Seed production and direct seeding to restore grassy understorey diversity at Mount Annan, NSW.

Peter Cuneo, Jordan Scott and Katharine Catelotti

Key words: direct seeding, grassy woodland restoration, seed production areas, Cumberland Plain woodland

Need for restoring grassy diversity. The rapid spread of African Olive (Olea europaea ssp. cuspidata) in the Cumberland Plain region of western Sydney in recent decades is now a significant conservation concern (Figs 1 and 2). Cumberland Plain Woodland (CPW) is now listed at the state and federal level as a critically endangered ecological community, and African olive invasion is recognised as the greatest invasive threat to CPW, and listed under the NSW TSC Act as a Key Threatening Process.

Dense monocultures of African olive are now established at a landscape scale in western Sydney, and there has been considerable use of mechanical mulching (‘forest mowing’) to control these highly degraded CPW remnants/monocultures (Fig 3). Often only remnant trees remain, and once these dense olive infestations are controlled, land managers are faced with several years of follow up olive control, degraded native soil seedbank and a profusion of annual weeds.

The Australian Botanic Garden, Mount Annan (ABGMA) has completed over 40 hectares of mechanical control of African Olive since 2009. Recent research (Cuneo & Leishman 2015) has indicated that a ‘bottom up’ approach restoration using native grasses as an early successional stage has potential to restore these transitional landscapes and achieve a trajectory towards CPW.

Fig 1. Hillside African Olive invasion

Fig 2. Nil biodiversity beneath dense African Olive canopy

Fig 3. Olive mulching machine

Like many landscape scale ecological restoration projects ABGMA faces a shortage of native grass seed, however a successful NSW Environmental Trust application provided the funding support to develop a 1500 sq metre native grass seed production area as part of the Australian PlantBank landscape. The key objective was to grow high quality weed free native grass seed (of known germinability) to direct sow on degraded African olive sites where the native grassy understory had been lost.

Seed production area. The seed production area was established by tubestock planting of four key local grasses, Dichelachne micrantha (Plume grass), Microlaena stipoides (Weeping meadow grass), Chloris truncata (Windmill grass) and Poa labillardieri (Tussock grass) (Figs 4 and 5). Seed was wild source collected from CPW and grasslands within ABGMA, which provides a reference vegetation type and condition to guide restoration. The seed production area which was irrigated and fenced to exclude rabbits was highly productive, even during the first summer season. Both hand and mechanical harvesting were used, and the total output over the 2014/15 summer was impressive 118 kg of seed material harvested. All seed batches were germination tested at PlantBank which indicated a total output of over 13 million viable seeds from the first harvest season.

Fig 4. Planting out seed production area

Fig. 5. Plumegrass in seed production area, almost ready for harvest

Direct seeding of grasses. Restoration challenges included large areas, profuse annual weeds and competitive olive seedlings on the transitional post-olive sites. A decision was made to focus the direct seeding across one fifth of the treatment area in a series of cultivated 2m wide strips at 8m spacing. The strips were created along contours to limit the erosive potential of the prepared areas. These seeded strips could then be managed in a similar way to surrounding cleared areas with broadleaf selective herbicide and slashing.

Seeded grass strips were prepared using a small track machine with surface tilling attachment to provide good soil/seed contact (Fig 6). Seed material (seed/stalks) were combined with compost (Fig 7) and sand and hand broadcasted. In an effort to create an ‘in situ’ seed production area and robust native grass populations, harvested grass seed was then used to high density (up to 3300 seeds/m²) direct sow a total of 5km x 2m wide strips throughout 5 hectares of cleared African olive sites at ABGMA in March 2015.

Favourable conditions during autumn 2015 resulted in excellent field germination, with established seedling densities of up to 608 seedlings/m² observed after 10 months (Fig 8). The combination of surface tilling and dense sowing rates has resulted in a dense and competitive grass layer, however some further broadleaf weed control along the strips will improve long term grass density and establishment.

These native grass strips will provide a ‘nucleus’ grass seed source for these degraded areas, maintaining soil stability, improving ecological resilience and accelerate the regeneration of these degraded areas.

Fig 6. Direct seeding strips prepared

Fig 7. Mixing bulk native grass seed

Fig 8. Seed strip established after one year

Lessons learned. Using known quality seed and achieving seed/soil contact through surface tilling was important to success, as cleared olive areas have a heavy mulch layer which limits seed contact. The use of both C3 and C4 grasses in the direct seeding mix worked well and is recommended, particularly for autumn sowing where cool season C3 can establish a quick cover followed by C4 grass establishment in summer. Some mechanical wild grass seed harvesting is also done at ABGMA, however practitioners should be aware of the risk of grassy weed contamination. Overall the project was relatively labour intensive, but some mechanisation of seed spreading could be achieved with a compost spreader. Steep terrain at ABGMA is a limiting factor for some machinery, and hand broadcasting can be a practical option.

Future directions would include scaling up the size of seed production areas, and refining mechanical harvesting techniques. Grass seed strips will be progressively managed, and seed either mechanically harvested or slashed to spread seed across the site. Once grasses are well established, the next phase will include direct seeding of CPW shrubs and trees. The well prepared and presented seed production area with mass plantings of native grasses, attracted considerable visitor interest at ABGMA and became a focus for several practitioner field days on olive control and ecological restoration.

Acknowledgements: Implementation of this NSW Environmental Trust project has relied significantly on working with industry partners, Greening Australia (Paul Gibson-Roy, Samantha Craigie, Chris Macris), Cumberland Plain Seeds (Tim Berryman) and Australian Land & Fire Management (Tom McElroy) who have bought additional technical expertise as well as on-ground implementation.

Contact person: Dr Peter Cuneo, Manager Seedbank & Restoration Research, Australian Botanic Garden, Mount Annan, NSW Australia. Locked Bag 6002, Mount Annan NSW, 2567. Email: peter.cuneo@rbgsyd.nsw.gov.au Phone: +61 (2)46347915