Invasion shadows in soil system overshadow the restoration of invaded ecosystems: implications for invasive plant management.
Recognizing the global urgency of restoring degraded ecosystems, the United Nations has proclaimed 2021-2030 as the Decade on Ecosystem Restoration. Among various global drivers of ecosystem degradation, the long-persisting impact of invasive species in the form of invasion shadow (or legacy effect) within belowground soil system, even after physical removal of plant invaders aboveground, is one of the major impediments to successful restoration. Using field manipulation experiment, we investigated the invasion shadow effects by evaluating ecological changes both in above- and belowground systems, following removal of Leucanthemum vulgare - a global plant invader - across multiple sites selected along an elevational gradient in Kashmir Himalaya. At each site, three types of plots were set up: plots invaded by L. vulgare- referred to as invaded reference, invader removal plots within the invaded area - referred to as removal plots, and spatially separated uninvaded plots outside the invaded area - referred to as uninvaded reference. The removal plots were significantly different from both invaded and uninvaded reference plots in terms of multiple biodiversity metrics. Both species richness and cover of exotic plants increased significantly in the removal plots when compared with the reference plots. However, in terms of integrated community composition metric, the removal plots differed significantly from the invaded reference plots only. Further, in terms of standard soil abiotic metrics, the removal plots were significantly different from both invaded and uninvaded reference plots, thus clearly suggesting invasion shadows. Our results reveal that, even after removal of invasive species aboveground, there are persistent invader-induced changes in soil chemistry, which impedes successful restoration of invaded ecosystem. We conclude that physical removal of invasive species aboveground alone is inadequate for restoration of invaded ecosystems, and call for additional management actions to anticipate invasion shadow effects in soil system. Building on the lessons learnt from this study, we propose a way-forward for effective management and policy interventions to guide ecological restoration of invaded ecosystems.