The restoration of Strandveld and Succulent Karoo degraded by mining: an enumeration of topsoil seed banks

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South African Journal of Botany 2004, 70(5): 717–725 Printed in South Africa — All rights reserved

SOUTH AFRICAN JOURNAL OF BOTANY ISSN 0254–6299

The restoration of Strandveld and Succulent Karoo degraded by mining: an enumeration of topsoil seed banks AJ de Villiers, MW van Rooyen* and GK Theron Department of Botany, University of Pretoria, Pretoria 0002, South Africa * Corresponding author, e-mail: [email protected] Received 10 November 2003, accepted in revised form 20 March 2004 This study was initiated to assess the potential of topsoil replacement to meet the objectives of the mining rehabilitation plan at Brand-se-Baai along the West Coast of South Africa. Seed bank size was determined seasonally for six vegetation units by means of the seedling emergence method. A mean seedling density of 2 725m–2 was recorded, dominated by annual species. Temporal variation in the seed bank was significantly higher than spatial variation at a vegetation unit level. Mean seedling densities ranged from 1 612–3 276m–2 between vegetation units, and from 838–7 772m–2

between seasons. The seasonally-germinable seed bank was significantly larger in autumn than in the other seasons. When compared to the potentially-germinable seed bank, the size of the summer, winter and spring seasonally-germinable seed banks greatly underestimated the true seed bank size. Topsoil replacement will go far to restore plant densities recorded in the pre-mining vegetation and will be essential for the successful re-vegetation of mined areas. It is critical to take seasonal changes in seed bank size into account when planning re-vegetation strategies.

Introduction Ecologists and evolutionary biologists have become increasingly aware of the role that seed banks play in maintaining species and genetic diversity in populations and communities (Gross 1990). For the applied biologist in particular, the aspect of greatest significance is the role of the seed bank in determining the future vegetation, especially after natural or deliberate disturbance (Van der Valk et al. 1992, Milberg and Persson 1994, Kotanen 1996). The seed bank of a plant community not only represents the ‘memory’ of previous conditions, but it is an important measure of the potential of the community to respond to conditions in the present and future (Coffin and Lauenroth 1989, Van der Valk et al. 1992, Valbuena and Trabaud 2001, Kalamees and Zobel 2002). Open cast mining of heavy minerals at Brand-se-Baai along the arid West Coast of South Africa destroys all the standing vegetation in the mined areas. The rehabilitation plan specifies that vegetation of the post-mining areas should conform as far as possible to pre-mining vegetation in terms of both species richness and abundance (Grindley and Barbour 1990). Recruitment by means of the soil seed bank contained in the topsoil, as well as seeding and/or transplanting of selected species are considered as viable means to re-vegetate the area (Mahood 2003). Only the role of the seed bank in the topsoil is explored in this study. Many studies in arid environments have indicated that seed banks in these areas are persistent and large (Van Rooyen and Grobbelaar 1982, Reichman 1984, Coffin and Lauenroth 1989, Von Willert et al. 1992, De Villiers et al.

1994, Van Rooyen 1999, Cabin and Marshall 2000). Revegetation by means of the topsoil contained seed bank should therefore theoretically produce good results. However, the species contributing most to the size of the seed bank in arid areas are usually annual and this could detract from the success of the revegetation operation. Topsoil replacement is also not favoured by the mining company because of the high content of heavy minerals in the upper soil layers. Numerous studies have reported on the spatial and temporal variation in soil seed banks (see Leck et al. 1989, Baskin and Baskin 1998 for reviews, Crawford and Young 1998, Elberling 2000, Gul and Weber 2001, Lortie and Turkington 2002 for more recent studies). However, in most regions of South Africa, soil seed bank studies have been neglected. In the arid areas of South Africa, seed bank studies have been done by Van Rooyen and Grobbelaar (1982), Dean et al. (1991), Esler et al. (1992) and Esler (1993). Only one study (De Villiers et al. 1994) relates directly to the Strandveld Succulent Karoo. This study was initiated to assess the potential of topsoil replacement to meet the objectives of the rehabilitation plan. The first step in this assessment was to investigate the spatial and temporal patterns in seed bank size and to determine the contributions of the different life forms (viz. annuals and perennials). The potential recruitment from the seed bank was then compared to the density of the pre-mining standing vegetation. The implications of the spatial and tem-

