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The survival of viable seeds in stored topsoil from opencast coal workings and its implications for site restoration
Biological Conservation 43 (1988) 257-265
The Survival of Viable Seeds in Stored Topsoil from Opencast Coal Workings and Its Implications for Site Restoration J. B. Dickie, Kamini H. Gajjar* Royal Botanic Gardens, Kew, Wakehurst Place, Ardingly, West Sussex RH17 6TN, Great Britain
P. Birch & J. A. Harris Environment and Industry Research Unit, Biology and BiochemistryDepartment, North East London Polytechnic, Romford Road, London El5 4LZ, Great Britain (Received I February 1987; revised version received 1 July 1987; accepted 9 July 1987)
ABSTRACT The viable seed populations of two topsoil stores, three months andfour years old, were estimated. Samples were taken for incubation from the surface and 1 and 2 m depth in each case. The methods used gave a good indication of species composition of the viable seed banks. Analysis revealed significant effects of age and depth on total seednumbers, as well as an interaction between the two. The results are discussed in relation to conditions in the stores, with implications for subsequent re-vegetation of the reinstated topsoil.
INTRODUCTION Conservation in its wider sense includes the rehabilitation o f degraded habitats. Restoration o f the original natural or semi-natural vegetation, though often technically very difficult or impossible to achieve, is frequently the ultimate aim o f rehabilitation programmes. In opencast coal mining, as * Present address: Environment and Industry Research Unit, Biology and Biochemistry Department, North East London Polytechnic, Romford Road, London El5 4LZ, Great Britain. 257 Biol. Conserv. 0006-3207/88/$03"50 © Elsevier Applied Science Publishers Ltd, England, 1988. Printed in Great Britain
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J. B. Dickie, Kamini H. Gajjar, P. Birch, .L A. Harris
well as other similar forms of mineral extraction, it is accepted that removal, storage and replacement of the original topsoil is essential to the restoration process; e.g. Johnson & Bradshaw (1979), who point out that very great care is required to ensure the maintenance of the original topsoil structure on reinstatement. The topsoil is stored in mounds, usually for several years and there is evidence for significant changes in its fertility over this period. In a study of soils stored for periods ranging from 1.5 to 7years, Abdul-Kareem & McRae (1984) have presented evidence for anaerobiosis at depth, as indicated by increases in ammonium, carbon dioxide, ethylene and methane, as well as extractable ferrous iron and manganese. In another study Harris & Birch (1987) found large decreases in the microbial biomass, bacterial and fungal numbers, particularly in the deeper zones of the store. Johnson & Bradshaw (1979) have drawn attention to the potential importance of the soil seed bank as a source of propagules of the orginal vegetation, either desirable wild species or troublesome weeds. The presence and composition of buried viable seed populations in the stored topsoil could affect the subsequent establishment of stable vegetation and its species composition during the rehabilitation phase, e.g. through competition and its interaction with any changes in soil fertility. During extended storage of topsoil there may be significant changes in the soil seed bank and this study describes an analysis of the effects of depth and age of store on populations of viable seeds. There have been a number of intensive studies of the buried viable seed content of topsoils over the years, exemplified by the work of Thompson & Grime (1979), and reviewed by Harper (1977), Grime (1979) and Fenner (1985). These studies have been largely concerned either with relations between buried viable seed floras and surface vegetation, e.g. Major & Pyott (1966), or with the effect of cultivation on the decline in numbers of buried viable weed seeds in arable soils (Roberts & Feast, 1973). However, little appears to have been done on buried viable seeds in topsoil which has been temporarily removed and stored, apart from some work on Australian coastal sands (Lewis, 1976), stored for relatively short periods (2-4 months). However, this cannot be compared directly with the work reported here, which refers to heavier, moist soils stored for extended periods. In most cases the methodology of these studies is essentially similar, in that soil samples are spread out and incubated under conditions likely to promote seed germination. Seedlings are removed, counted and identified as soon as they emerge. These studies can be time- and space-consuming. Together with heterogeneity in the distribution of seeds in soil, limitations on the effort that can be put into them often means that sampling is
Survival of seeds in topsoil
259
insufficient to allow rigorous statistical treatment of the data obtained, especially with regard to confidence limits on estimates of numbers per unit area. The present study is not exempt from this limitation. MATERIALS AND METHODS The site used was at Erin, Derbyshire (Grid ref: SK 440735) where opencast mining was resumed in 1980, being previously worked 20-25 years ago. Topsoil samples were taken from two soil stores, three months and four years old respectively, on 12 November 1985. Triplicate horizontal cores (4 cm diameter x 30 cm long: volume = 377 cm 2) were taken at each of three depths (surface, 1 m and 2 m) from each store, using a gouge auger applied to a vertical horizon, freshly exposed by a mechanical digger. All the samples were moist on collection and transported to the laboratory in polythene bags for treatment the following day. The cores were thoroughly mixed and then halved, one half being stored in black polythene bags in an incubator at 6°C for eight weeks to provide a stratification treatment. The remainder were passed through a 5 m m sieve into 17 cm plastic seed pans and were then incubated initially in a controlled environment cabinet (Conviron C M P 3023) under the following conditions: 8 h at 26°C in light, 16 h at 16°C