Designing Resilient and Sustainable Grasslands for a Drier Future: Adaptive Strategies, Functional Traits and Biotic Interactions

In many regions of the world, such as Southern Europe and most Mediterranean areas, the frequency and magnitude of droughts and heat waves are expected to increase under global warming and will challenge the sustainability of both native and sown grasslands. To analyze the adaptive strategies of species, genotypes and cultivars, we aim both: (1) to understand the composition and functioning of natural grasslands; and (2) to propose ideotypes of cultivars and optimal composition for mixtures of species/genotypes under water deficit and high temperatures. This review presents a conceptual framework to analyze adaptive responses of perennial herbaceous species, starting from resistance to moderate drought with growth maintenance (dehydration avoidance and tolerance of lamina) to growth cessation and survival of plants under severe stress (dehydration avoidance and tolerance of meristems). The most discriminating functional traits vary according to these contrasting strategies because of a trade-off between resistance to moderate moisture deficit and survival of intense drought. Consequently it is crucial to measure the traits of interest in the right organs and as a function of soil water use, in order to avoid misleading interpretations of plant responses. Furthermore, collaboration between ecologists, eco-physiologists, and agronomists is required to study the combination of plant strategies in natural grasslands as only this will provide the necessary rules for species and cultivars or ecotypes assemblage. This ‘agro-ecological’ approach aims to identify and enhance functional complementarity and limit competition within the multi-specific or multi-genotypic material associated in mixtures since using plant biodiversity should contribute to improving grassland resistance and resilience

La diversité fonctionnelle racinaire peut-elle favoriser la résilience des mélanges de graminées méditerranéennes sous sécheresses sévères ?

The sustainability of grasslands is threatened under climate change especially in Mediterranean areas. As biodiversity is increasingly recognized to enhance and stabilize processes within plant communities, we aimed to test whether the associations of forage species with contrasting belowground functional strategies improve soil water use and resilience of biomass productivity under increasing summer aridity. Monocultures and bi- or tri-specific mixtures of perennial grasses were compared in a 3-years field experiment under either an average or an extreme summer drought scenario in southern France. From the measured root traits, both the functional identity (mean traits of associated species) and the functional diversity (trait differences) were calculated for each mixture. Overyielding and resilience were assessed from seasonal aboveground biomass (AGB). Total Transpirable Soil Water (TTSW) was derived from monthly soil water content monitoring. Across all treatments and drought scenari, AGB productivity and resilience were highly correlated with TTSW and root depth. The functional identity of mixtures better explained overyielding and resilience responses than the functional diversity. These results provide sound agro-ecological rules to design suitable associations of species for drought-prone areas.La durabilité des prairies est menacée sous changement climatique surtout en zones méditerranéennes. Comme une biodiversité élevée est reconnue pour stabiliser les communautés végétales, cette étude a testé si des mélanges d’espèces fourragères avec des stratégies fonctionnelles racinaires contrastées pouvaient améliorer les prélèvements hydriques et la résilience des couverts sous sécheresses sévères. Des monocultures et des mélanges bi ou tri-spécifiques de graminées pérennes ont été comparés dans un essai au champ sous sécheresse estivale moyenne et extrême dans le sud de la France.Les traits racinaires ont permis de calculer l’identité fonctionnelle (traits moyens des espèces associées) et la diversité fonctionnelle (différence de traits) pour chaque mélange. Overyielding et résilience ont été estimés par des mesures de biomasses aériennes (AGB). La fraction de l’eau du sol transpirable par les plantes (TTSW) a été mesurée. Pour tous les traitements et niveaux de sécheresse, AGB et résilience sont très corrélés à TTSW et profondeur racinaire. L’identité fonctionnelle racinaire permet de mieux expliquer les réponses d’overyielding et de résilience que la diversité fonctionnelle. Ces résultats ont des implications pour la conception de mélanges fourragers adaptés aux zones sèches

Olive agroforestry shapes rhizosphere microbiome networks associated with annual crops and impacts the biomass production under low-rainfed conditions

Agroforestry (AF) is a promising land-use system to mitigate water deficiency, particularly in semi-arid areas. However, the belowground microbes associated with crops below trees remain seldom addressed. This study aimed at elucidating the effects of olive AF system intercropped with durum wheat (Dw), barely (Ba), chickpea (Cp), or faba bean (Fb) on crops biomass and their soil-rhizosphere microbial networks as compared to conventional full sun cropping (SC) under rainfed conditions. To test the hypothesis, we compared the prokaryotic and the fungal communities inhabiting the rhizosphere of two cereals and legumes grown either in AF or SC. We determined the most suitable annual crop species in AF under low-rainfed conditions. Moreover, to deepen our understanding of the rhizosphere network dynamics of annual crops under AF and SC systems, we characterized the microbial hubs that are most likely responsible for modifying the microbial community structure and the variability of crop biomass of each species. Herein, we found that cereals produced significantly more above-ground biomass than legumes following in descending order: Ba > Dw > Cp > Fb, suggesting that crop species play a significant role in improving soil water use and that cereals are well-suited to rainfed conditions within both types of agrosystems. The type of agrosystem shapes crop microbiomes with the only marginal influence of host selection. However, more relevant was to unveil those crops recruits specific bacterial and fungal taxa from the olive-belowground communities. Of the selected soil physicochemical properties, organic matter was the principal driver in shaping the soil microbial structure in the AF system. The co-occurrence network analyses indicated that the AF system generates higher ecological stability than the SC system under stressful climate conditions. Furthermore, legumes’ rhizosphere microbiome possessed a higher resilient capacity than cereals. We also identified different fungal keystones involved in litter decomposition and drought tolerance within AF systems facing the water-scarce condition and promoting crop production within the SC system. Overall, we showed that AF reduces cereal and legume rhizosphere microbial diversity, enhances network complexity, and leads to more stable beneficial microbial communities, especially in severe drought, thus providing more accurate predictions to preserve soil diversity under unfavorable environmental conditions.This research was carried out as part of the D4DECLIC Project, ARIMNet 2 Young Scientists Call 2017 (ERA-NET program), and Grant agreement no. 618127