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poral variation for the restoration of the vegetation are discussed. Material and Methods The study area covers approximately 9 400ha and is situated in the vicinity of Brand-se-Baai (31°18’S, 17°54’E) on the Cape West Coast, South Africa, some 350km north of Cape Town and about 80km northwest of the nearest major town, Vredendal. The Cape West Coast has a Mediterranean-type climate with hot dry summers (November–January) and rain during the winter months (April–July). Mean annual rainfall at the study area is 160mm. This meagre annual rainfall is supplemented by fog (c. 100 days per annum at the study area) and heavy dew-falls to a mean total annual precipitation of 282mm. Mean annual temperature is 15.8°C with a relatively small annual fluctuation due to the marine influence. Frequent easterly berg winds from the interior, bring hot, dry conditions to the coast. The vegetation at Brand-se-Baai is classified mainly as Strandveld Succulent Karoo (Low and Rebelo 1998), a vegetation type containing many drought deciduous and succulent species. In general, the shortest vegetation occurs at the coast, on exposed calcrete and coastal rocks and the tallest vegetation is found further inland, in areas with deep calcareous sand (Boucher and Le Roux 1990). The littoral vegetation of the study area is subject to heavy winds, salt spray and drift sands. These ecosystems are naturally fragile, with low resilience and are easily disturbed or destroyed (Boucher and Le Roux 1993). The vegetation of the study area at Brand-se-Baai was classified into six communities (De Villiers et al. 1999) (Figure 1). The vegetation units sampled for the seed bank study do not correspond directly to the six plant communities and are as follows: • Community 1, the Cladoraphis cyperoides–Lebeckia multiflora Coastal Strandveld, and community 2, the Cephalophyllum spongiosum–Odyssea paucinervis Coastal Strandveld, are the two coastal communities found in the protected zone along the coast where no mining is allowed. These communities were collective sampled as seed bank sampling unit 1. • Community 3, the Ruschia versicolor–Odyssea paucinervis Dwarf Shrub Strandveld, covers almost the entire western mining area. Because of the importance of this community to future rehabilitation efforts, the three variants were sampled separately for the seed bank study. Seed bank sampling unit 2 represents the Ehrharta calycina–Crassula expansa variant; seed bank sampling unit 3 the Ruschia caroli–Aspalathus divaricata variant, and seed bank sampling unit 4 the Tripteris oppositifolia– Cissampelos capensis variant. • The fourth community, the Salvia africana-lutea–Ballota africana Tall Shrub Strandveld, occurs mainly on the high white dunes to the west of the western mining area and constituted seed bank sampling unit 5. • Seed bank sampling unit 6 encompassed both variants of the Eriocephalus africanus–Asparagus fasciculatus Tall Shrub Strandveld.

De Villiers, Van Rooyen and Theron

• At the time of this study the eastern mining block was excluded from the mining operation. The Ruschia tumidula–Tetragonia virgata Tall Shrub Strandveld, which occurs almost exclusively in the eastern mining block (Figure 1), was therefore not sampled. The seed bank sampling units will further be referred to as vegetation units, to distinguish them from communities. Ten sampling locations were randomly selected within each of these six vegetation units and their positions recorded with a Global Positioning System (Figure 1). At each of the 60 sampling locations, 15 soil samples were taken along a transect at 2m intervals. Each sample consisted of a soil core with a diameter of 65mm taken to a depth of 100mm, totaling a volume of approximately 246cm3. Starting in June 1993 (winter), sampling was done every three months over a period of two years. From each of the 900 samples per season, a sub-sample of 100cm3 was spread evenly on top of sterile sand in a 1dm3 pot and placed under ambient conditions at the University of Pretoria. The seasonally-germinable seed content in the soil samples was estimated by means of the seedling emergence method. Samples were watered daily and emerged seedlings were marked with wooden toothpicks. Half strength Arnon and Hoagland’s complete nutrient solution (Hewitt 1962) was applied fortnightly. Examination of the samples continued for a period of six months, as recommended by Thompson (1993), whereafter the number of emerged seedlings (toothpicks) were counted. Species were categorised as either perennial or annual. Mean monthly temperatures at the experimental site at the University of Pretoria varied more between seasons than at Brand-se-Baai, but the extremes were within the range of those at Brand-se-Baai. Because the samples were watered daily to obtain optimum conditions for germination, the difference in rainfall regime between the two areas was considered not to be crucial. For each sampling location the remaining soil of three of the 15 samples was stored dry in paper bags under ambient laboratory conditions at the University of Pretoria until autumn 1995. These samples were re-examined in autumn because this season is considered as the peak season for germination of most Strandveld Succulent Karoo plant species (De Villiers 2002a, 2002b). The germinable seed density in each of these sub-samples was determined in the same manner as described above. The results obtained during this re-examination of samples are referred to as the potentially-germinable seed bank as opposed to the seasonally-germinable seed bank that was obtained by the examination directly after sampling. The re-examination of samples was carried out for six sampling seasons (with the exception of the first and last sampling season). Before the mining operation started, four sites were selected by the mining authorities to serve as benchmarks to measure the success of the re-vegetation process. Three of these sites were situated in the Ehrharta calycina–Crassula expansa variant of the Ruschia versicolor–Odyssea paucinervis Dwarf Shrub Strandveld community (seed bank sampling unit 2) and one in the Salvia africana-lutea–Ballota africana Tall Shrub Strandveld (seed bank sampling unit 5). At each of the benchmark sites, the standing vegetation in five replicate plots of 100m2 each was surveyed. During the