in dark in a constant RH of about 99%. They were examined at weekly intervals and any seedlings removed and counted. After each scoring the samples were stirred thoroughly to ensure that light-demanding seeds were brought to the surface and all samples were watered as necessary. Where not immediately identifiable, seedlings were transferred carefully to seedling compost and grown under glass until naming was possible (Clapham et aL, 1962; Chancellor, 1966). Incubation was completed on capillary-fed staging in a heated glasshouse; examination and cultivation being as before. After stratification the chilled subsamples were incubated in the same way. Incubation and cultivation was continued for about 200 days in all cases, after which time there had been no visible seedling emergence from any subsample for at least two weeks. After incubation all soils were further subsampled and examined for the presence of healthy ungerminated seeds, as a qualitative check on the efficiency of the incubation procedures. This was done by drying the soils and passing through a 2 m m sieve. The three replicates for each treatment were combined and thoroughly mixed, before taking three randomly drawn 100 ml samples from each. These were to provide one 300 ml subsample for each depth x age combination. The organic fraction from each of these samples was removed by flotation on tetrachloroethylene, the mineral fraction sinking. The organic matter was examined carefully for seeds and seed remains.
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J. B. Dickie, Kamini H. Gajjar, P. Birch, J. A. Harris
In many cases the numbers of seeds of any particular species in any one replicate were very small or zero and so data were pooled to give total counts of seedlings emerging for each age x depth combination. Where appropriate, statistical analysis was by contingency table on total counts, fitting a generalised linear model (log-linear) using the GLIM package (Baker & Nelder, 1978) to carry out Analysis of Deviance.
RESULTS A N D DISCUSSION Examination of the organic fractions of subsamples following incubation revealed only three intact seeds: one Rumex sp. from the surface of the old store; one grass caryopsis from the surface of the young store; and one Vicia sativa from 1 m in the young store.The empty remains of several Rumex sp. seeds were also recovered from the surface soil of both stores and from 1 m in the old store. The 1I. sativa required scarification for germination and positive identification. This highlights the possibility of underestimating buried seed populations of Leguminosae in this kind of study, due to the inadequacy of incubation techniques for complete removal of dormancy caused by seed-coat impermeability. Otherwise, the very few seeds recovered show the effectiveness of the techniques described in promoting the germination of a very high proportion of the viable seeds contained in the topsoil samples. A separate analysis, not shown, of total seedling emergence from chilled and non-chilled soil samples suggested a slight but significant reduction due to stratification. For simplicity and because the numbers emerging from chilled and non-chilled subsamples were quite similar, the two have been added together to give total numbers of seedlings emerging from each sample core, for the purposes of numerical analysis shown in Table 2. While significance is restricted because of limited sampling, the first two columns of Table 1 show eight species occurring in the stratified subsamples which were absent from the non-stratified, while the latter contained only one (Urtica dioica) which did not also emerge from the former. It indicates that, in general, pre-chilling of soil samples is probably desirable for a more complete representation of the buried viable seed flora. Table 1 shows the species and numbers of seedlings which emerged from the samples. Although a complete survey of species represented in the vegetation on the stores is not available, casual observations suggested that all those occurring were represented in the viable seed banks. At the time of sampling the young store was virtually devoid of vegetation. A number of species in the seed banks were not apparent as growing plants. The list is unfortunately botanically unexciting, probably reflecting the history of this
Survival o f seeds in topsoil
261
TABLE 1 List of Species Emerging as Seedlings from Samples of Topsoil Stored in Bulk. (The numbers shown are pooled from the triplicate cores at each age x depth combination. For stratification, + indicates presence of that species in any sample.) Species ~
Stratified Yes
Age o f soil store
No
3 months
4 years
Depth o f sarnple ( m ) 0 Atriplex hastata/patula Cirsium arvense C. vulgate Epilobiurn adenocaulon Galeopsis tetrahit Juncus bufonius Lapsana cornrnunis Matricaria rnatricarioides Plantago major Polygonum aviculare agg. P. convolvulus P. persicaria Ranunculus repens Rurnex crispus R. obtusi['olius Senecio jacobaea S. vulgaris Stellaria media Trifolium repens Tripleurosperrnum rnaritimurn spp. inodorurn Urtica dioica Grassesb Others c'a
+ + + + + + + + + + + + + + + + + + + + + +
1
2
7 1 9 1
1
0
I
2
1
14
1 + +
1
3
+ 1 + + + +
5 11
2 1 7
1 3 2
+ + + +
2 3 1
20
2
8
2
1
3 2
2
1
1
2 9 12
2 1 1
1
1 + + + +
21 1 33 22
17
6
30
2
82 18
21 6
58 6
2 2
1 2
° Throughout, the nomenclature is that of Clapham et al. (1962). b Because of numbers involved not every grass seed was grown on for identification, which is mostly impossible at an early seedling stage. Species represented under the heading 'grasses' were: Agrostis canina ssp. montana, A. stolonifera, A. tenuis, Bromus cornrnutatus, Holcus lanatus, and Poa annua, the last forming 30-60% of all grass seedlings observed. c Others: Dicotyledonous seedlings which died shortly after emergence and before positive identification was possible, probably comprising a mixture of the species in the Table. d An extra species revealed by the separation treatment at the end of the experiment (see Materials and Methods), but not included in the overall analysis, was one viable seed of Vicia sativa in the surface samples from the young store.
