Effects of clearing treatment on seed banks of the alien invasive shrubs Acacia saligna and Acacia cyclops in the Southern and South-Western Cape, South Africa

Published source details Holmes P.M., MacDonald I.A.W. & Juritz J. (1987) Effects of clearing treatment on seed banks of the alien invasive shrubs Acacia saligna and Acacia cyclops in the Southern and South-Western Cape, South Africa. Journal of Applied Ecology, 24, 1045-1051.
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Summary

Southern African endemic fynbos communities are highly susceptible to invasion by alien trees and shrubs. Two acacia species are amongst the most serious invaders, these are Acacia saligna and A.cyclops both of which are now widely distributed and often occur at high densities. Conservation of fynbos requires the control of these and other alien woody species. Many legumes, including Acacia spp. have become weeds partly as a result of large numbers of viable seeds persisting in the soil. Seed bank reduction thus provides a potential key to successful control of invasive acacias. This paper investigated the effects of different shrub clearing treatments on seed banks of these two Acacia species.

Study sites: Ten sites (all those available for this study) with dense Acacia infestation and well-documented past management were studied. Each site was subjected to shrub-clearing which involved felling, stacking and burning treatments in various combinations (as indicated):

1) Penhill, Cape Flats (33º59'S 18º43'E) - A. saligna infested c.30 years; fell only

2) Grootphisantekraal, Durbanville (33º48'S 18º41'E) - A. saligna infested c.25 years; fell only

3) Buffels Bay, Cape of Good Hope Nature Reserve (CGHNR) (34º19'S 18º41'E)
- A. saligna infested c.25 years; fell pile and burn piles

4) Silvermine Nature Reserve, Muizenberg (34º05 S 18º27'E) - A. saligna infested c.25 years; fell and burn

5) Cape Flats Nature Reserve, Cape Flats (33º56'S 18º37'E) - A. saligna infested c.50 years; fell and burn

6) Goukamma Nature Reserve, Sedgefield (34º03'S 22º57'E) - A. cyclops infested c.45 years; fell and stack in rows

7) Buffels Bay, CGHNR (34º19'S 18º27'E) - A. cyclops infested c.80 years; fell pile and burn piles

8) Rondevlei Nature Reserve, Cape Flats (34º03'S 18º29'E) - A. cyclops infested c.35 years; fell pile and burn piles

9) Walker Bay Forest Reserve, Hermanus (34º25'S 19º24'E) - A. cyclops infested c.40 years; burn live stand

10) Potbank, CGHNR (34º19'S 18º25'E) - A. cyclops infested c.40 years; fell, pile and burn area

Subsequent seedling control was undertaken to prevent further seed input.

Seed densities and viability: Densities of buried seeds were estimated in 1985 before and after clearing treatments, and germination experiments in the laboratory were undertaken to assess seed viability.

Acacia saligna: Felling alone had no effect on A.saligna seed banks after 1 year, but after 4-6 years they declined by 80%. Felling followed by burning decreased seed density by 90% after 1 year and by over 94% after 7-8 years. The lowest seed density recorded was seven seeds/m² 7 years after felling and burning (site 4). On part of site 4 where felling and burning had occurred, but where follow-up control of acacia seedlings had not been undertakne, the resultant developing stand restored the seed bank to its original size (7140 ±570 seeds/m²) within 7 years. Germination trials indicated high viability of A.saligna seeds remaining in the soil (86-l00%), with no significant differences between sites or clearing treatments

Acacia cyclops: A.cyclops seed banks declined exponentially after clearing, with greatest reduction in the first 1-2 years and little thereafter. Where sites were partially burnt (sites 7 and 8), seed densities were lower under burnt piles than in felled but unburnt areas. Similarly, the seed densities at site 6 were lower where brush was cleared than under the brush rows. Germination trials indicated a large range in viability of seeds between sites (46-95%), with site 9 the highest. Percentage viability of soil-stored seeds increased significantly with time elapsed since clearing at site 6.

Conclusions: Seed banks of A.cyclops but not A.saligna, were reduced by shrub felling alone after 1 year, apparently because a high proportion of seeds do not have seed-coat induced dormancy, and thus germinate immediately. Seed banks of both species declined significantly after 1 year with burning, but in general, not thereafter.

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