Social distancing between plants may amplify coastal restoration at early stage

Climate change and anthropogenic activities are jeopardising coastal ecosystems world-wide. Once degraded, these valuable ecosystems are not easy to recover. In their latest research, Hao Huang and colleagues conducted transplanting experiments to search for the optimal spatial design of coastal restoration.

Few ecosystems can equate to coastal wetlands in terms of connections with humans. They provide many ecosystem services that are vital to current societies, such as shoreline protection, water purification, and biodiversity maintenance. However, as unsustainable exploitation together with sea-level rise has induced serious degradation of these valuable ecosystems, ecosystem restoration becomes increasingly important in coastal areas across the world.

Degraded coastal ecosystems are often ‘trapped’ in an unwanted state that is hard to reverse, making vegetation re-colonization a critical bottleneck of restoration in degraded habitats characterized by bare substrates. In recent years, it has been suggested that artificial transplantation with clumped designs may serve as an easy-to-implement approach.

The rationale is based on the facilitation theory. Closely juxtaposed plants may produce factitive effects on one another, that could ameliorate physical stress by reducing soil salinity, increasing oxygen availability, and alleviating wave impacts. Such additional effects yielded by simply changing spatial pattern can substantially improve the survival, growth and expansion of coastal plants, as evidenced by experiments in European and American saltmarshes.

Is this clumped restoration approach a silver bullet everywhere? It would be naïve to expect that. Indeed, extensive theoretical and empirical work (especially in stressful dryland ecosystems) has clearly demonstrated that the net outcome of plant-plant interaction is highly elusive, depending on type and intensity of stress, species attributes, and development stage of plant individuals. This implies that we cannot count on plant-plant facilitation to solve the problem ubiquitously. However, direct evidence so-far remains very scarce when it comes to coastal restoration practices.

In our study, we selected two foundation plant species with contrasting traits, a stiff cordgrass (Spartina alterniflora) and a flexible sedge plant (Scirpus mariqueter), to set up a full factorial experiment in the saltmarsh ecosystems in the Yangtze Estuary, China. By mimicking the early-stage vegetation restoration, our experiment aimed at comparing the restoration success between these two species with different spatial designs.

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A drone photo of the experiment site with the bare mudflat on the left hand side and sedge vegetation on the right hand side. The transplanting plots are flagged with white PVC tubes, forming a line transect across the mudflat.

We transplanted the individuals onto bare mudflats subject to strong and relatively weak wave impacts. We manipulated the above-ground spatial patterns to be regular, random, and clumped, and the below-ground root systems to be highly connected, weakly connected, and non-connected. By doing so, we hoped to answer a simple question: which is a better way for the plants to re-colonize the bare mudflats, social aggregation or social distancing?

We found that the stiff cordgrasses performed better when they were transplanted with a clumped spatial pattern, in line with the previous work. However, we were surprised by the entirely opposite results from the flexible sedge plant. Scirpus with random or regular aboveground patterns performed much better, characterised by higher recolonization rate and plant cover in the 4 weeks after the transplantation. In contrast, the recolonization rate with the clumped pattern was only about half of that with the random and regular patterns. We obtained a similar finding when it comes to belowground patterns. We observed much poorer recolonization performance when sedge plants were transplanted with strongly inter-connected roots.

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The sedge plants (Scirpus mariqueter) growing in nature are often inter-connected with rhizome (the white parts) below-ground. 

Our results convey a clear message: social aggregation is good to the stiff cordgrass, but the flexible sedge plant prefers social distancing. This leaves the explanation that species traits may be responsible for the contrasting performance of the spatial design.

The cordgrass has high, dense and thick ramets as well as deep roots, thus can generate strong facilitative effects on neighboring plants by increasing wave attenuation and oxygen availability. Scirpus generally has much sparser ramets, lower statures, thinner, more flexible shoots, and shallower below-ground systems. These traits plausibly make Scirpus a poorer facilitator for early colonization against wave impacts. It could also well be that intra-specific competition for nutrients is sufficiently strong to cancel off the benefits from the facilitative interactions when the plants grow closely together, as supported by our experimental evidence published recently.

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The lead author Hao Huang measuring elevation on the bare mudflat. 

We therefore recommend that nature-based coastal restoration should seriously consider such context dependency underlying biotic interactions. As it is often difficult to tell a priori whether or not a net positive outcome would arise between focal interacting organisms, pilot experiments would provide critical information before applying particular spatial designs to large-scale restoration practices.

Read the full paper ‘Social distancing’ between plants may amplify coastal restoration at early stage in Journal of Applied Ecology.

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