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J. B. Dickie, Kamini H. Gajjar, P. Birch, J. A. Harris
TABLE 2 (a) Total Numbers of Seedlings Emerging from Three Replicate Samples at each of Three Levels in Two Topsoil Stores. (Approximate numbers, m -2 are shown in brackets.) Age o f store
Young Old
Depth (m)
0
1
2
117 (3 250) 130 (3 611)
179 (4 972) 10 (277)
54 (1 700) 22 (611)
(b) Analysis of Effects of Age and Depth upon the Counts Shown in (a) Factor
Age of store Depth of sample Age x depth
• 2a
df
70"7 98.2 127.2
1 2 2
The values of X2 represent the decrease in deviance due to each factor and all are highly significant (P < 0.001).
particular site (see Materials and Methods), and consists of a typical range of poor grasses and ruderal forbs to be expected from a disturbed site (cf. sites 9 and 10, Thompson & Grime, 1979). The total number of identified species was 29, with only about half that number in the deeper samples from the old store, which is of the same order as found by Thompson & Grime (1979) for a range of herbaceous vegetation types. The frequent disparity between species present in the soil seed bank and the overlying vegetation (usually more species in the seed bank) has been commented upon previously, e.g. Major & Pyott (1966). The high variability in both total and species seed counts, presumably reflecting large heterogeneity in the distribution of buried seed populations and possibly occasional inclusion of subsoil, means that the numbers presented in Table 2 must be treated with some caution. This is especially the case when the numbers emerging are extrapolated to numbers per square metre. Also, in comparing the latter with those obtained from other studies, care should be taken because of non-standardisation of core depths. In undisturbed soils the density of the buried seed population decreases sharply with depth, whereas the process of building topsoil stores leads to mixing,
Survioal of seeds in topsoil
263
with surface soil represented throughout the store profile. Table 2 shows the estimates of numbers per m 2 for the surfaces of both stores to be very similar at 3.25 x 103 and 3.61 x 103 from the young and old stores, respectively. These values are at the low end of the ranges quoted by Fenner (1985) for grasslands (103 to 106 per m 2) and arable soils (103 to l0 s per m2). The surface soil on both stores would contain seeds freshly dispersed during the late summer and autumn of 1985, but by the collection date in November there could already have been germination of non-dormant species with a transient soil seed bank (the Type I seed bank of Thompson & Grime, 1979). The analysis of total seedling counts in Table 2 reveals, not unexpectedly, a significant interaction between the effects of age and depth of store. There was an apparent slight increase in numbers in the young store at 1 m deep and it is possible that this reflects seeds dispersed immediately before construction of the store in summer 1985, which would have germinated immediately if left close to the surface, but which experienced dormancy induction by burial (Wesson & Wareing, 1969). In the old store there was a significant reduction in viable seed numbers in the two deep samples, assuming that the buried seed population when the store was formed four years previously was of the order of that observed from the surface of the store. With no opportunities for further input of seeds, such a reduction would be generally expected due to germination, predation or loss of viability. In undisturbed soil with 20 arable weed species Roberts & Feast (1973) have observed exponential decline in numbers averaging about 12% per annum. The corresponding rates at depth in the old store are of the order of 35-50% per annum, but more detailed studies would be required to establish whether seeds in general died at more rapid rates in the conditions of the store than they would in undisturbed topsoil. It is perhaps surprising that any seeds at all survive the conditions deep in the store for as long as four years. Current work by one of us (J.A.H.) has revealed significant anaerobiosis, indicated by ammonium accumulation, in the old store at Erin. Further evidence from another site suggests that this may occur inside three months. Seeds in soils are more or less fully imbibed for much of the time, their longevity depending upon aerobic respiration to provide energy for the repair o f cellular damage (Villiers, 1973; Ibrahim et al., 1983). However, Harper (1977) comments that in British upland vegetation acid and/or waterlogged conditions support large seed banks. At the same time he mentions that some of the species involved, including Juncus and Agrostis spp., have very high seed outputs. Table 1 shows that the presence of Juncus bufonius is restricted to the deep samples from the old store, suggesting a mechanism for seed longevity in this species in the conditions of the store, perhaps not shared by the grasses in general or Tripleurospermum maritimum, for example. In their classification of soil seed
