Overcoming restoration thresholds and increasing revegetation success for a range of canopy species in a degraded urban Mediterranean-type woodland ecosystem

Abstract

Widespread decline of Mediterranean-type ecosystem (MTE) woodlands can result in a loss of soil- and canopy-stored seed banks. This can drive woodlands across a biotic threshold, where natural regeneration cannot occur. Without management intervention, these woodlands will suffer local extinction. Using a Mediterranean-type, degraded woodland as a case study, we undertook field trials over 3 years, with the aim of increasing revegetation success by (1) introducing propagules of key canopy species to overcome this biotic threshold and (2) applying commonly used revegetation treatments (abiotic treatments such as the addition of nutrient and water resources, two types of tree guards, and combinations of these). We found that (1) control plants had low establishment success, confirming the crossing of a biotic threshold and the practical irreversibility of the degraded state without intervention, (2) plant establishment was often significantly higher for treated than for control seedlings and (3) supplementation of nutrient and water resources seems to be critical in terms of increasing early seedling establishment for some species. We suggest that in declining woodlands that have crossed biotic thresholds, merely adding propagules does not ensure successful revegetation. The present study has practical implications for restoration activities in degraded MTE communities where biotic thresholds may have already been crossed.