'Social distancing' between plants may amplify coastal restoration at early stage.
Ecological restoration is crucial to counteract the degradation of coastal ecosystems. Recent studies increasingly suggest that coastal restoration success can be amplified by harnessing positive interactions between individuals of foundation species, with clumped spatial designs as an easy-to-implement approach. However, positive interactions are strongly context dependent, making it often difficult to tell a priori if clumped configurations would necessarily enhance survival and growth. Another uncertainty is that the relevance of below-ground spatial configuration remains unclear. Here, using the clonal plant species Scirpus mariqueter as the material, we conducted a full-factorial transplanting experiment mimicking short-term restoration of coastal vegetation to test for the effects of above-ground spatial pattern and below-ground (rhizome) connectance on restoration efficiency. An unexpected finding is that clumped above-ground spatial patterns and high below-ground rhizome connectance resulted in lower restoration efficiency for Scirpus, reflected by lower recolonization rates and vegetation cover. Such effect of spatial configuration was especially pronounced during the early post-transplantation stage, and was robust in both high and low wave-stressed conditions. Particularly, the recolonization rate in the clumped treatment was only about half of that in the random and regular treatments for Scirpus in the 4 weeks after the transplantation. The stark contrast between our results and previous work may be explained by the differences in plant traits that can substantially shape the net outcomes of plant-plant interactions. Synthesis and applications. Our in situ experiment using flexible Scirpus demonstrates that higher extents of plant distancing represented by regular above-ground patterns and lower below-ground connectance can result in amplified restoration efficiency. In contrast to previous work showing the advantage of clumped designs in coastal restoration, our results suggest that optimal spatial designs may be context dependent, and can be pursued through thoughtful manipulative experiment in the field. Our work highlights the necessity of considerations on biotic attributes for restoration designs by means of harnessing positive interactions, and advances the applications of facilitation theory in ecological restoration practices.