264
J. B. Dickie, Kamini H. Gajjar, P. Birch, J. A. Harris
banks, Thompson & Grime (1979) place several other Juncus spp. in Type IV, i.e. those with persistent soil seed banks, large in relation to the annual output of seeds. It would be interesting to compare the seed storage physiology of this group with other less resistant species under the presumably adverse conditions of anaerobic soil. Further work including more intensive sampling of a wider range of soil stores needs to be done to establish the general applicability of any conclusions from this work for re-vegetation using stored topsoil. However, the results of this survey suggest two main consequences for this aspect of conservation. First, in topsoil stored for several years, rather than months, the overall viable seed population will be significantly reduced except near the surface of the store. In addition, the longer the stores remain the more will the surface be subject to invasion by widely dispersed and undesirable ruderal and weed species. These will then add their seeds to the bank unless they are effectively controlled. Sole reliance on buried viable seed from redistributed topsoil would not guarantee rapid establishment of closed vegetation, with further opportunities for undesirable species to enter the consequent gaps. Secondly, as well as a reduction in total numbers of viable seeds, there may be a reduction in the number of species represented. Any potential for a reduced diversity would be unwelcome in a conservation context. At the site described here deep storage for four years appeared to favour one species in particular, Juncus bufonius, generally regarded as undesirable; although a similar change in the balance of species favouring desirable ones cannot be ruled out in a different situation. It seems that the soil seed bank of stored topsoil can be maintained throughout the store, so long as the storage period is no longer than a few months. Because of the time taken for mineral extraction, the latter condition cannot often be met and it would seem unwise to rely on seed survival deep in a store for several years. This would be especially so for an important species when its capability for survival under the inhospitable conditions deep in the store is unknown. In this case alternative measures could be taken; for example, viableseeds could be collected from the site at an appropriate time before topsoil stripping. These collections could then be held under optimum conditions in a seed bank until required for reintroduction.
ACKNOWLEDGEMENTS The authors would like to thank E. Brent-Jones, Restoration and Research Officer British Coal Opencast Executive, for his help especially regarding
Survival of seeds in topsoil
265
access to sites and the Opencast Executive for financial support (J. A. H. and P. B.). T h e y are also grateful to R. D. Smith for constructive comments on the manuscript.
REFERENCES AbduI-Kareem, A. W. & McRae, S. G. (1984). The effects on topsoil of long-term storage in stockpiles. Plant & Soil, 76, 357-63. Baker, R. J. & Nelder, J. A. (1978). The GLIM system release 3 manual, Oxford, Numerical Algorithms Group. Chancellor, R. J. (1966). The identification of weed seedlings of farm and garden. Oxford, Blackwell. Clapham, A. R., Tutin, T. G. & Warburg, E. F. (1962). Flora of the British Isles, 2nd edn. London, Cambridge University Press. Fenner, M. (1985). Seed ecology, London, Chapman and Hall. Grime, J. P. (1979). Plant strategies and vegetation processes, Chichester, Wiley. Harper, J. L. (1977). Population biology of plants, London, Academic Press. Harris, J. A. & Birch, P. (1987). The effects on topsoil of storage during opencast mining operations. J. Sci. Food agric., 40, 220-1. lbrahim, A. E., Roberts, E. H. & Murdoch, A. J. (1983). Viability of lettuce seeds, 2. Survival and oxygen uptake in osmotically controlled storage. J. exp. Bot., 34, 631-40. Johnson, M.S. & Bradshaw, A. D. (1979). Ecological principles for the restoration of disturbed and degraded land. App. Biol., 4, 141-200. Lewis, J. W. (1976). Regeneration of coastal ecosystems after mineral sand mining. Australian Mining, July 1976, 27-29. Major, J. & Pyott, W. T. (1966). Buried viable seeds in two California bunchgrass sites and their bearing on the definition of a flora. Vegetatio, 13, 253-82. Roberts, H. A. & Feast, P. M. (1973). Emergence and longevity of seeds of annual weeds in cultivated and disturbed soil. J. appl. Ecol., 10, 133-43. Thompson, K. & Grime, J. P. (1979). Seasonal variation in the seed banks of herbaceous species in ten contrasting habitats. J. Ecol., 67, 893-921. Villiers, T. A. (1973). Ageing and longevity of seeds in field conditions. In Seed ecology, ed. by W. Heydecker, 265-88. London, Butterworths. Wesson, G. & Wareing, P. F. (1969). The induction of light sensitivity in weed seeds by burial. J. exp. Bot., 20, 414-25.
