Urban Ecosyst (2008) 11:107–120 DOI 10.1007/s11252-007-0044-5
Influence of a parking area on soils and vegetation in an urban nature reserve Peter Shaw & Nigel Reeve
Published online: 22 December 2007 # Springer Science + Business Media, LLC 2007
Abstract We studied the influence of a car park on soil and vegetation within Richmond Park, UK, before and after imposition of fenced boundaries restricted public access. Soil and vegetation samples were taken before (once) and after (twice) access restrictions were enforced. The over-riding trend in all the data was for soil adjacent to the car park to be less acidic and more fertile than pristine local soils, accompanied by a more eutrophic grassland community (Lolium perenne/Trifolium repens, compared with Agrostis/Festuca grassland away from the car park). The chemical influence of the car park extended at least 50 m from its boundary. A common acidophilic collembole Folsomia quadrioculata was replaced by Cryptopygus thermophilus adjacent to the car park. There was little evidence from the vegetation data that car park closure benefitted the ecosystem, but chemical data showed signs of progressive recovery in the 2 years following restrictions. Possible explanations for the car park's influence on the local landscape are suggested to include calcareous chippings and canine faecal deposits. Keywords Roadside . Parking . Acid grassland . Soil eutrophication . Canine faeces . Collembola
Introduction Road transport constitutes a dominant environmental influence in the developed world, causing damaging impacts from their gaseous and saline pollution (e.g. Angold 1997; Czerniawska-Kusza et al. 2004; Bernhardt-Romermann et al. 2006) in addition to their deleterious aesthetic and safety effects. One aspect that has hitherto been little researched is P. Shaw (*) Centre for Research in Ecology and Environment, School of Human and Life Sciences, Whitelands College, Roehampton University, Holybourne Avenue, London SW15 4JD, UK e-mail:
[email protected] N. Reeve Holly Lodge, Richmond Park, Richmond TW10 5HS, UK
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the influence of parking zones (as distinct from highways) on surrounding ecosystems, despite the obvious potential for impacts from trampling, dog fouling and oily/saline runoff from parked vehicles. Shaw et al. (1995) studied a car park on Headley Heath, Surrey and found clear changes in both soil and vegetation near the car park edge. The car park area was more grassy with less Calluna vulgaris and a higher pH than adjoining heathland. By contrast, there is a body of literature on the effects of major roads on soils and flora. The most prominent effect is due to winter salt applications leading to local halophyte floras (Gilbert 1989), often manifest as a strip of Atriplex, Puccinellia and Spergularia species at the very outermost edge of a grassed verge by a main road (Piernik 2003). Wrobel et al. (2006) also noted salt-tolerant cultivars of Dactylis glomerata and Poa pratensis by roadsides. In a series of four papers Rutter and Thompson (1986), Thompson and Rutter (1986) and Thompson et al. (1986a, b) detailed variation in salt concentrations in soils adjacent to motorways and assessed their impact on putative restoration scrub species. Their models did not explicitly include spatial variation, but a re-analysis of their data show soil sodium levels to decay away from the roadside exponentially with half distances of approximately 3 m (outside edges) or 1 m (central reservation), and that peak levels (c. 50 μM/g) killed some plants by root (as opposed to spray) damage. They also noted that sodium was retained in the soil better than chloride, a standard observation during soil salinisation (due to soils containing significant cation exchange capacity, but negligible anion exchange capacity) that may lead to alkalinity from a build-up of sodium carbonate (Wild 1993). In acid soils the mere introduction of calcareous materials will eventually lead to soil changes; Milleret (1963, in Scheit 1967) mentions a strip between Orleans and Sens where limestone foundations were used in a roman road crossing acid (podsolic) soils, creating a 20-m wide strip of base-rich soils manifest as different woodland. Other roadside influences include mechanical disturbance and nitrogenous pollution. The creation of a new road generates a disturbed boundary zone where non-native plants (Rentch et al. 2005; Pauchard and Alaback 2006) and insects (Stiles and Jones 2004) can colonise and establish in pristine environments. In addition to spray and physical disturbance, road-derived nitrogenous pollution has been shown to affect vegetation up to 200 m from a major road (Angold 1997; Bernhardt-Romermann et al. 2006). Where measured, soil pH tends to be elevated by roadsides due to salinity and