South African Journal of Botany 2004, 70: 717–725

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Goerap River 0

5 000m

1 000

N

59 57

60

Salt Pan Road

58

56

55

54

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52

39

36 38 48

37 50

35 43 42

tern

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32

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New Tar Road

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27

45

10

ATLANTIC OCEAN

49

44

rea

gA

47

33

46 8 9

26

7

29 24

6

31

25

Seed bank Plant Communities vegetation unit

5 28

23 4

Brand-se-Baai

30

om

eK

Di

11

32

Ruschia tumidula–Tetragonia virgata Tall Shrub Strandveld

21

12

22

13

6

Eriocephalus africanus–Asparagus fasciculatus Tall Shrub Strandveld

5

Salvia africana-lutea–Ballota africana Tall Shrub Strandveld

19 14

18 16

20

15

Ruschia versicolor–Odyssea paucinervis Dwarf Shrub Strandveld

1

17

4

Tripteris oppositifolia–Cissampelos capensis Variant

3

Ruschia caroli–Aspalathus divaricata Variant

2

Ehrharta calycina–Crassula expansa Variant

1

Cephalophyllum spongiosum–Odyssea paucinervis Coastal Strandveld

1

Cladoraphis cyperoides–Lebeckia multiflora Coastal Strandveld Agriculture

Figure 1: Vegetation map of the study area and corresponding seed bank sample units, indicating 60 seed bank sampling points

Table 1: Mean number of emerged seedlings m–2, for samples taken in six vegetation units (n = 80 per vegetation unit). Within a category, or within the mean for the study area, values followed by the same letter are not significantly different at α ≤ 0.05 Category Perennials Annuals Mortalities Total

1 225.0 bc 742.6 a 644.5 a 1 612.1 a

2 253.8 c 1 767.8 bc 842.5 ab 2 864.0 b

Vegetation unit 3 4 170.0 ab 221.6 bc 1 870.2 c 1 717.1 bc 1 060.7 bc 1 155.4 bc 3 100.9 b 3 094.1 b

5 187.8 b 1 826.2 c 1 262.0 c 3 276.0 b

6 115.9 a 1 056.5 ab 1 232.4 c 2 404.8 ab

Mean for study area 195.7 x 1 496.7 z 1 032.9 y 2 725.3

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De Villiers, Van Rooyen and Theron

Overall seed bank size

survey the species composition in each plot was recorded per square metre and in the case of the perennial species (with the exception of the stoloniferous grass species), all individuals in the plot were counted. The density of the perennial plants in the standing vegetation obtained during the field survey, is compared with the density of perennial seedlings in the seed bank. Spatial and temporal data on seed bank size were analysed using a factorial analysis of variance (ANOVA) and Fischer’s LSD post hoc tests of the Statistica 6 computer program (StatSoft Inc., Tulsa Oklahoma USA), at α ≤ 0.05. Spatial data for the factorial ANOVAs were analysed at two scales: (a) by using the 10 locations per vegetation unit as replicates per sampling time, each replicate being the mean of 15 samples; and (b) by using all 150 samples as replicates for each vegetation unit per sampling time.