The Survival of Viable Seeds in Stored Topsoil from Opencast Coal Workings and Its Implications for Site Restoration J. B. Dickie, Kamini H. Gajjar* Royal Botanic Gardens, Kew, Wakehurst Place, Ardingly, West Sussex RH17 6TN, Great Britain
P. Birch & J. A. Harris Environment and Industry Research Unit, Biology and BiochemistryDepartment, North East London Polytechnic, Romford Road, London El5 4LZ, Great Britain (Received I February 1987; revised version received 1 July 1987; accepted 9 July 1987)
ABSTRACT The viable seed populations of two topsoil stores, three months andfour years old, were estimated. Samples were taken for incubation from the surface and 1 and 2 m depth in each case. The methods used gave a good indication of species composition of the viable seed banks. Analysis revealed significant effects of age and depth on total seednumbers, as well as an interaction between the two. The results are discussed in relation to conditions in the stores, with implications for subsequent re-vegetation of the reinstated topsoil.
INTRODUCTION Conservation in its wider sense includes the rehabilitation o f degraded habitats. Restoration o f the original natural or semi-natural vegetation, though often technically very difficult or impossible to achieve, is frequently the ultimate aim o f rehabilitation programmes. In opencast coal mining, as * Present address: Environment and Industry Research Unit, Biology and Biochemistry Department, North East London Polytechnic, Romford Road, London El5 4LZ, Great Britain. 257 Biol. Conserv. 0006-3207/88/$03"50 © Elsevier Applied Science Publishers Ltd, England, 1988. Printed in Great Britain
258
J. B. Dickie, Kamini H. Gajjar, P. Birch, .L A. Harris
well as other similar forms of mineral extraction, it is accepted that removal, storage and replacement of the original topsoil is essential to the restoration process; e.g. Johnson & Bradshaw (1979), who point out that very great care is required to ensure the maintenance of the original topsoil structure on reinstatement. The topsoil is stored in mounds, usually for several years and there is evidence for significant changes in its fertility over this period. In a study of soils stored for periods ranging from 1.5 to 7years, Abdul-Kareem & McRae (1984) have presented evidence for anaerobiosis at depth, as indicated by increases in ammonium, carbon dioxide, ethylene and methane, as well as extractable ferrous iron and manganese. In another study Harris & Birch (1987) found large decreases in the microbial biomass, bacterial and fungal numbers, particularly in the deeper zones of the store. Johnson & Bradshaw (1979) have drawn attention to the potential importance of the soil seed bank as a source of propagules of the orginal vegetation, either desirable wild species or troublesome weeds. The presence and composition of buried viable seed populations in the stored topsoil could affect the subsequent establishment of stable vegetation and its species composition during the rehabilitation phase, e.g. through competition and its interaction with any changes in soil fertility. During extended storage of topsoil there may be significant changes in the soil seed bank and this study describes an analysis of the effects of depth and age of store on populations of viable seeds. There have been a number of intensive studies of the buried viable seed content of topsoils over the years, exemplified by the work of Thompson & Grime (1979), and reviewed by Harper (1977), Grime (1979) and Fenner (1985). These studies have been largely concerned either with relations between buried viable seed floras and surface vegetation, e.g. Major & Pyott (1966), or with the effect of cultivation on the decline in numbers of buried viable weed seeds in arable soils (Roberts & Feast, 1973). However, little appears to have been done on buried viable seeds in topsoil which has been temporarily removed and stored, apart from some work on Australian coastal sands (Lewis, 1976), stored for relatively short periods (2-4 months). However, this cannot be compared directly with the work reported here, which refers to heavier, moist soils stored for extended periods. In most cases the methodology of these studies is essentially similar, in that soil samples are spread out and incubated under conditions likely to promote seed germination. Seedlings are removed, counted and identified as soon as they emerge. These studies can be time- and space-consuming. Together with heterogeneity in the distribution of seeds in soil, limitations on the effort that can be put into them often means that sampling is
Survival of seeds in topsoil
259
insufficient to allow rigorous statistical treatment of the data obtained, especially with regard to confidence limits on estimates of numbers per unit area. The present study is not exempt from this limitation. MATERIALS AND METHODS The site used was at Erin, Derbyshire (Grid ref: SK 440735) where opencast mining was resumed in 1980, being previously worked 20-25 years ago. Topsoil samples were taken from two soil stores, three months and four years old respectively, on 12 November 1985. Triplicate horizontal cores (4 cm diameter x 30 cm long: volume = 377 cm 2) were taken at each of three depths (surface, 1 m and 2 m) from each store, using a gouge auger applied to a vertical horizon, freshly exposed by a mechanical digger. All the samples were moist on collection and transported to the laboratory in polythene bags for treatment the following day. The cores were thoroughly mixed and then halved, one half being stored in black polythene bags in an incubator at 6°C for eight weeks to provide a stratification treatment. The remainder were passed through a 5 m m sieve into 17 cm plastic seed pans and were then incubated initially in a controlled environment cabinet (Conviron C M P 3023) under the following conditions: 8 h at 26°C in light, 16 h at 16°C in dark in a constant RH of about 99%. They were examined at weekly intervals and any seedlings removed and counted. After each