled burning has undoubtedly been the principal factor leading to this growth of scrub. In his 1962 paper Moore said `Probably the most important ecological eect of the decline in rough grazing has been the virtual extinction of controlled burning that used to accompany it'. The decline identi®ed by Moore (1962) has continued and the 1978 Survey showed that both grazing and burning had been largely abandoned as the principal methods of vegetation management (Webb, 1990). Over the same period the control and management of many of these heaths has passed out of the hands of farmers in to those of conservation agencies. Recently, agric-environment schemes have been introduced as a conservation measure to compensate for the decline in traditional management. Free-range grazing (mostly cattle and ponies) has also been introduced on a number of nature reserves. This type of grazing will maintain existing open heathland, but will not recover areas invaded by scrub. Ponies may be eective in curtailing the development of scrub as they graze the young shoots of gorse Ulex europaeus and birch Betula spp., but invasion by pine Pinus spp. is unlikely to be checked by the animals. During the very dry summers of 1975 and 1976, 775 ha (9.8%) of the Dorset heathlands were burnt. The occurrence of these large ®res led to the introduction of stringent protection measures and an almost complete cessation of controlled burning. Surprisingly, no adequate alternative form of vegetation management was substituted. As a consequence, at the present time, these heaths carry extensive areas of mature heather and increasing amounts of scrub. This vegetation provides a greater quantity of fuel and when ®res do occur they are
hotter and more intense (Allchin, 1997). The increased temperatures attained during these ®res may impede the recovery of heathland vegetation and result in the establishment of an increased number of invasive species. The situation is analogous to that in many ®re-dominated communities in other parts of the world, especially in North America (e.g. Keiter and Boyce, 1991). Here also, ®re protection measures have disrupted natural cycles of ®re and regeneration to which the vegetation was adapted. Ultimately excessive ®re protection has proved harmful to the regeneration of the vegetation. Controlled burning aims to maintain a mosaic of different aged stands of heather and a range of habitats. To maintain a 25-year cycle by ®re management 4% of the total area of heathland (295 ha in 1996) should be burnt annually. This rotation would result in a number of stands of dierent ages of which 20% was either bare soil or in the post-burn or pioneer phase, 40% building phase and 40% mature heathland. In each of the survey years the proportion of mature heath has been considerably higher than the theoretical ideal (Table 6). In 1978 the combined area of bare soil and pioneer heath (17.8%) was less than the prescribed ®gure and in 1987, when this same area of vegetation was in the building phase, it fell short of the target of 40%. In 1996 the proportions of bare soil and the three main vegetation phases were close to matching a 60-year burning cycle where 8.3% was either bare soil or in the pioneer phase, 16.7% in the building phase and 75% was mature heathland. This cycle assumes that each area is burnt in strict rotation. However, most of the ®res that do occur are not managed (wild-®res) and tend to be most frequent on the heaths in the urban fringes, whilst many heaths may not be burnt for considerably longer than the average rotation of 60 years. There is now a clear case for the reinstatement of programmes of controlled burning because other methods of management have failed to check succession to scrub and woodland. The total number of individual heaths has continued to rise and their size has decreased (Table 3). The median heath size in 1996 is now below the 4 ha minimum size unit used by Moore (1962) in his analysis of the
Table 6 The total areas of bare soil and dwarf shrub heathland (dry heath, humid heath, wet heath and peatland) in each of the three main vegetation phases Year Bare soil Area % (ha) 1978 1987 1996
612 359 106
Pioneer (0±5 years)
Building (6±15 years)
Mature (over 16 years)
Area (ha)
%
Area (ha)
%
Area (ha)
%
7.8 5.7 5.2
940 1510 922
15.5 26.1 17.7
4039 3585 3919
66.6 62.0 75.1
10.1 474 6.2 332 2.0 270
R.J. Rose et al. / Biological Conservation 93 (2000) 117±125
fragmentation of the Dorset heathlands (see Fig. 1.). Using the 10 ha minimum size of site suggested for quali®cation as a Site of Special Scienti®c Interest (Nature Conservancy Council, 1989), 11 of 62 sites fell below this threshold between 1987 and 1996. These sites are still noti®ed because in Dorset most Sites of Special Scienti®c Interest are noti®ed for reasons other than habitat area such as the presence of nationally rare or nationally scarce species. The degree of isolation of these small heaths is not great; indeed, the spatial separation of the fragments shows a small decrease in each of the surveys. However, the proportion of heathland in the vicinity of each individual fragment is declining, and likewise the opportunities for dispersal by heathland species is reduced. The transitory nature of heathland is demonstrated in Tables 4 and 5 which give details of those