Flooding and Soil Properties Control Plant Intra- and Interspecific Interactions in Salt Marshes

Abstract

The stress gradient hypothesis (SGH) states that plant-plant interactions shift from competition to facilitation in increasing stress conditions. In salt marshes, edaphic properties can weaken the application of the SGH by amplifying the intensity of flooding and controlling plant zonation. We identified facilitative and competitive interactions along flooding gradients and tested the role of edaphic properties in exacerbating stress and shaping plant-plant interactions. Morphological traits of two target halophytes (Limonium narbonense and Sarcocornia fruticosa), flooding intensity, soil texture and soil organic C were recorded. The relative plant fitness index was assessed for the two species based on the relative growth in plurispecific rather than monospecific plant communities. Plant fitness increased with increasing stress supporting the SGH. L. narbonense showed larger fitness in plurispecific stands whereas S. fruticosa performed better in conspecific stands. Significant intra- or interspecific interactions were observed along the stress gradient defined by the combination of flooding and clay content in soil. When considering the limited soil organic C as stressor, soil properties were more important than flooding in defining plant-plant interactions. We highlight the need for future improvements of the SGH approach by including edaphic stressors in the model and their possible interactions with the main abiotic drivers of zonation.

New perspective for the upscaling of plant functional response to flooding stress in salt marshes using remote sensing

“…Clay content did not affect plant community traits, while it contributes for the shaping of S. fruticosa response, that, in turn, can facilitate other species along the abiotic stress gradient caused by flooding depth and soil clay, causing plausible feedback at the community level 24 , 49 .…”

“…Our findings suggest that these feedbacks might also drive the remote sensing-vegetation relationships for the monitoring of the ecosystem properties. Moreover, these relationships suggest for a strong complexity occurring between the soil-individual-community responses, also involving facilitation/competition interactions 24 . This complexity probably contributes to determining the low variance of the remote sensing indices explained by singular traits of the key species and the community.…”

“…Our findings confirmed flooding depth to be the main driver shaping plant communities in tidal salt marshes. Interestingly, flooding depth was the only driver directly affecting plant community features (i.e., biomass, plant height, and diversity), while its interaction with soil clay content shaped the functional response of Salicornia fruticosa (i.e., shoot length, shoot carotenoid content), probably via increasing soil anoxia soil 24 .…”

“…This is indicating that remote sensing monitoring over time might reveal the acclimation of saltmarshes to sea level rise by intercepting the response of the key species S. fruticosa , opening new perspective in the multispectral image interpretation. Salt marsh key species has been already linked to the community response by plant trait-soil interactions 24 , 27 . Our findings suggest that these feedbacks might also drive the remote sensing-vegetation relationships for the monitoring of the ecosystem properties.…”

“…Tidal flooding is a pivotal factor driving plant species growth, distribution, and zonation in salt marshes 19 – 22 . In an equilibrium state, tidal flooding is mainly determined by land morphology 23 and soil texture (i.e., soil permeability) 24 . As a consequence, flooding influences crucial soil features, such as the availability of oxygen 25 , 26 , triggering intense plant-soil feedbacks 27 .…”