calcareous leachates (Czerniawska-Kusza et al. 2004; Kocher et al. 2005). Lowland dry acid grasslands are a priority habitat under the UK Biodiversity Action Plan (UK Biodiversity Group 1998, http://www.ukbap.org.uk/), and face decline from a number of factors of which the major problems are neglect (leading to succession), and agricultural ‘improvement’ (removing most of the biodiversity). Additionally, in common with other oligotrophic terrestrial systems, grasslands are experiencing compositional shifts caused by anthropogenic nitrogen deposition (Wilson et al. 1995; Smart et al. 2005). Within the London conurbation, the largest area of lowland acid grassland lies in Richmond park, where over 900 ha remain as part of a royal deer park dating back to 1630. We report here on vegetation and edaphic changes in this grassland habitat radiating outwards from a central car park, and the effects on these of the subsequent introduction of strict access controls. The work described below focuses on Richmond Park National Nature Reserve (London, UK), which contains 955 ha of park woodland (primarily oak Quercus robur) on acid grassland, grazed by Fallow deer Dama dama and Red Deer Cervus elaphus. Close to the centre of the park lie two artificially dammed water bodies called Pen Ponds, for which the closest parking access is the Pen Ponds car park 500 m away. For many years this was an unfenced, unsurfaced semi-formal parking zone from which informal footpaths ran
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out in all directions. In early 2004 this car park was formalised by introduction of boundary fences, which halved the available parking area and constrained visitors to choosing between just two formal exit paths. The intention was to reduce the impact of the car park on its environs, in particular to control the network of informal paths radiating out from the car park. The effectiveness of this intervention was assessed by a monitoring programme consisting of one pre-intervention and two post-intervention annual surveys of soil and vegetation conditions around the car park. The decisions to impose the change and to undertake a monitoring programme were made too late in 2003 for a summer sampling programme to be set in place, and the pre-intervention survey was started in December 2003. For interyear consistency each survey was therefore taken in mid-winter. Here we report the results of this monitoring, focusing on the impacts of the car park on local vegetation and soil chemistry. An additional linked project is also reported here, monitoring the distribution of Collembola in soils around the study site. These arthropods are among the commonest soil fauna, but are also sensitive enough to changes in soil properties to be useful as pollution monitors (Hopkin 1997).
Materials and methods Site description Work was undertaken around the Pen ponds car park, Richmond Park, UK (OS grid ref TQ204727). The soil is loamy Thames drift of the Wickham 3 series (Soil Survey of England and Wales 1983), overlying London clay. The history of this car park is not well documented, but from c. 1950 until 2004 it was an unsurfaced, unfenced sandy zone, with approximate dimensions 90×70 m. In January 2004 this was altered in three ways; it was reduced in size by half (to c. 45×70 m), surfaced in tarmac, and fenced in by rigid boundaries to direct pedestrians onto official paths. The surface was once topped with an unknown thickness of mixed gravel, of which scattered chips remained (mainly flint, but some calcareous material too), but for at least 20 years the surface has been almost bare soil. Four footpaths were selected for study, one per cardinal point. There is no evidence that any chippings have ever been spread on any of these paths. Sampling points were established at five distances (0, 25, 50, 100 and 200 m) from the car park along each path (Fig. 1). The 0 m position was always defined by the hard road edge for the north and east paths, while the south and west paths started at the eroded edge of the car park itself in 2003. The exact same south and west 0 m positions were used in subsequent years, but they were then lying at the base of a new exclosure fence. Main sample collections Field visits were made in three consecutive winters; December 2003, January 2005 and December 2005 for collection of soil samples (surface 5 cm), and identification of associated flora. On each visit, three replicate quadrats (50×50 cm) were placed randomly at each sampling location, subject to the constraint of being within 2.5 m of the path centre. Percentage cover of all flowering plants was recorded, plus moss (pooled), and a soil sample taken (5 cm depth) for chemical analysis, so that each winter's collection consisted of 60 values for all data. Soils were stored at −20°C prior to analyses.