squares from which heathland has been lost and those where it has been established or re-established. These squares are often heathland edge squares that are important not only for their own heathland content but as buer zones for the other squares within the heathland block. They are often the closest points to adjacent heaths and form natural links with other blocks of squares. Therefore, in terms of fragmentation and encroachment of other vegetation types they are an important part of the heathland landscape. The loss of 100 ha of heathland between 1987 and 1996 to urban and industrial development, roads etc. is an irreversible change. However, losses to mineral extraction, agricultural improvement and aorestation (140 ha between 1987 and 1996) may be reversed using existing re-establishment procedures (Putwain and Rae, 1988). A further 140 ha that has been lost to scrub and semi-natural woodland could be re-claimed as heathland using good heathland management practice (Gimingham, 1992). The gains are squares where heathland has either naturally regenerated or has been restored. These areas include abandoned open-cast mineral workings, felled woodland, cleared scrub and abandoned agricultural land. Some of these areas are being managed as heathland for restoration, others are reverting naturally to heathland, and some in the felled woodland category are a short-term transition between tree crops. In general these squares have a relatively small proportion of dry and humid heath and rarely any wet heath or peatland. There is often a high proportion of other vegetation or bare ground and, as can be seen from the 1987 survey `new squares' they are particularly prone to the re-invasion by other species. The 1996 Survey revealed dramatic changes in the composition of the heathland vegetation with a reduction in the area of both peatland and wet heath vegetation types. These changes may be attributed to a number of environmental factors. On Dutch heathlands ericaceous communities have been replaced by grassdominated communities as a result of inputs of nitrogen
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in excess of the critical loads of 15±20 kgN haÿ1 yearÿ1 for dry heath and 17±22 kgN haÿ1 yearÿ1 for wet heath (Bobbink et al., 1992). The estimated current deposition for the Dorset heaths is 16 kg N haÿ1 yearÿ1 (Pitcairn et al., 1995) which is just below the critical load for these heaths. However, the response of vegetation to increased inputs of nitrogen on the Dorset heaths is limited by the low availability of phosphorus in these soils (Chapman et al., 1989b). Changes in hydrological conditions are a more likely explanation than increases of inputs of atmospheric nitrogen for the observed vegetation changes. Three factors may be responsible. First, after 20 years of unmanaged growth the heathlands carry a larger standing crop of vegetation with a greater structural diversity. As a consequence, both interception of precipitation and evapo-transpiration may have increased and less water reaches the wet heath and peatland communities. Secondly, rainfall in Dorset has shown a downward trend, particularly in the summer months, during the last decade (Paxman, 1992). Thirdly, land use changes, such as the growth of coniferous plantations, outside of the heathlands yet within the heathland catchments may aect ground water levels and lateral ¯ows that are essential to the maintenance of wet and valley mire communities. The most remarkable feature of these changes is the relatively rapid response, detectable over the entire Dorset heathland landscape, of the heathland vegetation. The expectation might have been that such a response would be slow in view of the low growth rates of the plants. It also remains an interesting point for speculation whether or not a series of wet years would result in a simple reversal of these vegetation changes or whether the recovery of wet heath and peatland vegetation would show a hysteresis eect or for the vegetation to develop in a new direction. The rate at which wet heath and peatland vegetation would recover or change is also a matter for speculation. Throughout the 20 years of the Dorset Heathland Survey the value of using a comparable recording protocol and a repeatable methodology for each Survey has been demonstrated. It is by using this approach that change can be detected successfully and accurately. Nevertheless, the analysis of change becomes increasingly complex as new data are added to the Survey. The dynamic nature of the biotope becomes increasingly evident and it is more dicult to distinguish between changes arising from succession, those due to management and those induced by external factors such as climate and pollution. The relatively rapid response, equivalent to the rate of successional change and detectable at the landscape scale, which the Dorset Heathlands have shown to changes in the weather is, perhaps, the most interesting feature of the new Survey. If the trend for less rainfall and drier summers continues then the conservation of the wet heath and peatland
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communities may not only depend on the management vegetation but on controlling the hydrological conditions that are required. This provides a basis for further experimental study.