scoring the samples were stirred thoroughly to ensure that light-demanding seeds were brought to the surface and all samples were watered as necessary. Where not immediately identifiable, seedlings were transferred carefully to seedling compost and grown under glass until naming was possible (Clapham et aL, 1962; Chancellor, 1966). Incubation was completed on capillary-fed staging in a heated glasshouse; examination and cultivation being as before. After stratification the chilled subsamples were incubated in the same way. Incubation and cultivation was continued for about 200 days in all cases, after which time there had been no visible seedling emergence from any subsample for at least two weeks. After incubation all soils were further subsampled and examined for the presence of healthy ungerminated seeds, as a qualitative check on the efficiency of the incubation procedures. This was done by drying the soils and passing through a 2 m m sieve. The three replicates for each treatment were combined and thoroughly mixed, before taking three randomly drawn 100 ml samples from each. These were to provide one 300 ml subsample for each depth x age combination. The organic fraction from each of these samples was removed by flotation on tetrachloroethylene, the mineral fraction sinking. The organic matter was examined carefully for seeds and seed remains.
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J. B. Dickie, Kamini H. Gajjar, P. Birch, J. A. Harris
In many cases the numbers of seeds of any particular species in any one replicate were very small or zero and so data were pooled to give total counts of seedlings emerging for each age x depth combination. Where appropriate, statistical analysis was by contingency table on total counts, fitting a generalised linear model (log-linear) using the GLIM package (Baker & Nelder, 1978) to carry out Analysis of Deviance.
RESULTS A N D DISCUSSION Examination of the organic fractions of subsamples following incubation revealed only three intact seeds: one Rumex sp. from the surface of the old store; one grass caryopsis from the surface of the young store; and one Vicia sativa from 1 m in the young store.The empty remains of several Rumex sp. seeds were also recovered from the surface soil of both stores and from 1 m in the old store. The 1I. sativa required scarification for germination and positive identification. This highlights the possibility of underestimating buried seed populations of Leguminosae in this kind of study, due to the inadequacy of incubation techniques for complete removal of dormancy caused by seed-coat impermeability. Otherwise, the very few seeds recovered show the effectiveness of the techniques described in promoting the germination of a very high proportion of the viable seeds contained in the topsoil samples. A separate analysis, not shown, of total seedling emergence from chilled and non-chilled soil samples suggested a slight but significant reduction due to stratification. For simplicity and because the numbers emerging from chilled and non-chilled subsamples were quite similar, the two have been added together to give total numbers of seedlings emerging from each sample core, for the purposes of numerical analysis shown in Table 2. While significance is restricted because of limited sampling, the first two columns of Table 1 show eight species occurring in the stratified subsamples which were absent from the non-stratified, while the latter contained only one (Urtica dioica) which did not also emerge from the former. It indicates that, in general, pre-chilling of soil samples is probably desirable for a more complete representation of the buried viable seed flora. Table 1 shows the species and numbers of seedlings which emerged from the samples. Although a complete survey of species represented in the vegetation on the stores is not available, casual observations suggested that all those occurring were represented in the viable seed banks. At the time of sampling the young store was virtually devoid of vegetation. A number of species in the seed banks were not apparent as growing plants. The list is unfortunately botanically unexciting, probably reflecting the history of this
Survival o f seeds in topsoil
261
TABLE 1 List of Species Emerging as Seedlings from Samples of Topsoil Stored in Bulk. (The numbers shown are pooled from the triplicate cores at each age x depth combination. For stratification, + indicates presence of that species in any sample.) Species ~
Stratified Yes
Age o f soil store
No
3 months
4 years
Depth o f sarnple ( m ) 0 Atriplex hastata/patula Cirsium arvense C. vulgate Epilobiurn adenocaulon Galeopsis tetrahit Juncus bufonius Lapsana cornrnunis Matricaria rnatricarioides Plantago major Polygonum aviculare agg. P. convolvulus P. persicaria Ranunculus repens Rurnex crispus R. obtusi['olius Senecio jacobaea S. vulgaris Stellaria media Trifolium repens Tripleurosperrnum rnaritimurn spp. inodorurn Urtica dioica Grassesb Others c'a
+ + + + + + + + + + + + + + + + + + + + + +
1
2
7 1 9 1
1
0
I
2
1
14
1 + +
1
3
+ 1 + + + +
5 11
2 1 7
1 3 2
+ + + +
2 3 1
20
2
8
2
1
3 2
2
1
1
2 9 12
2 1 1
1
1 + + + +
21 1 33 22
17
6
30
2
82 18
21 6
58 6
2 2
1 2
° Throughout, the nomenclature is that of Clapham et al. (1962). b Because of numbers involved not every grass seed was grown on for identification, which is mostly impossible at an early seedling stage. Species represented under the heading 'grasses' were: Agrostis canina ssp. montana, A. stolonifera, A. tenuis, Bromus cornrnutatus, Holcus lanatus, and Poa annua, the last forming 30-60% of all grass seedlings observed. c Others: Dicotyledonous seedlings which died shortly after emergence and before positive identification was possible, probably comprising a mixture of the species in the Table. d An extra species revealed by the separation treatment at the end of the experiment (see Materials and Methods), but not included in the overall analysis, was one viable seed of Vicia sativa in the surface samples from the young store.
