Scalable mangrove rehabilitation: Roots of success for Rhizophora stylosa establishment

Abstract

Large-scale mangrove restoration initiatives have been attempted worldwide but have often suffered from low success rates and high costs. Direct seeding is increasingly used as a viable and cost-effective strategy for achieving restoration at scale for other coastal habitats yet has been little used for mangroves. Planting mangrove propagules instead of saplings can reduce costs and labour associated with the collection, growing out, and re-planting involved in conventional restoration methods. In this study, we document research into direct seeding for mangrove restoration, focussing on early establishment processes and identifying recruitment enhancement strategies that will improve natural recruitment success rates. The elongated propagules produced by Rhizophoraceae species can establish by self-planting into the substrate, or after grounding flat as the tide recedes. An aquaria experiment showed that vertically sown (to simulate self-planting) Rhizophora stylosa propagules grew significantly longer and more roots than propagules sown horizontally. After 35 days the vertical propagules grew roots 46.3 ± 20.5 mm in length while horizontal propagules grew roots 17.4 ± 16.6 mm in length. A field study showed that specially designed bamboo structures facilitate vertical self-planting, thus enhancing successful establishment. Propagules grounding in a vertical orientation successfully established 52.6 % of the time, whereas propagules grounding horizontally had a 10 % success rate. Results from this study suggest that grounding orientation, and the hypocotyl being embedded into the substrate, prompt root initiation and may lead to R. stylosa reaching an establishment threshold quicker than naturally stranding propagules. As such we propose that direct seed planting represents a viable alternative for large-scale restoration of Rhizophora.