The soil seed bank of the Strandveld Succulent Karoo yielded a mean of 2 725 seedlings m–2 for samples collected in six vegetation units at different seasons (Table 1). Seedling mortalities were high and occurred primarily in the first month before identification was possible. Annuals constituted 88% of the total surviving seedling numbers and at a mean density of 1 497 seedlings m–2, were significantly more abundant than perennials, at a mean density of 196 seedlings m–2 (Table 1). Spatial variation The factorial ANOVA using the ten locations per vegetation unit as replicates indicated that both spatial and temporal variation significantly affected total seed bank size as well the sizes of the annual and perennial components (Table 2). A significant interaction was only found in the case of the perennial component. When the spatial data were analysed at the level of the individual samples (150 replicates) no significant difference (P > 0.05) could be detected on the spatial scale.

Results Expression of spatial and temporal variation depends on the scale of sampling. In this study, spatial distribution was expressed at the level of a vegetation unit, and temporal distribution on a seasonal scale.

Table 2: Factorial analysis of variance (α ≤ 0.05) for the mean number of emerged seedlings in samples taken in different vegetation units and seasons Category

Spatial variation (between vegetation units) P-value 0.0011 0.0098 0.0016

MEAN NUMBER OF EMERGED SEEDLINGS (m–2)

Perennials Annuals Total

Temporal variation (between seasons) P-value 0.0000 0.0000 0.0000

10 000

Unit 1

k

k

Unit 2

jk

Unit 3

ijk

8 000

Interaction P-value 0.0000 0.8569 0.6218

ijk

Unit 4

hij

hij

Unit 5

6 000

ghi

Unit 6

e-h

e-h d-g

4 000 b-f

2 000 a

abcabc bc abc ab

Winter 93

a

abc ab ab

abc

abc

c-g a-e

a-e

a-e a-e a-d

a-d

a-d

abc

abc

ab

Spring 93

f-i

a

Summer 93 Autumn 94

Winter 94

ab

a

abcabcabc

Spring 94

ab

a a

abc

abc abc

Summer 94 Autumn 95

SAMPLING SEASON Figure 2: Mean (n = 150) number of emerged seedlings for samples collected in different vegetation units and seasons. Bars with the same letter are not significantly different at α ≤ 0.05

South African Journal of Botany 2004, 70: 717–725

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Table 3: Mean number of emerged seedlings m–2 of different life forms, for samples taken in different seasons (n = 60 per season). Within a category, values followed by the same letter are not significantly different at α ≤ 0.05 Category Perennials Annuals Mortalities Total

Winter 1993 23.7 a 216.5 a 730.8 bc 971.0 a

Season Summer 1993 Autumn 1994 Winter 1994 146.6 b 562.8 c 146.6 b 1 072.6 ab 4 933.0 d 1 283.4 b 852.6 c 2 275.9 f 1 389.5 d 2 071.8 b 7 771.7 d 2 919.5 b

Spring 1993 42.9 a 500.8 ab 335.0 a 878.6 a

(a)

10 000

Vegetation unit 2 Vegetation unit 3

gh

MEAN NUMBER OF EMERGED SEEDLINGS (m–2)

Summer 1994 Autumn 1995 47.4 a 544.7 c 306.8 a 3 266.1 c 483.8 ab 1 774.0 e 838.0 a 5 584.9 c

Vegetation unit 1

h

fgh

8 000

Spring 1994 50.8 a 394.7 a 421.8 ab 867.3 a

Vegetation unit 4 efg

Vegetation unit 5

def

6 000

Vegetation unit 6 cde

4 000

bcd abc abc

abc abc

abc

2 000 a

a

a

a

ab

a

a

abc

abc

abc a

a

a

a

a

a

a

a

a

a

a

a

a a

m

10 000

(b) lm j-m

8 000

h-m

g-m

klm lm

f-m e-l

6 000

d-l b-l

a-k a-k

4 000 2 000

a-h

a-h a-d a-e

abc

a-e

ab

Spring 93

a-d

c-l a-k

a-j

a-i

a-h

a-h a-g

a-g

a-f a-e

a-d

a-i

a-i

a-e

a

Summer 93

Autumn 94

Winter 94

Spring 94

Summer 94

SAMPLING SEASON Figure 3: Mean (n = 30) number of emerged seedlings of samples collected in different vegetation units and seasons, a) examined directly after collection and b) examined at peak time for field germination (autumn). Bars with the same letter are not significantly different at α ≤ 0.05