262
J. B. Dickie, Kamini H. Gajjar, P. Birch, J. A. Harris
TABLE 2 (a) Total Numbers of Seedlings Emerging from Three Replicate Samples at each of Three Levels in Two Topsoil Stores. (Approximate numbers, m -2 are shown in brackets.) Age o f store
Young Old
Depth (m)
0
1
2
117 (3 250) 130 (3 611)
179 (4 972) 10 (277)
54 (1 700) 22 (611)
(b) Analysis of Effects of Age and Depth upon the Counts Shown in (a) Factor
Age of store Depth of sample Age x depth
• 2a
df
70"7 98.2 127.2
1 2 2
The values of X2 represent the decrease in deviance due to each factor and all are highly significant (P < 0.001).
particular site (see Materials and Methods), and consists of a typical range of poor grasses and ruderal forbs to be expected from a disturbed site (cf. sites 9 and 10, Thompson & Grime, 1979). The total number of identified species was 29, with only about half that number in the deeper samples from the old store, which is of the same order as found by Thompson & Grime (1979) for a range of herbaceous vegetation types. The frequent disparity between species present in the soil seed bank and the overlying vegetation (usually more species in the seed bank) has been commented upon previously, e.g. Major & Pyott (1966). The high variability in both total and species seed counts, presumably reflecting large heterogeneity in the distribution of buried seed populations and possibly occasional inclusion of subsoil, means that the numbers presented in Table 2 must be treated with some caution. This is especially the case when the numbers emerging are extrapolated to numbers per square metre. Also, in comparing the latter with those obtained from other studies, care should be taken because of non-standardisation of core depths. In undisturbed soils the density of the buried seed population decreases sharply with depth, whereas the process of building topsoil stores leads to mixing,
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with surface soil represented throughout the store profile. Table 2 shows the estimates of numbers per m 2 for the surfaces of both stores to be very similar at 3.25 x 103 and 3.61 x 103 from the young and old stores, respectively. These values are at the low end of the ranges quoted by Fenner (1985) for grasslands (103 to 106 per m 2) and arable soils (103 to l0 s per m2). The surface soil on both stores would contain seeds freshly dispersed during the late summer and autumn of 1985, but by the collection date in November there could already have been germination of non-dormant species with a transient soil seed bank (the Type I seed bank of Thompson & Grime, 1979). The analysis of total seedling counts in Table 2 reveals, not unexpectedly, a significant interaction between the effects of age and depth of store. There was an apparent slight increase in numbers in the young store at 1 m deep and it is possible that this reflects seeds dispersed immediately before construction of the store in summer 1985, which would have germinated immediately if left close to the surface, but which experienced dormancy induction by burial (Wesson & Wareing, 1969). In the old store there was a significant reduction in viable seed numbers in the two deep samples, assuming that the buried seed population when the store was formed four years previously was of the order of that observed from the surface of the store. With no opportunities for further input of seeds, such a reduction would be generally expected due to germination, predation or loss of viability. In undisturbed soil with 20 arable weed species Roberts & Feast (1973) have observed exponential decline in numbers averaging about 12% per annum. The corresponding rates at depth in the old store are of the order of 35-50% per annum, but more detailed studies would be required to establish whether seeds in general died at more rapid rates in the conditions of the store than they would in undisturbed topsoil. It is perhaps surprising that any seeds at all survive the conditions deep in the store for as long as four years. Current work by one of us (J.A.H.) has revealed significant anaerobiosis, indicated by ammonium accumulation, in the old store at Erin. Further evidence from another site suggests that this may occur inside three months. Seeds in soils are more or less fully imbibed for much of the time, their longevity depending upon aerobic respiration to provide energy for the repair o f cellular damage (Villiers, 1973; Ibrahim et al., 1983). However, Harper (1977) comments that in British upland vegetation acid and/or waterlogged conditions support large seed banks. At the same time he mentions that some of the species involved, including Juncus and Agrostis spp., have very high seed outputs. Table 1 shows that the presence of Juncus bufonius is restricted to the deep samples from the old store, suggesting a mechanism for seed longevity in this species in the conditions of the store, perhaps not shared by the grasses in general or Tripleurospermum maritimum, for example. In their classification of soil seed