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Pen Ponds
N Spankers Hill wood
Key Sample position (from GPS)
Lawn plantation
c
Major trackway Unofficial pathway
0
100
200
300m
new car park (with previous extent the pale shading to the W)
Fig. 1 Site map. The point of the arrow in the lot is at GPS coordinate TQ20431 72716 (51° 26′ 24″N, 0° 16′ 1″W)
Chemical analyses Determinations were performed for pH (10 g:25 ml), conductivity (10 g:25 ml), organic matter content (dry to 105°C then 2 h at 500°C (LOI)), aqueous-extractable ammonium, nitrate (by ion-selective electrode) and Truog's extractable phosphate (molybdenum blue method). Procedures followed Allen (1974). Soil compaction This was only measured on one occasion (14/12/2005), when an ELE proving ring penetrometer took 10 replicate measurements (kPa) from each of the 20 sampling locations. Soil fauna Six-centimeter diameter soil cores (4.5 cm deep) were taken on 17/03/05 (North and East paths) and 12/05/05 (south and west paths) for extraction of soil arthropods in a high gradient Tullgren funnel. Five replicate cores were collected from random locations around the 0 m or 200 m sampling points shown in Fig. 1. Collembola were preserved in 70% ethanol for identification using a pre-print of Hopkin (2007). Statistical methods Significance of year, footpath and distance from car park were assessed by two-way ANOVA with distance from car park as a covariate, unless data were irredeemably non-
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normal (such as the Collembola counts) in which case each effect was assessed separately by Kruskal Wallis ANOVA. Chemical data (except pH) were found to be best handled as their logarithmic transform. The chemical variables proved to be highly colinear, and were reduced to one dimension by principal components analysis (PCA). Collembola data (being skewed) were transformed by log(1+X) before analyses, then back-transformed when quoting values. Multiple linear regression was used to relate vegetation community to distance from the site edge, and the resulting equation used to calculate a predicted distance from site edge for each observation in turn. The changing relationship between actual and inferred distance is used to assess recovery after access restrictions were imposed. Relationships between vegetation and environmental data were assessed using Canonical Correspondence Analysis (ter Braak 1986) on log-transformed data, in order to portray objectively the gradation of vegetation away from the hard edge. As with all techniques based on multiple regression (Shaw 2003b), CCA is sensitive to colinearity between explanatory variables, and we followed ter Braak (1986) in excluding colinear variables, leaving just one environmental variable (log-transformed distance from edge). This allowed testing of the null hypothesis of no connection between data matrices using Monte-Carlo testing (Manly 1991). Calculations were performed on SPSS 14, except for CCA (run on PcOrd, McCune and Mefford 1999). Throughout, significance is abbreviated as follows: *p<0.05; **p<0.01; ***p<0.001.
Results Edaphic properties Soil chemical properties are summarised in Table 1, which also lists the results of test of the edge effects, of inter-year and inter-path differences. All chemical parameters measured changed significantly with distance from car park, the effect being greatest for pH (Fig. 2, Tables 2 and 3). Closest to the car park the soil was richest in all nutrients, had the highest dissolved salts and pH (Tables 2 and 3). Organic matter behaved similarly but peaked at 25 m rather than 0 m (Fig. 2d). Conductivity correlated positively with phosphate (r178 = 0.53) and ammonium (r178 =0.46, or r177 =0.61 for the model combining both nutrients) but
Table 1 Summary chemical data for the soil collected from Pen Ponds car park, giving mean values over all samples Chemical
Mean±1 SE
Distance (ANOVA)
Distance (linear)
Year (ANOVA)
Year (linear)
Path (ANOVA)
pH LOI (%)a P (mg kg−1) NO3 (mg kg−1) NH4 (mg kg−1) Cond. (μS cm−1)
5.0±0.04 11.6±0.5 14.3±1.7 44.4±6.1 6.9±0.5 66.2±3.6
** ** ** * ** **
** ** ** NS ** **
** * ** ** * **
NS NS ** NS * **
** ** ** ** ** **
Analyses given are ANOVA (testing H0: no difference between category means) and regression (testing H0: no linear trend between data and distance or time). In all cases where the regression was significant the sign was negative; hence significant values always declined with time and with distance a
Loss on ignition, hence organic matter content
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b
20 0
4.6
4.8
10
5.0
pH
5.2
P mg/Kg
30
5.4
5.6
a
40
112
0 25 50 100 200 Distance from car park edge, m
0
60 0
20
40
NO3-, mg/Kg
6.0 4.0
NH4+ mg/Kg
2.0
80
100
10.0
d
8.0
c
0 25 50 100 200 Distance from car park edge, m
0 25 50 100 200 Distance from car park edge, m
100 120 80 60
Conductivity, µS cm-1.