Only a two-fold difference in spatial variation between vegetation units was observed (Table 1) when seasonal data were combined. Vegetation unit 1 yielded the lowest mean number of emerged seedlings (1 612m–2). Compared with other vegetation units, low densities of annuals were recorded in unit 1, but perennials had the highest percentage contribution (23% of the surviving seedlings) in this unit. Vegetation unit 6 also had a relatively small seed bank (Table 1, Figure 2) and yielded the lowest density of perennial seedlings (116 seedlings m–2). Seed bank size did not

differ significantly between vegetation units 2, 3, 4 and 5 when seasonal data were lumped (Table 1). Temporal variation The mean number of seedlings emerging from soil collected and examined in different seasons varied up to nine-fold (Table 3). Autumn sampling yielded significantly more seedlings than other sampling seasons, when vegetation unit data are combined (Table 3). The factorial ANOVA con-

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firmed the high temporal variation in the seasonally-germinable seed bank and its annual and perennial components (Table 2). Significant interyear differences in size were found in the winter, summer and autumn seasonally-germinable seed bank (Table 3), whereas the spring seed bank did not show interyear differences. When samples were stored and re-examined during the following autumn (Figure 3), the size of the seasonally-germinable soil seed bank of summer, winter and spring collected samples increased noticeably. The values given in Figure 2 differ slightly from those in Figure 3a because in the latter figure the mean values for only the 30 samples that were re-examined are given, whereas Figure 2 represents the mean of all 150 samples per vegetation unit. Standing vegetation Mean density of perennial plants (except for stoloniferous perennial grasses) in vegetation units 1 and 5 was 47 240 ± 13 808 individuals ha–1 and ranged from 15 840–83 180 individuals ha–1. Mean density of the surviving perennial seedlings recruited from the seed bank for these two communities was 2 208 000 seedlings ha–1. Discussion Overall seed bank size A mean seed density of 2 725 seeds m–2 was obtained in this study of the Strandveld Succulent Karoo. This value compares well with the seed bank densities of 100–4 000 seeds m–2, reported for the Northern Cape Province, South Africa (Dean et al. 1991), but is lower than seed densities reported for the annual-rich Upland Succulent Karoo in Namaqualand, which ranged from 5 000–41 000 seeds m–2 (Van Rooyen and Grobbelaar 1982). The seed bank estimates in this study were considerably higher than those for the southern Succulent Karoo (17–426 seeds m–2) (Esler et al. 1992). Seed densities in other desert regions such as the Sonoran Desert (4 000–15 000 seeds m–2, Reichman 1984), North American Great Basin (45–3 940 seeds m –2, Parmenter and MacMahon 1983), desert grassland in New Mexico (8 068 seeds per m–2, Henderson et al. 1988) and Loma Ancon of Central Peru (5 000–8 000 seeds m–2, Ohga 1992) are of the same order of magnitude. Annuals dominated the seed bank at Brand-se-Baai, contributing to 88% of the surviving seedlings. The dominance of annual species is common in seed banks (Coffin and Lauenroth 1989, Bertiller 1998), especially in arid regions, and has also been reported for other areas of the Succulent Karoo in South Africa (Van Rooyen 1999). Spatial variation Under homogeneous soil and management conditions, the soil seed content has been reported to vary spatially by a factor of ten and more (Albrecht and Forster 1996). However, low spatial variability in the size of the seed bank has also been reported (Coffin and Lauenroth 1989). These differences in spatial variation may be largely scale-related.

