Restoration treatments enhance early establishment of native forbs in a degraded temperate grassy woodland

Abstract

We investigated effects of three understorey restoration treatments, carbon (sugar) addition, spring burning and re-establishment of the perennial native grass Themeda australis (R.Br.) Stapf, on early establishment of eight species of native forbs in a degraded white box (Eucalyptus albens Benth.) woodland dominated by exotic annuals, in central New South Wales, Australia. Forb seeds and seedlings were introduced to treatments in a randomised, irrigated field trial and monitored for 4–8 months. Treatments significantly altered vegetation structure and composition, resulting in higher native grass and lower exotic grass cover on sugar and burnt plots (especially if seeded with Themeda), a proliferation of broadleaf exotic annuals on burnt plots, and continued high cover of exotic annual grasses and broadleaf annuals on control plots. Forb germination was largely confined to three large-seeded forbs, Bulbine bulbosa (R.Br.) Haw., Dichopogon fimbriatus (R.Br.) J.F.Macbr. and Microseris lanceolata (Walp.) Schultz-Bip., and few germinants of three small-seeded forbs were observed. Overall, germination (% emergence, and seedling numbers after 4 months) was significantly greater on sugar than burnt and control plots, while transplant survival was significantly enhanced by both sugar and burning treatments for up to 8 months after planting. Re-introduced Themeda swards significantly promoted seedling germination and transplant survival in burnt plots. While long-term seedling survival was extremely low owing to severe drought, we concluded that (i) the applied restoration treatments (especially carbon addition) have considerable potential for promoting forb establishment in degraded woodlands, (ii) reduced competition with exotic annuals associated with carbon addition outweighed potential disadvantages of reduced nutrient levels, (iii) competition with broadleaf annuals and/or exposure to more extreme environments associated with burning may be problematic for forb germination and (iv) Themeda swards may be more conducive to establishment of native forbs than swards of exotic annuals, owing to their contrasting growth strategies.