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Integrated coastal zone management proposes nature-based mitigation strategies based on the replacement of artificial coastal stabilization and protection structures with dunes stabilized with plant species. These psammophytes stabilize sands and act as supporters, increasing dunes’ ability to reduce storm damages and effectively minimize erosion with minimal negative impacts to natural ecosystems. That is why searching for native salt-tolerant plants with extensive root systems and studying their capacity for erosion and flooding control is fundamental to the practice of ecologically-sound ecosystem services. The aim of the present study is to define the effects of flooding stress on a number of wheatgrass (Thinopyrum ponticum) plant life aspects (survival ability, viability, and growth response) in order to determine wheatgrass’s capacity as dune stabilizer. Conducted experiments established that T. ponticum was very tolerant to immersion impact and salt and oxygen deficiency stress, and its rhizomes were able to regenerate after 30 days in seawater. The temporal expression of its survival is presented as critical decomposition time (CDT) by linking the maximum duration of floods along the Bulgarian Black Sea Coast and the resilience of tall wheatgrass in flooding simulations. A statistical analysis of the experimental data demonstrated that immersion in sea water increases rhizome viability, biomass, and allocation to root biomass, whereas other factors, such as the duration of immersion, salinity, and temperatures of sea water have no significant effect. According to flood resilience and growth response to sea water submergence, T. ponticum demonstrated high potential to be a dune stabilizer.

Leymus racemosus (Lam.) Tzvelev subsp. sabulosus (M. Bieb.) Tzvelev (mammoth wildrye) and Ammophila arenaria (L.) Link (marram grass) are perennial, psammophilous species which dominate sand dunes due to their biological characteristics. Their communities have an important role in the formation of the natural vegetation cover of coastal sand strips along the Bulgarian Black Sea Coast. This study provides the first systematic analysis of tolerance of L. racemosus subsp. sabulosus and A. arenaria to sea water immersion along the Bulgarian Black Sea Coast. It aims to determine effects of flooding stress on whole plants and how long rhizomes can remain viable in sea water. Two experiments establish that decomposition of leaves of immersed plants starts from the 7th day. A growth of stems and root sprouts is observed on the same day. There are no visible decompositions of stems, roots and rhizomes till the end of the experiment (20th day). Rhizomes from the investigated species are able to regenerate after 30 days of submergence and bud viability appears to be enhanced slightly by sea water. L. racemosus subsp. sabulosus shows higher bud viability than A. arenaria. The possible negative consequences of flooding to L. racemosus and A. arenaria communities based on experimental results, hydrodynamical modelling, detailed topography, bathymetry surveys and detailed GIS mapping on Panorama Beach are discussed.Key words: Leymus racemosus subsp. sabulosus, Ammophila arenaria, immersion tolerance, viability, IDF modelsThis study is performed with the financial support of the THESEUS project "Innovative technologies for safer European coasts in a changing climate" (GA No 244104). 211Introduction. Leymus racemosus (Lam.) Tzvelev subsp. sabulosus (M. Bieb.) Tzvelev (mammoth wildrye) and Ammophila arenaria (L.) Link (marram grass) are erect, perennial, rhizomatous grass species [ 1 ]. They are major psammophilous species which dominate sand dunes due to their biological characteristics and their communities have an important role in the formation of the natural vegetation cover of coastal sand strips along the Bulgarian Black Sea Coast. These closely-related species form an extensive system of horizontal and vertical rhizomes that support sand stabilization. A good example of the high potential of the root system of these two species to accumulate sands and prevent from washout is the storm in March 2010 in Panorama Natural Beach situated north of the Golden Sands Resort near the town of Varna. A great amount of sediments are preserved within the beach boundaries (Fig. 1), unlike the neighbouring Golden Sands urbanized beach.Despite their high resistance, L. racemosus subsp. sabulosus and A. arenaria are vulnerable and sensitive to the impact of waves and storms. These factors in combination with anthropogenic impact may lead to various negative consequences. Plants could be affected by direct storm-wave damage, during which they are uprooted or buried by sediments. Furthermore, leaf, stem and root decomposition process...

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