De Villiers, Van Rooyen and Theron

Seed densities in desert soils have been shown to be highly variable in space at a micro-topographical level (Van Rooyen and Grobbelaar 1982, Reichman 1984, Esler 1993, Van Rooyen 1999). A 98-fold variation in seedling density was recorded among the 15 samples at a single location in this study with many samples producing no emerged seedlings. However, on a vegetation unit scale, the spatial variation found in seed bank size in this study was relatively small (Table 1). Variation in spatial distribution between vegetation units was apparently masked by the large variation within units (compare ANOVA results at two spatial scales). Temporal variation In contrast to the low spatial variation at vegetation unit scale, temporal variation was large with the mean number of seedlings emerging in different seasons varying up to ninefold (Table 3). In general, the size of the seasonally-germinable seed bank of summer, winter and spring collections were poor predictors of the size of the potentially-germinable seed bank. Autumn sampling, which was done before the onset of the rainy season, yielded significantly more seedlings than other sampling seasons (Table 3). In autumn, seeds of most species have completed their period of after-ripening (De Villiers et al. 2002b) and should either be non-dormant or in a state of conditional dormancy (Baskin and Baskin 1998). Environmental conditions, notably the temperature regime, are favourable for germination in autumn (De Villiers et al. 2002a) and most local species germinate naturally in this season, provided there is sufficient rainfall. The percentage seedling loss due to mortalities was the lowest for the autumn sampling. Winter sampling yielded significantly less emerged seedlings than autumn sampling in all vegetation units and years (Figure 2, Table 3). The reduction in seed bank size in winter is caused largely by losses due to autumn germination without any appreciable inputs being made by seed production. Viable seeds that had not germinated in autumn, could also have entered secondary dormancy (Baskin and Baskin 1998). In winter rainfall areas, sampling of the soil seed bank during winter, usually gives a good estimate of the size and composition of the persistent seed bank. The seasonally-germinable spring seed bank was also significantly smaller than in autumn (Figure 2, Table 3). At the time of spring sampling, many species had not fully completed seed production and dispersal of seeds. This, as well as seed dormancy were probably responsible for the low numbers of seedlings in the spring seasonally-germinable seed bank. Although most plants had completed their seed production and released their seeds by the summer sampling time, the summer seasonally-germinable seed bank was also significantly smaller than in autumn (Figure 2, Table 3). The low seedling numbers recorded during summer were probably due to seed dormancy as well as unfavourably high temperatures for germination (De Villiers et al. 2002a, 2002b). Seasonal variation in soil seed bank densities as a result of seasonal inputs and losses of seeds has been reported elsewhere (Reichman 1984, Henderson et al. 1988, Coffin

South African Journal of Botany 2004, 70: 717–725

and Lauenroth 1989, Esler 1993, Lavorel et al. 1993, Malo et al. 1995, Milberg and Andersson 1998). In this study it was found that the temporal pattern of soil seed density was more pronounced than that of the spatial pattern. Populations that experience more temporal variation in the soil seed bank are predicted to have lower germination fractions and a higher fraction of their seeds in between-year seed banks than populations that experience less temporal variation (Pake and Venable 1996). A further source of temporal variation in seed bank size is the annual variation found in samples of the same season. With the exception of the spring sampling time, significant differences were found between 1993 and 1994 sampling in the same seasons. In arid ecosystems germination, growth and productivity of annual as well as perennial plants are regulated primarily by water availability and in arid ecosystem rainfall is highly variable in timing, amount and space. For example, Gutierrez et al. (2000) recorded a six-fold increase in the size of the seed bank after an ENSO-driven wet year in Chile. When compared with the potentially-germinable seed bank, the size of the summer, winter and spring seasonallygerminable seed banks greatly underestimated the true size. For example, the seasonal estimate of the summer seed bank was only approximately 13% of the potential estimate. The large differences between these estimates indicate the importance of dormancy and environmental conditions in regulating seed germination in arid ecosystems. These differences also illustrate how ‘snapshot’ studies, done at a single point in time and space may give a distorted view of real soil seed densities (Cabin and Marshall 2000). When planning revegetation programmes both estimates are essential. Although the seasonally-germinably seed bank underestimates the true size of the seed bank, this is the only fraction of the seed reserves that will germinate and that restoration efforts during that particular season can rely on. A low seasonal estimate compared with the potential estimate indicates that restoration efforts during that season may be largely wasted. Re-vegetation In the final Environmental Impact Report (Grindley and Barbour 1990) a suggestion was made as to how the success of the restoration programme could be measured. It was proposed that the programme could be considered as successful if at least 50% of the mean number of species recorded per 100m2 benchmark plot could be returned and if these plants were established at densities as close as possible to pre-mining densities. Mean seedling density of only the perennial component of the seed bank (2 208 000 seedlings ha–1) exceeded mean density of perennials in the standing vegetation (47 240 individuals ha–1) by two orders of magnitude. Even with a further 98% mortality rate of these seedlings, a density comparable to that of the standing vegetation can be obtained. Because the mean seasonally-germinable seed bank size was used in the above comparison and only the surviving seedlings were considered, this value is a conservative estimate of seedling density. Restoration efforts during autumn could potentially produce 5 million

