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This leads to partial surface stabilization by vegetation, visible by contraction of the blowout shape, and can culminate in its closure, e.

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We define blowout closure, through vegetated incipient foredune development across the throat of the blowout, which prevents further erosion of the deflation basin or erosional walls. Blowout closure marks the transition from the bio-geomorphological to the ecological phase described in Figure 3. Through the ability of vegetation to engineer blowout topography, it plays an important role in blowout closure [ 44 ]. The influence of vegetation on sand mobility results in a hysteresis [ 66 , 85 , 86 , 87 ]. More specifically, wind energy needed for blowout initiation in a vegetation-covered area is significantly higher than the wind energy threshold at which vegetation can re-establish.

This was previously also linked to vegetation-altered physical sediment structure, such as adding organic matter by plant roots or trapping relatively fine inorganic sediment particles of the aboveground biomass [ 88 , 89 ]. Thus maintenance or reactivation of blowouts needs disturbances such as overwash, high wind, fire, aridity, biogenic, or other disturbance [ 15 , 66 , 90 ].

Moreover, studies on inland dune fields showed that dunes are constantly adjusting and are therefore lagging behind a continuously changing climate. Lag times between morphological adaptation of dune fields, being typically out of sync with present climate, also affect closure and reactivation dynamics and are strongly dependent on vegetation characteristics [ 91 ].

Re-colonization of bare blowout areas can originate from germinating seeds, broken of rhizome fragments or clonal lateral rhizome growth [ 92 ]. The deflation basin, being the highest disturbed area within a blowout, is most likely to be re-colonized through clonal rhizomes or rhizome fragments of bordering vegetation. This is due to the fact that plants originating from rhizome nodes grow faster and are much less vulnerable than seedlings [ 73 ].

Clonal rhizomes usually grow perpendicular to the stem, but can also grow vertical if burial occurs, and new sprouts can form at each node, leading to re-colonization from the blowout edges. Rhizome fragments originate from mass slumping from the rim of the deflation basin walls. The walls of the deflation basin tend to oversteepen due to lateral wall erosion and vegetation stabilization at the top of the wall [ 80 ] Figure 4 b.

Rhizome fragments reaching the blowout through mass-slumping can form new islands of colonized vegetation within the blowout i. Apart from colonization by rhizomes, colonization by seeds or seedlings is potentially more important at the depositional lobe being less disturbed by wind-sand blasting.

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  • Moreover disturbance by accretion potentially favors establishment of burial-tolerant and burial dependent species [ 24 ]. For instance, saucer blowout closure was observed through colonization by Ammophilia sp. Another study observed recolonization by vegetation in the deflation basin, which could potentially lead to formation of embryo dunes through increased sediment capture and closing of the blowout from the beach [ 93 ]. These examples illustrate that blowout closure depends on interactions between colonizing plant species and environmental factors.

    Previously three main factors driving blowout stabilization where identified: i availability of sediment outside the blowout, ii vegetation type in relation to plant-sediment-flow interaction, burial tolerance, and colonization rates , and iii climate variability magnitude and duration of precipitation, i.

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    As already seen in the above-mentioned examples, the consequences of these factors for blowout development are very specific on the governing abiotic-biotic interactions. For instance, increased soil moisture generally increases plant growth e. However, since plant stress tolerance, as shown above, is dependent on the life-stages i. More specifically, across temperate systems seedling emergence is either related to an increase in temperature in early spring, or to a coinciding decrease in temperature and increase in rainfall in autumn. Most herbaceous plants in temperate systems, such as Ammophila breviligulata or Cakile edentula , reach maturity and start seed dispersal in summer and autumn.

    This leads to seedling emergence in late spring of the next year. Plants like Aira caryophyllaea or Saxifraga tridactylites , in contrast, reach maturity and start seed dispersal in spring, leading to seedling emergence in autumn [ 73 ]. Thus, not only magnitude and frequency but also timing of disturbances, such as storms, will constitute a major constraint on the survival of blowout colonizing species [ 94 ].

    Moreover, the combination of factors such as strong winds, low precipitation, short growing season, high physical disturbance e. As a possible explanation for this phenomenon, the windows of opportunities concept was proposed. It states that seeds need a disturbance free period to build up their stress resilience which consequently drastically increases their survival [ 96 ].

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    • Consequently, climate conditions and their potential to create disturbance free recruitment periods i. Plants able to tolerate sediment burial not only differ in their morphology, as explained above Figure 4 , they also differ in metabolic pathways which has important ramification on their competitive strength.

