The restoration of floodplain grasslands has benefited from many studies of the underlying mechanisms. Among the operational tools that resulted, hay transfer is now used increasingly to alleviate the effects of limited seed dispersal and recruitment. To improve this method, we still need to understand how it can affect restoration trajectories, and particularly their direction and magnitude during the early stages of restoration. Based on concepts from the field of community ecology theory, we investigated the effects of early‐stage management through grazing or mowing on restoration trajectories after soil harrowing and hay transfer. We established a randomized block design experiment and quantified several community‐related metrics to formalize restoration trajectories for 3 years after hay transfer on a previously arable alluvial island in southwestern France. Whatever the management treatment, the species richness and evenness were significantly higher in hay‐inoculated than in control plots. This effect was linked to the recruitment of species originating not only from the reference grassland through hay transfer, but also from the seed bank, a well‐known effect of soil harrowing. Although generally oriented toward the reference grassland, the origin, direction, and magnitude of the trajectory of hay‐inoculated plots all depended on the management applied. Sheep grazing applied at the same time as hay transfer enhanced the recruitment of reference species as from the first experimental year, because it controlled aboveground competition and maintained the window of opportunity open for a sufficiently longer period of time. Our findings show that the type of management applied simultaneously to hay transfer influences the origin of a grassland trajectory, while its direction and magnitude are dependent on the management applied in subsequent years. Grazing immediately after hay transfer may be appropriate to accelerate the recruitment of species from the reference grassland.

Alfalfa was cultivated in two potted soil series obtained from two sandy soils contaminated by Cu (SM) and metal(loids)/PAH (CD). Shoot production was monitored for 8 weeks. Then, larvae of Spodoptera exigua were reared on alfalfa of both soil series for eight days. A biotest (using Phaseolus vulgaris) was used to assess the soil phytotoxicity. Increasing soil contamination reduced P. vulgaris growth, but alfalfa growth was only reduced on the SM soil series. Exposure to the SM soil was mirrored by shoot Cu and Cr concentrations of alfalfa (respectively, in mg kg DW, Cu and Cr ranged from 11.9 and 0.4 in the CTRL soil to 98.5 and 1.2 in the SM one). Exposure to the CD soil series was mirrored by shoot Zn concentrations (i.e., 48-91.6 mg kg DW). Internal metal(loid) concentrations of S. exigua remained generally steady across both soil series (respectively Cd 0.05-0.16, Cr 0.5-3.3, Cu 5.8-98.5, Ni 0.6-1.6, Pb 0.4-1.3, and Zn 57-337 mg kg DW), and most of the associated transfer factors were lower than 1. Here, due to the excluder phenotype of alfalfa across our TE contamination gradients, S. exigua could cope with high total metal(loid) concentration in both contaminated soils.

Nous étudions expérimentalement les effets de la gestion initiale sur les premiers stades de restauration d’une prairie alluviale à partir d’une ancienne culture de maïs, dans le sud-ouest de la France. Une expérimentation par blocs randomisés a été définie pour tester les effets du transfert de foin associé au hersage superficiel du sol et de la gestion par pâturage appliquée, soit immédiatement, soit plusieurs mois après le transfert de foin, ou par fauche appliquée annuellement en début ou en fin d’été. La richesse en espèces était toujours supérieure dans les placettes ayant reçu du foin, par rapport aux placettes témoin. L’origine, la direction et l’amplitude de la trajectoire de chaque placette étaient dépendantes du type de gestion appliquée. Le pâturage initial par les brebis, appliqué en même temps que le transfert de foin, a augmenté le recrutement des espèces de la communauté de référence, par un effet de contrôle de la compétition des espèces pré-existantes et le maintien d’une fenêtre d’opportunité pour le recrutement de ces espèces suffisamment longue. Le type de gestion appliqué dès le transfert conditionne l’origine de la trajectoire de restauration, tandis que sa direction et son amplitude sont liées à la gestion sur le moyen terme. À partir de la cinquième année d’expérimentation, ce sont des événements stochastiques liés à l’environnement ou la démographie des espèces qui semblent donner l’avantage aux contingences locales (banque de graines et/ou pluie de graines) aboutissant à une déviation de la trajectoire de restauration. Nous montrons néanmoins que le pâturage initial, très tôt dans le processus de restauration, semble approprié pour accélérer le recrutement des espèces de l’écosystème de référence.