Ecosystem engineering by plants on wave-exposed intertidal flats is governed by relationships between effect and response traits

In hydrodynamically stressful environments, some species—known as ecosystem engineers—are able to modify the environment for their own benefit. Little is known however, about the interaction between functional plant traits and ecosystem engineering. We studied the responses of Scirpus tabernaemontani and Scirpus maritimus to wave impact in full-scale flume experiments. Stem density and biomass were used to predict the ecosystem engineering effect of wave attenuation. Also the drag force on plants, their bending angle after wave impact and the stem biomechanical properties were quantified as both responses of stress experienced and effects on ecosystem engineering. We analyzed lignin, cellulose, and silica contents as traits likely effecting stress resistance (avoidance, tolerance). Stem density and biomass were strong predictors for wave attenuation, S. maritimus showing a higher effect than S. tabernaemontani. The drag force and drag force per wet frontal area both differed significantly between the species at shallow water depths (20 cm). At greater depths (35 cm), drag forces and bending angles were significantly higher for S. maritimus than for S. tabernaemontani. However, they do not differ in drag force per wet frontal area due to the larger plant surface of S. maritimus. Stem resistance to breaking and stem flexibility were significantly higher in S. tabernaemontani, having a higher cellulose concentration and a larger cross-section in its basal stem parts. S. maritimus had clearly more lignin and silica contents in the basal stem parts than S. tabernaemontani. We concluded that the effect of biomass seems more relevant for the engineering effect of emergent macrophytes with leaves than species morphology: S. tabernaemontani has avoiding traits with minor effects on wave attenuation; S. maritimus has tolerating traits with larger effects. This implies that ecosystem engineering effects are directly linked with traits affecting species stress resistance and responding to stress experienced

Quantifying critical conditions for seaward expansion of tidal marshes: a transplantation experiment

The alternative stable states theory is increasingly applied to tidal marsh shorelines, where the two opposing stable states – a dense vegetated state on the one hand and a bare tidal flat on the other hand – can coexist in time but differ in space. The shift from the bare to vegetated state by the establishment of individual plants (seedlings, rhizome-grown shoots) on the bare tidal flat is known to be triggered by the occurrence of windows of opportunity. These are periods when species- and life stage-dependent thresholds, such as sediment dynamics or wave impact, are not exceeded. One controlling environmental parameter in intertidal wetlands is elevation as many important stressors for plants – such as hydroperiod, sediment dynamics and wave properties (wave period and wave height) – are typically correlated to it. Disentangling the respective impact of these correlated stressors remains challenging. In this paper, we present the results of a transplantation experiment where the establishment of three different life stages (seedlings, rhizome-grown shoots and patches) of the brackish pioneer <i>Scirpus maritimus</i> was tested over an elevation gradient at two locations of contrasting wave exposure. This gradient reached from the bare tidal flat into the marsh and covered an elevation range at which continuous <i>S. maritimus-</i>dominated pioneer marsh is known to occur. We found that erosion stress influences seedling survival on tidal flats while drought stress seems to limit long-term establishment of individual shoots and seedlings in the marsh. Furthermore, survival of transplants was more successful on the tidal flat of the sheltered site compared to the tidal flat of the exposed site whereas survival time within the marsh did not differ between sites. This highlights the attenuation of waves and currents in exposed marshes. However, no long-term establishment occurred on the tidal flat, emphasizing the importance of clonal integration for tidal flat colonization

Effects of Wind Waves versus Ship Waves on Tidal Marsh Plants: A Flume Study on Different Life Stages of Scirpus maritimus

International audienceRecent research indicates that many ecosystems, including intertidal marshes, follow the alternative stable states theory. This theory implies that thresholds of environmental factors can mark a limit between two opposing stable ecosystem states, e.g. vegetated marshes and bare mudflats. While elevation relative to mean sea level is considered as the overall threshold condition for colonization of mudflats by vegetation, little is known about the indi- vidual driving mechanisms, in particular the impact of waves, and more specifically of wave period. We studied the impact of different wave regimes on plants in a full scale flume exper- iment. Seedlings and adult shoots of the pioneer Scirpus maritimus were subjected to two wave periods at two water levels. Drag forces acting on, and sediment scouring occurring around the plants were quantified, as these are the two main mechanisms determining plant establishment and survival. Depending on life stage, two distinct survival strategies emerge: seedlings present a stress avoidance strategy by being extremely flexible, thus lim- iting the drag forces and thereby the risk of breaking. Adult shoots present a stress toler- ance strategy by having stiffer stems, which gives them a higher resistance to breaking. These strategies work well under natural, short period wind wave conditions. For long peri- od waves, however, caused e.g. by ships, these survival strategies have a high chance to fail as the flexibility of seedlings and stiffness of adults lead to plant tissue failure and ex- treme drag forces respectively. This results in both cases in strongly bent plant stems, po- tentially limiting their survival

Survival of the thickest? Impacts of extreme wave-forcing on marsh seedlings are mediated by species morphology

Although tidal marshes are known for their coastal defense function during storm surges, the impact of extreme wave forcing on tidal marsh development is poorly understood. Seedling survival in the first season after germination, which may involve exposure to extreme wave events, is crucial for the natural establishment and human restoration of marshes. We hypothesize that species-specific plant traits plays a significant role in seedlings survival and response to wave induced stress, i.e., through stem bending and uprooting. To test this hypothesis, seedlings of pioneer species (Bolboschoenus maritimus, Schoenoplectus tabernaemontani, Spartina anglica, and Puccinellia maritima) with contrasting biophysical traits were placed in the Large Wave Flume in Hannover (Germany) and exposed to storm wave conditions. Seedlings of P. maritima and S. anglica experienced a lower loss rate and bending angle after wave exposure compared to S. tabernaemontani and especially B. maritimus. The higher loss rates of B. maritimus and S. tabernaemontani result from deeper scouring around the stem base. Scouring depth was larger around stems of greater diameter and higher resistance to bending. Here, B. maritimus and S. tabernaemontani have both thicker and stiffer stems than S. anglica and P. maritima. Our results show that especially seedlings with thicker stems suffer from erosion and scouring, and have the highest risk of being lost during extreme wave events. This implies that for successful seedling establishment and eventually the establishment of a mature tidal marsh vegetation, the species composition and their capacity to cope with storm wave disturbances is crucial