Higher temperature and nutrient concentrations may decrease black mangrove seedling stability during coastline recovery

Mangrove forests provide a wide range of benefits for natural and human communities in tropical and subtropical coastlines. Mangrove restoration using seedlings is vital to counteract the loss of mangrove forests, currently estimated at 3.4% over the past 24 years, with mangroves disappearing at a rate twice as large as their gain. Restoration success depends on the successful establishment of seedlings in the new environment, which may be influenced by unexplored factors such as nutrient status and temperature in selected sites. In order to determine the role of these two factors on mangrove seedling growth and development once they have been outplanted, we set up a laboratory experiment consisting of two climate rooms at 25ºC and 30ºC mimicking a standard and a future global warming scenario. These two temperature levels were combined with two different nutrient levels representing a standard and an eutrophication scenario. We assessed the influence of increased nutrients and temperature on the development and photosynthetic performance of black mangrove seedlings over a period of 10 weeks, and studied their responses to the individual and combined effects of these drivers. Seedling growth was measured as leaf area, plant length and above/ below biomass development. In addition, we determined the photosynthetic performance of seedlings by applying the Pulse Amplitude Modulated (PAM) fluorometry technique. Our results showed reduced root growth and disproportionally long, bent stems, with lower net assimilation rates under the combined effects of high temperature and nutrient concentrations. These outcomes, specially the nearly-negligible root growth suggest a low stability as the seedlings fail to anchor themselves to the sediment, which may translate into a greater vulnerability to physical disturbance leading to their dislodgement. On this basis, we recommend that local hydrodynamic conditions and nutrient status are taken into consideration for improving future site selection and increasing restoration success under a warming climate. Our results also suggest that the impacts of global-scale processes such as global warming could be dampened by the implementation of local policies, such as preventing coastal eutrophication.