Active restoration after three decades: Seed addition increases native dominance compared to landscape-scale secondary succession

Authors: O'REILLY-NUGENT, ANDREW - University Of Canberra; Blumenthal, Dana; WANDRAG, ELIZABETH - University Of New England; DUNCAN, RICHARD - University Of Canberra; CATFORD, JANE - King'S College

Submitted to: Journal of Applied Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/3/2024
Publication Date: 10/7/2024
Citation: O'Reilly-Nugent, A., Blumenthal, D.M., Wandrag, E.M., Duncan, R.P., Catford, J.A. 2024. Active restoration after three decades: Seed addition increases native dominance compared to landscape-scale secondary succession. Journal of Applied Ecology. 61(12):2997-3006. https://doi.org/10.1111/1365-2664.14778.
DOI: https://doi.org/10.1111/1365-2664.14778

Interpretive Summary: Ecological restoration is central to the global target of restoring at least 30 per cent of degraded terrestrial ecosystems by 2030. Ecological restoration often involves active interventions to assist natural regeneration. Our findings indicate that active restoration of degraded grasslands is only necessary where natural succession to native dominance has either slowed, stalled or diverged. Using data that is both long-term and spatially extensive, we show how chronosequences of passively regenerating communities provide successional context for assessing restoration outcomes. Accounting for regional-scale trends in secondary succession allows practitioners to target sites that maximise effectiveness of active restoration and avoid wasted effort at sites that would transition to native dominance without intervention.

Technical Abstract: Are efforts to actively restore ecosystems always worthwhile? They may not be if the immediate effects of restoration are soon overwhelmed by natural successional change. Experimentally assessing enduring outcomes of restoration can be challenging when control plots are themselves affected by restoration interventions albeit indirectly (e.g., through local spread of sown species). Suitable reference sites that can be directly compared to, but are independent of, restoration experiments are rare. We assessed whether seed addition increased native dominance within a controlled restoration experiment in old fields abandoned from agriculture. We assessed the efficacy of active restoration over three decades, comparing the abundance of four functional groups in our experiment against the secondary succession of nearby old fields abandoned at different times. Overall, seed addition of native grassland species, coupled with herbicide application, burning and tilling, led to higher abundance of native grasses and native forbs and lower abundance of non-native species 27 years after restoration, compared to reference plots in 21 fields undergoing passive regeneration. However, findings about magnitude and duration of restoration effects varied widely depending on the choice of reference plots. Restored plots had higher native abundance than control plots set within the original restoration experiment for the first two years, but these differences were no longer evident after 27 years. Based on community composition of two reference fields of the same age, seed addition did accelerate the transition to native dominance. Lastly, a chronosequence of abandoned old fields in the area showed that many naturally regenerating communities transition to native grass dominance without active restoration, but that active restoration greatly increases native forb abundance.