Topsoil Seed Bank – Worth its Weight in Gold?
Following on from her series of four articles on Mine Rehabilitation and Closure – Are you getting the Basics Right?, Astron’s Rehabilitation Principal Scientist, Katina De Sousa, discusses loss of seed from the topsoil seed bank in the rehabilitation process. Katina has worked in the mining industry for over 10 years and has focused her career on mine rehabilitation and closure. She is also a member of the Management Committee for the Revegetation Industry Association of Western Australia.
The Value of the Topsoil Seed Bank
In rehabilitation there are three potential sources of propagules:
- Topsoil containing a soil seed bank
- Application of seed
- Planting of tubestock (nursery produced seedlings).
Of these three sources one of them is effectively free – if you are applying topsoil to your rehabilitation areas then the seed in the soil seed bank comes with it for no additional cost. Therefore, to minimise the price tag of conducting your rehabilitation and maximise your chances of success it is important to get as much vegetation establishment from the soil seed bank as possible.
The topsoil seed bank may also provide species for which seed is not commercially available or for which availability is low or variable. Koch and Ward (1994) found that at Alcoa’s sites, 77% of species found in rehabilitated pits germinate from seed of the topsoil rather than from applied seed. However, the efficiency of use of seed in the topsoil for mine restoration appears to be low. In 1996, at Alcoa the highest efficiency of seedling establishment from topsoil stored seed was about 2% (Koch et al. 1996).
So, where has all the seed gone?
Part 1 - Where has all the Seed Gone?
In this article, Katina summarises the findings of a study conducted by Alcoa which pinpoints at which steps of rehabilitation seed can be lost from the seedbank. In Part 2 – Save the Seeds, practical recommendations will be provided for how to minimise loss of seed during the rehabilitation process.
Alcoa’s study on the effects of topsoil handling indicates that substantial losses of seed can occur during soil handling activities, particularly when soil is stockpiled (Koch et al. 2006). Prior to disturbance, the average density of germinable seed stored in the surface soil was found to be 352 per square metre (Koch et al. 1996). For areas where the topsoil had to be stockpiled, the action of clearing the vegetation, stripping the topsoil and creating stockpiles resulted in a significant loss of seed with the average density reducing to 31% of the density at the uncleared sites. After the topsoil was stockpiled for 10 months and then respread, Alcoa found that the seed store was reduced to 13% of the original forest store (Koch et al. 2006).
After deep ripping, the seed store was 15% of the original forest seed store for the stockpiled topsoil. Alcoa found that ripping does not appear to cause any further loss of seed of seed once the topsoil is respread. In two out of the three sites studied there was an apparent increase in seed density after ripping, thought to be due to ripping operations providing the scarification required to germinate “hard” seeds (Koch et al. 2006).
Where topsoil could be stripped and immediately respread the seed store reduced to 32% of the original seed store (compared with 13% for topsoil stockpiled for 10 months). After deep ripping the seed store was 51% of the original seed store (compared with 15% for topsoil stockpiled for 10 months).
However the story does not end there. Not all of the remaining seed may germinate to produce vegetation in your rehabilitation area ….
To germinate, seeds must be sown sufficiently deep to ensure continual moisture supply and good seeding anchorage but not too deep to cause depletion of remaining seed nutrient reserves and increased time between seed germination and seedling emergence (Grant et al. 1996). Alcoa determined that most seeds buried deeper than five centimetres are unable to germinate and establish – particularly for species with small seeds (Grant et al. 1996). During the process of stripping, respreading and ripping the soil is mixed and the entire seed store is distributed evenly through the zero to 20 centimetre profile instead of being concentrated in the surface two to five centimetres as at undisturbed sites (Tacey and Glossop 1980, Koch et al. 2006). Therefore, in the case of the direct returned topsoil (no stockpiling), only 14.7% of the original seed store was within five centimetres of the soil surface (31 seeds per square metre) (Koch et al. 2006). This reduced to 5% (22 seeds per square metre) for the stockpiled topsoil. This explains why Alcoa were only recording two to ten plants per square metre establishing from the topsoil, even when direct return of topsoil was carried out (Koch et al. 2006).
Loss of seed during the rehabilitation process is attributed to a number of factors:
- physical damage during soil handling procedures
- loss of seed while soil is stockpiled
- dilution of seed when soil is respread
- burial of seed at a depth too deep for emergence.
Stay tuned for recommendations on how to reduce loss of seed from the soil seed bank in the second article of this series Part 2 – Save the Seeds.
Grant, CD, Bell, DT, Koch, JM and Loneragan, WA 1996, ‘Implications of seedling emergence to site restoration following bauxite mining in Western Australia’, Restoration Ecology, vol. 4 pp. 146-154.
Koch, JM and Ward, SC 1994, ‘Establishment of understorey vegetation for rehabilitation of bauxite mined areas in the jarrah forest of Western Australia’, Journal of Environmental Management, vol 41, pp. 1-15.
Koch, JM, Ward, SC, Grant, CD and Ainsworth, GL 1996, ‘Effects of Bauxite Mine Operations on Topsoil Seed Reserves in the Jarrah Forest of Western Australia’, Restoration Ecology, vol. 4, pp. 368-376.
Tacey, WH and Glossop, BL 1980, ‘Assessment of topsoil handling techniques for restoration of sites mined for bauxite within the jarrah forest of Western Australia’, Journal of Applied Ecology, vol. 17, pp. 195-201.
Vhlahos, S and Bell, DT 1986, ‘Soil seed-bank components of the northern jarrah forest of Western Australia’, Australian Journal of Ecology, vol. 11, pp. 171-179.
Image: Acacia tetragonophylla.