20
40
16 14 12 10 8 6
0
0
2
Organic matter content %
f
4
e
0 25 50 100 200 Distance from car park edge, m
0 25 50 100 200 Distance from car park edge, m
0 25 50 100 200 Distance from car park edge, m
Fig. 2 Soil chemical data in relation to distance from paths (N=4 paths). Data are means±1 SE, from 2003 before access was restricted. a pH, b phosphate, c ammonium, d nitrate, e organic matter, f conductivity
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Table 2 Soil chemical data from 2003 (pre-intervention) as mean±1SE Distance
pH
LOI
P
NO3
NH4
Cond.
0 25 50 100 200
5.6±0.1 5.0±0.1 5.0±0.1 4.7±0.1 4.7±0.1
12.0±2.5 14.9±1.3 10.6±1.6 10.2±1.5 8.4±2.5
3.2±0.7 0.9±0.2 1.9±0.6 3.2±1.7 0.4±0.1
4.5±1.3 1.9±0.4 3.7±0.8 4.1±1.2 3.9±1.1
0.8±0.2 1.0±0.1 0.8±0.1 0.8±0.2 0.8±0.2
172.5±15.5 96.8±8.1 96.8±17.4 93.5±16.7 76.1±8.8
not with pH. All chemical data also varied significantly between years, with the first year (pre-access restrictions) producing high values for pH, phosphate, ammonium and conductivity compared with the end of the survey (Tables 2 and 3). In addition to using ANOVA to test whether there were differences between locations or years, these data were entered into regression models testing for a linear change with distance or with time (Tables 1, 2 and 3). All except nitrate were significant with distance, while phosphate, conductivity and ammonium were also significant with year. In all significant regression models the regression coefficient was negative, showing chemical values to have declined approximately linearly with distance from car park or over the three year period. The chemical data were ordinated by PCA to seek underlying trends, and the first axis of variation (43.0%) was found to correlate well (r167 =−0.46, p<0.001) with distance from car park (Fig. 3), showing the main trend of variation in the soil chemistry to be a continuous, progressive change with increasing distance from the car park. When distance from car park was analysed as the dependent variable to be predicted from independent chemical data, the regression model was Distance ¼ 512:3 64:6 pH 33:2 cond: 74:6 LOI 38:3 P þ 31:8 NO3 0:45 NH4 r2 ¼ 0:40
(where each variable takes the units in Table 1 but is log10-transformed, except for pH.) Using this model it was possible to assign a predicted distance from the car park to each soil sample analysed, based only on its composition. Hence each of the 20 locations sampled generated 3 values of predicted distance from edge (one per year). Changes over time in these predicted distances can be used as a summary indicator of soil recovery trends. The ‘soil-predicted’ distances increased significantly (around 10 m per year; r165 = 0.22**) after access restrictions were imposed (Fig. 4). Compaction was only measured on one occasion, when it was found to differ significantly between transects, but did not change significantly with distance from car park. The mean compaction was 7.7 (1 SD) 3.5 kPa.
Table 3 Soil chemical data from 2005 (post-intervention) as mean±1SE Distance
pH
LOI
P
NO3
NH4
Cond.
0 25 50 100 200
5.2±0.2 4.9±0.1 4.9±0.1 4.7±0.1 4.9±0.0
12.6±2.8 12.8±1.8 8.9±1.4 8.3±1.4 8.9±1.1
1.8±0.7 0.8±0.2 0.6±0.2 1.4±0.6 0.2±0.0
5.9±2.0 1.1±0.4 8.4±6.4 1.9±0.6 1.2±0.3
1.1±0.3 0.8±0.2 0.8±0.2 0.5±0.1 0.3±0.1
79.7±12.3 45.2±2.7 52.6±7.7 40.7±3.8 29.7±1.0
114
-1
-0.5
0
0.5
1.0
Impacted state
1st principal axis (chemical data).