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banks, Thompson & Grime (1979) place several other Juncus spp. in Type IV, i.e. those with persistent soil seed banks, large in relation to the annual output of seeds. It would be interesting to compare the seed storage physiology of this group with other less resistant species under the presumably adverse conditions of anaerobic soil. Further work including more intensive sampling of a wider range of soil stores needs to be done to establish the general applicability of any conclusions from this work for re-vegetation using stored topsoil. However, the results of this survey suggest two main consequences for this aspect of conservation. First, in topsoil stored for several years, rather than months, the overall viable seed population will be significantly reduced except near the surface of the store. In addition, the longer the stores remain the more will the surface be subject to invasion by widely dispersed and undesirable ruderal and weed species. These will then add their seeds to the bank unless they are effectively controlled. Sole reliance on buried viable seed from redistributed topsoil would not guarantee rapid establishment of closed vegetation, with further opportunities for undesirable species to enter the consequent gaps. Secondly, as well as a reduction in total numbers of viable seeds, there may be a reduction in the number of species represented. Any potential for a reduced diversity would be unwelcome in a conservation context. At the site described here deep storage for four years appeared to favour one species in particular, Juncus bufonius, generally regarded as undesirable; although a similar change in the balance of species favouring desirable ones cannot be ruled out in a different situation. It seems that the soil seed bank of stored topsoil can be maintained throughout the store, so long as the storage period is no longer than a few months. Because of the time taken for mineral extraction, the latter condition cannot often be met and it would seem unwise to rely on seed survival deep in a store for several years. This would be especially so for an important species when its capability for survival under the inhospitable conditions deep in the store is unknown. In this case alternative measures could be taken; for example, viableseeds could be collected from the site at an appropriate time before topsoil stripping. These collections could then be held under optimum conditions in a seed bank until required for reintroduction.
ACKNOWLEDGEMENTS The authors would like to thank E. Brent-Jones, Restoration and Research Officer British Coal Opencast Executive, for his help especially regarding
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access to sites and the Opencast Executive for financial support (J. A. H. and P. B.). T h e y are also grateful to R. D. Smith for constructive comments on the manuscript.
REFERENCES AbduI-Kareem, A. W. & McRae, S. G. (1984). The effects on topsoil of long-term storage in stockpiles. Plant & Soil, 76, 357-63. Baker, R. J. & Nelder, J. A. (1978). The GLIM system release 3 manual, Oxford, Numerical Algorithms Group. Chancellor, R. J. (1966). The identification of weed seedlings of farm and garden. Oxford, Blackwell. Clapham, A. R., Tutin, T. G. & Warburg, E. F. (1962). Flora of the British Isles, 2nd edn. London, Cambridge University Press. Fenner, M. (1985). Seed ecology, London, Chapman and Hall. Grime, J. P. (1979). Plant strategies and vegetation processes, Chichester, Wiley. Harper, J. L. (1977). Population biology of plants, London, Academic Press. Harris, J. A. & Birch, P. (1987). The effects on topsoil of storage during opencast mining operations. J. Sci. Food agric., 40, 220-1. lbrahim, A. E., Roberts, E. H. & Murdoch, A. J. (1983). Viability of lettuce seeds, 2. Survival and oxygen uptake in osmotically controlled storage. J. exp. Bot., 34, 631-40. Johnson, M.S. & Bradshaw, A. D. (1979). Ecological principles for the restoration of disturbed and degraded land. App. Biol., 4, 141-200. Lewis, J. W. (1976). Regeneration of coastal ecosystems after mineral sand mining. Australian Mining, July 1976, 27-29. Major, J. & Pyott, W. T. (1966). Buried viable seeds in two California bunchgrass sites and their bearing on the definition of a flora. Vegetatio, 13, 253-82. Roberts, H. A. & Feast, P. M. (1973). Emergence and longevity of seeds of annual weeds in cultivated and disturbed soil. J. appl. Ecol., 10, 133-43. Thompson, K. & Grime, J. P. (1979). Seasonal variation in the seed banks of herbaceous species in ten contrasting habitats. J. Ecol., 67, 893-921. Villiers, T. A. (1973). Ageing and longevity of seeds in field conditions. In Seed ecology, ed. by W. Heydecker, 265-88. London, Butterworths. Wesson, G. & Wareing, P. F. (1969). The induction of light sensitivity in weed seeds by burial. J. exp. Bot., 20, 414-25.