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Changes in the shrub stratum

Prior to 1988 patches of Bursaria in the woodland were generally associated with tree bases or steeper slopes which were not slashed. Our 1988 woodland photos showed an open grassy ground cover with previously slashed Bursaria rootstocks beginning to regrow.

These Bursaria spinosa shrubs grew very vigorously to form dense thickets within about 5-7 years after slashing. After the initial regrowth, lateral spread was slow, apart from longer spreading lateral branches as the plants increased in size, with seedlings mainly recruiting under parent plants. As a result, in 2006, many of the areas between Bursaria spinosa clumps are still relatively open.

Bursaria spinosa can resprout from its base after cutting and fire. Where rootstocks have persisted in the soil after grazing or slashing vigorous recovery takes place resulting in plants up to 2-3 m high in say 5-7 years. Total rooted frequency in our 8 measured plots (200 1 m x 1 m subplots) increased from 25% in 1988 to a peak of 47% 12 years later, though it subsequently declined to 38% in the following 3 (dry) years. This measurement includes both mature 2-3 m high plants, as well as smaller plants and seedlings 5-30 cm high.

Our interpretation is that growth of Bursaria spinosa rapidly recovers where it has previously been suppressed, but that invasion into new areas is a slow process. We have previously thought that lateral spread was by rootsuckers, but extensive digging around established plants has not verified this as a possibility. Spread does appear to occur mainly adjacent to established plants, though occasional plants appear at distances of 10-20 m from the main edge by seedling recruitment. How successfully Bursaria recruits from seed, is not clear. Viable seed is produced each year, and there is no dormancy or soil-stored seedbank. We have only observed occasional seedlings, i.e. that can be identified from presence of cotyledons, but these have all subsequently disappeared. Nursery-grown plants and small plants dug up show a tapering carrot-like rootstock develops early in the establishment phase. There may also be some suppression of young plants by established adults.

While Bursaria spinosa generally resprouts vigorously after fire, regrowth of some plants burnt in the 2002 and 2005 burns has been very slow, presumably because of the low rainfall over this period.

Bursaria spinosa is very common in many Cumberland Plain Woodland remnants and there is the perception that it will crowd out other species. On the other hand its dense prickly canopy may protect associated species from stock grazing and damage from trampling, and with its relatively open canopy allows light for other species to grow. As a major component of Cumberland Plain Woodland and because of its importance in remnant sites, more research on the ecology of Bursaria spinosa is needed.

See our image gallery of shrub species in the woodland.

December 1988 to March 2006: the shrub stratum returns!

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