Fig. 3 PCA scores for chemical data as a function of distance from hard edge, mean±1 SE
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Native state
0
25
50
100
200
Distance from car park edge, m
Vegetation data There were, unsurprisingly, rather few plant species present in these pathside communities in mid-winter. Only 21 species of higher plant were recorded, listed in Table 4. To a first approximation there were two distinct plant communities; one around the car park edge dominated by Lolium perenne (perennial rye grass) and white clover Trifolium repens (a Lolium perenne–Trifolium repens ley, fitting the UK description of MG7 Lolium grassland in Rodwell 1992), being replaced by typical acid grassland species (approximating to a Festuca–Agrostis–Rumex acetosella U1 grassland sensu Rodwell 1992) at increasing distance from the car park (Fig. 5). The most car-park avoiding species (determined by stepwise MLR) were Festuca rubra and Agrostis capillaris, the fine-leaved grasses that make up much of the park's grassland. Moss and Rumex acetosella cover also increased with distance from the hard edge. Overall 23 tests of the edge effect generated 10 significant results, a pattern that is itself significant at p<0.001. Apart from the edge species Lolium perenne, Trifolium repens and Polygonum aviculare, all taxa increased in cover with increasing distance from the edge. By contrast there was little evidence of between-year changes in plant cover (Table 4), apart from an increase in Poa annua over time, with a concomitant decrease in Agrostis capillaris. Regression between distance and log-transformed vegetation data was used to generate “flora-predicted” distances for each quadrat. These distances did not differ significantly between years, suggesting that no recovery in the vegetation community was observed. Floral data were ordinated by CCA, using distance from car park as the constraining environmental variable. (This was preferred to using the chemical data since these are strongly colinear, but their underlying trend is approximately linear with distance from car park.). The first axis of this ordination (presented in Fig. 6) corresponded with distance from car park (significant by Monte-Carlo testing at p=0.002), with Lolium perenne and Trifolium repens at the negative end (representing low distance from edge) while Rumex acetosella and Festuca rubra were located at the positive end (high distance from car park).
90 80 70 60 50
Mean predicted distance ± 1 se
Fig. 4 Evidence for a recovery in soil chemistry, shown as increases in chemical-inferred distance from hard edge after access restrictions in early 2004 (the overall mean distance=75 m)
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100
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2003
2004
2005
Year
Table 4 Mean (± sd) % cover for plant species along transects from car park including the significance of distance and year effects Species
Number of records>0
Mean % cover
Bare soil Agrostis capillaris Bellis perennis Carex sp Catapodium rigidum Crepis capillaris Festuca rubra Geranium molle Holcus mollis Hordeum murinum Hypochaeris radicata Juncus effusus Lolium perenne Rumex acetosella Poa annua Polygonum aviculare Potentilla reptans Spergula arvensis Ranunculus acris Stellaria holostea Trifolium repens Moss, pooled faeces
142 104 1 8 1 1 19 1 4 1 5 1 107 13 54 9 1 2 1 8 9 25 5
23.5 30.1 <1 <1 <1 <1 1.8 <1 <1 <1 <1 <1 30.7 <1 10.6 <1 <1 <1 <1 <1 <1 4.1 <1
*p<0.05; **p<0.01
sd 23.6 sd 34.5
Distance from car park effect rs, p df=4
Year effect χ2, p df=2
+0.42**
10.3**
+0.16*
sd 8.1
+0.41** +0.18* +0.19*
sd 34.6
−0.52** +0.29**
sd 22.8 −0.27*
48.2** 19.1**
9.0* sd 12.5
−0.16** +0.41**
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b
0
25
50
100
2.0 1.0 0
% cover Trifolium repens
50 25 0
% cover Lolium perenne
75
a
200
0
100
200
Distance from CP, m
% cover A. capillaris
8 6 4 0
0
2
25
d 10
c % cover F. rubra
50
50
Distance from CP, m
25
0
25
50
100
200
0
25
Distance from CP, m
50
100
200
Distance from CP, m
3 2
Poa / bare Acid grassland
1
Bare
Dist Fr
0 -1
Second canonical axis
Poa
Lp Tr
Ac
Ra
Roadside
-2
Fig. 6 CCA on vegetation data, constrained to fit distance from car park. Axis 1 approximates to distance from car park. Ac Agrostis capillaries, Dist distance from car park, Fr Festuca rubra, Lp Lolium perenne, Poa Poa annua, Ra Rumex acetosella, Tr Trifolium repens
4
Fig. 5 Percentage cover of vegetation, mean±1 SE. a Lolium perenne, b Trifolium repens, c Festuca rubra, d Agrostis capillaris
