Establishing native species from seed within exotic grasslands

Authors: Nick Ledgard, Marie Charru, Harlon Davey
Publication: New Zealand Journal of Forestry, Volume N.Z.J.For. 2008, Issue N.Z.J.For. 53(1) 2008, pp 23-32, May 2008
Publisher: New Zealand Institute of Forestry

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Abstract: There is an increasing interest in converting existing vegetation, often dominated by exotic grasses, through to native (usually woody) plant communities. This may happen by natural processes of spread by seed, by physically planting nursery- grown seedlings, or by direct sowing of seed. We explored whether native species could be established from seed into ex- pasture grassland environments at seven sites in North Canterbury and Banks Peninsula. Particular emphasis was placed on site preparation, via management of the grass cover (grazing or mowing), soil disturbance, and the use of herbicides to control competition. Seed of twenty-three species was sown in late June 2006 in the form of locally collected pure seed and forest litter. After 1 year, almost without exception, native seedlings were only found where seed had been sown. At most sites there was a definite trend for increased seedling numbers where herbicides were applied, particularly where the grass was short, but the greatest increases occurred where ground disturbance had created a bare soil surface. The only situations where there were more seedlings in herbicide treated plots than in disturbed plots were where the grass was naturally short or had been mown. At the one site where it was known how many seeds had been sown, kohuhu (Pittosporum tenuifolium) had the best emergence in the short grass, followed by poroporo (Solanum aviculare) and broadleaf (Griselinia littoralis). Kohuhu also had the best mean emergence in the long grass, followed by cabbage tree (Cordyline australis), broadleaf and kowhai (Sophora tetraptera). Seedlings tended to be taller in disturbed plots. Seventy percent of the seedling numbers that were recorded in mid-season were still present at the season’s end, with the best survival in the disturbed treatment. We conclude that the establishment of native species by direct seeding has potential as a cheaper technique for restoring an indigenous woody cover in ex-pasture grassland environments, particularly where large areas have to be treated. However, the results of seeding will vary between sites and seasons, and weed control after seedling emergence remains a critical factor. More research is needed in these areas.

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