Aerial surveys reveal biotic drivers of mangrove expansion along a Thai salt flat ecotone

Abstract

Rapid sea level rise (SLR) could overwhelm the adaptive capacity of many mangrove forests. The conservation of functional mangrove ecosystems will rely on their expansion into upper intertidal areas, including salt marshes and salt flats, in advance of SLR. The development of new mangrove habitats could be accelerated by amplifying positive biotic interactions among herbaceous plants, macrofauna, and mangrove trees, such as trapping of hydrochorous propagules or site amelioration. It was investigated how biotic and abiotic factors influenced the expansion of mixed-species mangrove stands along a mangrove-salt flat ecotone. Multi-temporal unmanned aerial vehicle (UAV) imagery (< 1 cm/pixel) was used to monitor seedlings of Avicennia alba, Bruguiera cylindrica, and Lumnitzera racemosa in three sample plots (0.7-0.9 ha) over 2 years. The aerial images were used to map mangrove crowns, sedges (Fimbristylis tetragona), substrate elevation, and microtopography. The spatial distribution of seedlings was modeled using point pattern processes; mixed-effects models were used to analyze how environmental factors influenced seedling survival and growth. The spatio-temporal patterns of mangrove regeneration revealed that sedges and macrofauna facilitated the regeneration of mangroves into higher-elevated areas of low tree density. Sedges not only trapped seeds but also increased the likelihood of survival among seedlings of all species. Large-sized propagules established in undulating microtopography created through the bioturbation of macrofauna. Herbaceous vegetation expanded the range of macrofauna in the absence of trees. Thus, the migration of mangroves into hypersaline retreat areas may be accelerated by enhancing the growth of herbaceous vegetation and promoting burrowing macrofauna.

Establishment and survival of coastal mangrove trees under mechanical disturbances

“…Ultimately, in trying to achieve optimal wave attenuation function, one should be careful to consider mangrove forest width and mangrove forest density as measurement tools rather than end goals. Mangrove forests are complex ecosystems that are affected by numerous physical (Chapter 1), biotic (Biswas et al, 2018;Pranchai et al, 2022) and anthropogenic drivers (van Bijsterveldt et al, 2021(van Bijsterveldt et al, , 2020. Hence, selecting the species with for example the highest drag coefficient or the best establishment success is not a guarantee for an optimal nature-based flood defence in the long run.…”