Acknowledgements We are especially grateful to the Royal Society for the Protection of Birds for their interest in and generous support for this project both in the provision of funds and for deploying members of their Dorset Heathland Project Team for the 1996 ®eld Survey. In particular, we thank John Waldon, RSPB Regional Ocer for south west England and Nigel Symes, Manager of the RSPB Dorset Heathland Project. We are also grateful for ®nancial support and advice from English Nature and the Dorset County Council. In particular we thank Jim White and Dr Andrew Nicholson of English Nature, and Dr Phil Stirling of Dorset County Council. Appendix A Non-heathland land These three attributes were added as main cover land types, Arable, Urban/Industrial and Other were added to record the areas of arable, urban/industrial and other land within the square. The previous surveys did not record these categories directly and they were added to improve the accuracy of all estimates of area and thereby those of the heathland land types. Potential heathland Codes for the area and most common species within the existing scrub, woodland, grassland and bare ground categories were added and the most common understorey species in woodland noted. Notable species Twelve species were added to the list of species recorded, these were; petty whin (Genista anglica), silver-studded blue butter¯y (Plebejus argus), sand lizard (Lacerta agilis), smooth snake, (Coronella austriaca), Dartford warbler (Sylvia undata), woodlark (Lullula arborea), stonechat (Saxicola torquata), skylark (Alauda arvensis), linnet (Carduelis cannabina), lapwing (Vanellus vanellus), reed bunting (Emberiza schoeniculus) and snipe (Galinago galinago). These species were recorded if seen or heard within the square but no systematic search was made for them.
Heathland and potential heathland management The original heathland grazing attribute was retained but to cover recent changes in heathland management practices sub-categories for sheep, cattle and horses were added. These attributes were repeated for the grazing of potential heathland for which an additional code for fencing was also added. Reverting land Codes for scattered heather and felled+scattered heather were added to the grassland and woodland vegetation types and litter was added to the bare ground category. Invasive species The introduced shrub, shallon (Gaultheria shallon) was added to the dry heath, scrub and woodland categories. References Allchin, E.A., 1997. Vegetation dynamics following management burning of lowland heathland. PhD Thesis, University of Liverpool, 208 pp. Anon, 1995. Biodiversity: The UK Steering Group Report, Vol. 2 Action Plans, HMSO, London. Auld, M., Davis, S., Pickess, B., 1992. Restoration of lowland heaths in Dorset. RSPB Conservation Review No. 6 1992. Royal Society for the Protection of Birds, Sandy, pp. 68±73. Bannister, P., 1965. Biological Flora of the British Isles. Erica cinerea L.. Journal of Ecology 53, 527±542. Bannister, P., 1966. Biological Flora of the British Isles. Erica tetralix L.. Journal of Ecology 54, 795±813. Bobbink, R., Boxman, D., Fremstad, E., Heil, G., Houdijk, A., Roelofs, J., 1992. Critical loads for nitrogen eutrophication of terrestrial and wetland ecosystems based upon changes in vegetation and fauna. In: Greenfelt, P., ThoÈrneloÈf, E. (Eds.), Critical Loads for NitrogenÐA Workshop Report. ThemaNord 1992/42. Nordic Council of Ministers, Copenhagen, pp. 113±159. Bullock, J.M., Webb, N.R., 1995. Response to severe ®res in heathland mosaics in southern England. Biological Conservation 73, 207±214. Chapman, S.B., Webb, N.R., 1978. The productivity of Callunaheathland in southern England. In: Heal, O.W., Perkins, D.F. (Eds.), The Ecology of some British Moors and Montane Grasslands. Springer-Verlag, Berlin, pp. 246±262. Chapman, S.B., Clarke, R.T., Webb, N.R., 1989a. The survey and assessment of heathland in Dorset for conservation. Biological Conservation 47, 137±152. Chapman, S.B., Rose, R.J., Bassanta, M., 1989b. Phosphorus adsorption by soils from heathlands in southern England in relation to successional change. Journal of Applied Ecology 26, 673±680. Diemont, W.H., Degn, H.J., Webb, N.R., 1996. A pan-European view on heathland conservation. Proceedings National Heathland Conference 1996. English Nature, Peterborough, pp. 21±32. Gimingham, C.H., 1972. Ecology of heathlands. Chapman and Hall, London. Gimingham, C.H., 1992. The lowland heathland management handbook. English Nature, Peterborough. Keiter, R.B., Boyce, M.S. (Eds.), 1991. The Greater Yellowstone Ecosystem. Yale University Press, New Haven. Moore, N.W., 1962. The heaths of Dorset and their conservation. Journal of Ecology 50, 369±391.
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