-3
-2
-1 0 1 2 First canonical axis
3
4
Distances: 200 100 50 25 0
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The CCA also separated out a distinct Poa annua-bare ground community, largely found within 50 m of the car park. Repeating the CCA with year as the explanatory variable did not produce a significant ordination (Monte Carlo p>0.05). Collembola Eighteen species were recorded, of which the 8 most frequently encountered species are listed in Table 5. The most common species was Folsomia quadrioculata, widely distributed in acid or organic-rich soils (Poole 1961, Usher 1970, Hopkin 2007) but absent from the car park edge soils, where it was replaced by Cryptopygus thermophilus. Both these differences were significant (p<0.001 by Kruskal–Wallis ANOVA). Parisotoma notabilis also had a significantly (p<0.05) higher density away from the edge. Total Collembola were significantly (p<0.05) lower adjacent to the car park, but this effect disappeared if F. quadrioculata were excluded from the total. Species richness did not differ significantly between distances.
Discussion This project was set up with two aims: to ascertain whether any edge effects could be detected around the focal car park, and to examine whether restrictions on visitor numbers ameliorated the impact in any way. The principal finding from our results is that the parking area exerted a profound influence both on soil chemistry and on ground flora, detectable up to at least 50 m, and that this influence was to create eutrophic soils. Levels of nutrients and pH values both rose towards the hard edge (Fig. 2), and concomitantly the native acid grassland was replaced by a Lolium ley, more typical of agriculturally improved land. Similar results were found by Shaw et al. (1995) on a heath, where the vegetation gradient ran from Deschampsia grassland adjacent to the parking area, to a matrix of Calluna heath. The influence of the car park on soil chemistry could be explained by the inclusion of calcareous material (concrete chips or chalk gravel) in the surface material of the original car park. This would then have been gradually displaced by people walking, leading to the observed changes in pH. This does not directly explain elevation of nutrient levels, although liming acid soils is known to increase the activity of lumbricids and hence their Table 5 Mean (±SE) density (individuals m−2) Collembola data at 0 or 200 m from car park edge, along with the significance of this difference by Kruskal Wallis ANOVA (1 df) Species
0m
200 m
P value
Cryptopygus thermophilus Entomobrya nivalis Folsomia quadrioculata Friesea mirabilis Isotoma viridis Lepidocyrtus cyaneus Parisotoma notabilis Protaphorura armata Pseudosinella alba Total Collembola
830±157 12±12 0 84±45 328±127 279±113 12±13 968±173 209±100 3,488±203
0 69±44 6375±245 116±40 630±113 409±109 239±119 339±114 344±159 10,070±228
*** NS *** NS NS NS * NS NS *
NS: p>0.05; *p<0.05; ***p<0.01
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nitrogen turnover (Robinson et al. 1992). Lumbricids are known to avoid pH values below 5.5 (Satchell 1955), so may only be expected to occur in the immediate vicinity of the car park (although they were not surveyed in this study). Hence the pH shift may account for the peak in nitrate levels at the car park edge (Fig. 2e) mediated via earthworm activity, but other eutrophic effects remain unexplained. Overall conductivity is a measure of total dissolved salts, so that the one explanation for the peak by the car park edge (Fig. 2f) is that it derives from winter road salt, carried into the park on tyres. This mechanism alone could explain the higher pH at the edge, since residual sodium will raise the pH by the formation of sodium carbonate (Wild 1993). However if this were the principal mechanism at work one would expect a high correlation between conductivity and pH, while in fact this was very weak at r=0.12 NS. A potential eutrophying influence is nitrogen fixation by Trifolium repens, which was strongly edge-associated, albeit with low cover values (Fig. 5). Assuming fixation to add c.10 g m−2 yr−1 (Alexander 1977), a 1% Trifolium cover will add c. 0.1 g N m−2 yr−1 or c. 0.5 mg N kg−1 yr−1 (0.6 mg ammonium or 2.2 mg nitrate). This seems too low to explain the nitrate peak at the edge (an excess of 50 mg kg−1NO 3 ), though could contribute to the smaller ammonium peak (5 mg kg−1NHþ 4 ). A final factor capable of acting as