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      For instance, the burial-dependent C3-plant Ammophilia brevingulata has a lower efficient nitrogen- and water use efficiency than the burial tolerant C4-plant Uniola paniculata. This results in a competitive advantage for the latter during co-occurrence. As shown at the North American Atlantic coast, a climate-induced change in habitat range can result in a transition of the dominating species depending on metabolic efficiency [ 97 ]. Moreover, it was shown that direct interactions between burial-tolerant plants might lead to reduced functional traits, for instance leaf elongation [ 98 ].

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      Thus in the context of blowout closure, it is expected that during the transition between the bio-geomorphological and ecological phase the importance of physical interactions will reduce, while the importance of ecological interactions e. Several studies conceptualize environmental factors driving dune formation and dynamics, e. Here, we combine current knowledge on foredune blowout development by focusing on the feedbacks between abiotic and biotic processes based on the above-described concept of successional stages Figure 3.

      Although previous studies, e. The conceptual model including erosive and accretive trends follows the approach of Hesp [ 10 ] for dune formation and consists of two main parts. Color of the arrows between development stages indicates whether biological biotic or physical abiotic factors need to be dominant to provoke a stage change.

      A newly incised blowout starts its development at the geomorphologic stage 1 where blowout development is mainly driven by erosional sediment transport processes increasing the depth d and the width w of the blowout Figure 6 a, geomorphological, along-shore, dashed and closed line Figure 4 [ 18 , 82 ].

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      At the initial stage, an increase in wind speed leads to an increase in blowout erosion d and w through morphological funneling suppressing the establishment of erosion intolerant plant species IV , which in turn facilitates wind speed Figure 6 b , e. Increased funneling and wind speed leads to the extension of the depositional lobe Figure 6 a, geomorphological, cross-shore, dashed to close line. This leads to the transitional stage 1. As soon as vegetation establishment reached a sufficient level to reduce wind and entrap sediment, a shift to the bio-geomorphological stage 2 takes place.

      At the bio-geomorphological stage vegetation is able to colonize the walls of the deflation basin, constraining the width of the blowout, and starts to establish in the deflation basin and the depositional lobe engineering its environment Figure 6 a. At this stage, a reversal to the geomorphological stage can take place through major disturbances e. Morphological development is governed by the interaction between biological and physical processes, where erosion suppresses plant establishment while accretion facilitates it Figure 6 a, bio-geomorphological, cross-shore, dashed to closed line. Blowout development is then heavily dependent on the stress-tolerance of the colonizing species, wind magnitude, and sediment transport.

      As previously proposed by Maun [ 73 ], accretion tolerance ranges from I burial-intolerant, II burial-tolerant, and III burial-dependent for details see Figure 5. However, since airflow and sediment transport in the deflation basin is dependent on sediment availability and morphological flow acceleration, erosional tolerance will also be crucial.

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      Based on Maun [ 78 ] and McLeod et al. Erosion-tolerant V species only reduce their fitness after a certain erosion threshold is surpassed, whereas erosion-intolerant species are immediately affected by the occurrence of erosion IV Figure 6 b. In the bio-geomorphological stage, vegetation can establish on the depositional lobe, which can lead to transforming its morphological configuration into a parabolic dune [ 83 ].

      This transformation will reduce deposition along lateral edges of the parabolic dune, facilitating a competition driven transition to burial-tolerant II and burial-intolerant I plants. If more and more vegetation is able to establish in the blowout, shear stresses are further reduced at the sand surface and thus higher wind stresses are needed to induce erosion and vegetation removal.

      Continuous sedimentation will reduce blowout depth and width resulting in a transition to the ecological stage 3 [ 66 , 85 , 87 ]. At the ecological stage, the established vegetation induces sedimentation in the deflation basin and thereby reduces sediment transport to the depositional lobe Figure 6 a, Ecological cross-shore and along-shore dashed and closed line. This is caused by the positive feedback between sediment accretion and plant biomass, which consequently reduces wind speed Figure 6 b.

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      The positive feedback between accretion and plant biomass does not only influence the morphology, it also influences species composition within the blowout. At the start of vegetation colonization only burial-dependent III plants are able to colonize due to high levels of accretion, but with reducing accretion rates and wind speeds an ecological succession from burial-dependent III to burial-tolerant II to burial-intolerant I takes place. This is caused by the inverse relation of stress tolerance to competitive strength with the most tolerant burial-dependent III plant species possessing the least competitive strength and vice versa Figure 6 b.

      The vegetation succession and simultaneous soil development results in higher soil stability and reduces the susceptibility of sand to erosion. However, increased competitive strength of successional species is accompanied by an increased vulnerability to physical disturbances physical disturb. In this stage, big storms generating high input of aeolian sand can kill the established burial-tolerant II and burial-intolerant I vegetation, facilitating the onset of erosion and reversing the blowout back into the bio-geomorphological stage.

      Thus, occurrence of a disturbance can reverse the positive feedback between accretion and biomass and move the system to transitional stage 2.