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seedlings of perennial species ha–1. Theoretically, topsoil replacement should therefore be able to meet the objectives of the restoration programme in terms of densities. During topsoil replacement at the study site, the density of the resulting vegetation will be fairly homogeneous. The high spatial variation that was originally present on a micro-topographical scale (intra-vegetation unit) will be greatly reduced, if not entirely lost, during the re-vegetation process. Although differences in seed bank size were small between vegetation units, the differences in species richness and composition between units will be important in achieving the proposed re-vegetation goals (De Villiers et al. 1999, 2001). As a result of the domination of the soil seed bank of the Strandveld Succulent Karoo by annuals, topsoil replacement as a means of re-vegetation will yield a high density of annuals. Although annuals will contribute to post-mining vegetation efforts, and are essential in the initial sand stabilising phase of rehabilitation, these species are of less importance to long-term re-vegetation goals than perennial species. The latter species dominate the pre-mining standing vegetation in terms of abundance and species richness (De Villiers et al. 1999). The large temporal variation in the seasonally-germinable seed bank indicates that the timing of re-vegetation efforts will be crucial. Topsoil collection and replacement during the period of highest soil seed density, i.e. summer and autumn, is likely to ensure the largest possible reserve of genetic diversity in post-mining restored areas. During summer and autumn, the soil seed bank contains species with transient seed banks as well as those that accumulate persistent seed banks (Thompson and Grime 1979, De Villiers et al. 2002c). Although the summer potentially-germinable seed bank is large, germination is prevented during summer by unfavourable environmental conditions and the presence of seed dormancy. During autumn the size of the seasonallygerminable and potentially-germinable seed bank are almost equal, and at their highest levels. Restoration efforts should strive to make the most of favourable autumn temperature conditions, but because autumn rains at the study site are variable in exact timing and amount, rainfall can be supplemented by irrigation. However, irrigation of areas where topsoil replacement and sowing have been completed should not commence before autumn because a large percentage of the seeds are dormant in other seasons (De Villiers et al. 2002b). Irrigation in other seasons will yield poor results because only a fraction of the potentially-germinable seed bank will respond. Mechanisms to preserve replaced topsoil and/or sown seeds, such as hydromulch or sand-binding techniques, should be applied to keep the topsoil in place until irrigation in autumn. Transplanting of selected species can be done during autumn or winter and should be completed before the end of the rainy season. Stockpiling of soils before they are used in restoration can negatively affect recruitment in at least two ways. Short-lived viable seeds may be lost if the soil is stored too long, and environmental conditions, particularly temperatures, in the stockpiled soil may be so unfavourable that seeds are killed (Abdul-Kareem and McRae 1984, Van der Valk et al. 1992, Mahood 2003). Topsoil should therefore be replaced direct-

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ly after stripping on another area where mining has been completed and restoration starts. In conclusion, the relatively large soil seed bank in the Strandveld Succulent Karoo indicates that topsoil replacement can meaningfully contribute to the re-vegetation of mined areas. Although annual species dominate the seed bank, the potential contribution of perennial seed bank species should not be underestimated in re-vegetation efforts. Environmental conditions (soil moisture and temperature) can greatly influence recruitment from the seed bank, and the success or failure of a project can depend as much on environmental conditions as on the size and composition of the seed bank. Acknowledgements — The authors would like to thank Hester Steyn, Marie Pretorius, Helena du Plessis, Helga Rösch, Hendrik Wentzel and Magda Nel for their assistance during the field work, the Mazda Wildlife Fund for transport, the University of Pretoria for funding and facilities and the Anglo American Corporation for financial assistance. This material is based on work supported by the National Research Foundation (GUN 2053522).

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