a eutrophying influence around a car park is the deposition of canine faeces. Such deposition undoubtedly occurs on a near-daily basis; during the sampling programme faecal material was found in quadrats five times (Table 4), always within 50 m of the car park. The best correlate of conductivity was a combination of ammonium and phosphate, which are the expected residues from faecal material on an acid soil where nitrification is generally expected to be negligible. This suggests that faecal contamination is not just the most credible source for the phosphate enrichment seen, but may be a major determinant of dissolved salts in the soil solution in soils close to the car park. The excess of nitrate over ammonium is consistent with nitrification taking place, despite the low pH. Although nitrification has long been known to be pH sensitive (Wild 1993), it does occur in acid soils (Brierley et al. 2001, Islam et al. 2007). The most favourable conditions for nitrification would have been at 0 m (the tarmac edge) where the pH and ammonium were both highest, which is consistent with the sharp peak in nitrate at this distance (Fig. 2). Given the strongly acidifying nature of nitrification, if this nitrate derived purely from ammonium this would have acidified the soil equivalent to the removal of c. 100 g lime per square metre. This can only be reconciled with the high pH and high nutrient status of the edge zone by assuming the deposition of a material simultaneously rich in bases, nitrogen and phosphate, consistent with canine faeces. Given the chemical changes exhibited by the car park edge soil, some change in biota was to be expected. The greatest effect was in the vegetation type, with Lolium perenne, Trifolium repens or Poa annua dominating the most eutrophic soils. The association between Lolium perenne and rich soils has been known since at least Saxon times (Russell 1973). There was also a shift in the soil faunal community (in 2005, after 2 years of access restrictions), as measured by the Collembola with a replacement of the commonest Collembolan in the site, Folsomia quadrioculata, with Cryptopygus thermophilus at the edge of the car park. This led to a significant decline in total numbers of Collembola (but not their species richness). Studying the succession of Collembola on an industrial waste, Shaw (2003a) found C. thermophilus to be a common early successional species, but that it declined as the succession proceeded, with F. quadrioculata appearing after 30 years, when woodland soil started to form and the pH fell. Also affected by the edge was the common isotomid Parisotoma notabilis, which was primarily found only in the acid grassland 200 m from the edge.
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By contrast to the clear impact of the car park on its surroundings, the evidence for a recovery towards pristine conditions following access restrictions was confined to trends in the chemical data. Phosphate, ammonium and conductivity values fell steadily over the period of study (Tables 1, 2 and 3), summarised in the significant (p<0.05) rise in chemistry-derived distance from car park (Fig. 3), and consistent with a reduction in faecal deposition after path closure. By contrast, there was very little in the vegetation data to suggest recovery after path closure; in particular the flora-derived distances for each quadrat did not change significantly over the period of survey. Changes in the vegetation may follow, if the chemical changes persist. The wider implication of this work is that the introduction of parking zones into a pristine environment (especially where the native soil is infertile and acidic) should be expected to have a landscape-level impact beyond the immediate and visual, leading to long-lasting changes in soil chemistry and the vegetation community. In this case, in addition to the 0.6 ha of land occupied by the car park itself, an additional 2.7 ha (the land within 50 m) fell within the zone experiencing eutrophication. This impact may be seen as having two components; an increase in pH and salinity which is generally associated with roadsides, and an increase in nitrogen and phosphorus, which is suggested to be a residue of canine faeces hence associated with the parking zones used by dog walkers.
Acknowledgements Thanks are due to Katharine Lankey who performed the chemical analyses and provided technical and taxonomic assistance.
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