Presence of Trifolium repens Promotes Complementarity of Water Use and N Facilitation in Diverse Grass Mixtures

Legume species promote productivity and increase the digestibility of herbage in grasslands. Considerable experimental data also indicate that communities with legumes produce more above-ground biomass than is expected from monocultures. While it has been attributed to N facilitation, evidence to identify the mechanisms involved is still lacking and the role of complementarity in soil water acquisition by vertical root differentiation remains unclear. We used a 20-months mesocosm experiment to investigate the effects of species richness (single species, two- and five-species mixtures) and functional diversity (presence of the legume Trifolium repens) on a set of traits related to light, N and water use and measured at community level. We found a positive effect of Trifolium presence and abundance on biomass production and complementarity effects in the two-species mixtures from the second year. In addition the community traits related to water and N acquisition and use (leaf area, N, wateruse efficiency, and deep root growth) were higher in the presence of Trifolium. With a multiple regression approach, we showed that the traits related to water acquisition and use were with N the main determinants of biomass production and complementarity effects in diverse mixtures. At shallow soil layers, lower root mass of Trifolium and higher soil moisture should increase soil water availability for the associated grass species. Conversely at deep soil layer, higher root growth and lower soil moisture mirror soil resource use increase of mixtures. Altogether, these results highlight N facilitation but almost soil vertical differentiation and thus complementarity for water acquisition and use in mixtures with Trifolium. Contrary to grass-Trifolium mixtures, no significant overyielding was measured for grass mixtures even those having complementary traits (short and shallow vs. tall and deep). Thus, vertical complementarity for soil resources uptake in mixtures was not only dependant on the inherent root system architecture but also on root plasticity. We also observed a time-dependence for positive complementarity effects due to the slow development of Trifolium in mixtures, possibly induced by competition with grasses. Overall, our data underlined that soil water resource was an important driver of over-yielding and complementarity effects in Trifolium-grass mixtures

CLIMAGIE: A French INRA Project to Adapt the Grasslands to Climate Change

Climate change in France, central and southern Europe is expected to provoke more frequent and more intense summer water deficits, with increased amplitude in temperatures, exposing the same perennial crops to frosts as well as to heat waves and severe droughts. The impacts on sown monospecific grasslands have been assessed using crop models (Durand et al. 2010) but with less accuracy in extreme situations. Since less work has been done on intra-specific genetic variability there is urgent need to investigate both ranges of climate conditions and genetic variability (Poirier et al. 2012). Phenology and plant productivity responses to water, temperature and nitrogen (N) in particular need to be re-assessed over the full range of temperatures projected in the future

Intra and inter-annual climatic conditions have stronger effect than grazing intensity on root growth of permanent grasslands

Understanding how direct and indirect changes in climatic conditions, management, and species composition affect root production and root traits is of prime importance for grassland C sequestration service delivery. We studied during two years the dynamics of root mass production with ingrowth-cores and annual above- and below-ground biomass (ANPP, BNPP) of upland fertile grasslands subjected for 10 years to a gradient of herbage utilization by grazing. We observed strong seasonal root production across treatments in both a wet and a dry year but response to grazing intensity was hardly observed within growing seasons. In abandonment, spring and autumn peaks of root growth were delayed by about one month compared to cattle treatments, possibly due to later canopy green-up and lower soil temperature. BNPP was slightly lower in abandonment compared to cattle treatments only during the dry year, whereas this effect on ANPP was observed the wet year. In response to drought, the root-to-shoot biomass ratio declined in the abandonment but not in the cattle treatment, underlining higher resistance to drought of grazed grassland communities. Rotational grazing pressure and climatic conditions variability had very limited effects on root growth seasonality although drought had stronger effects on BNPP than on ANPP

Incorporating Biodiversity into Biogeochemistry Models to Improve Prediction of Ecosystem Services in Temperate Grasslands: Review and Roadmap

Multi-species grasslands are reservoirs of biodiversity and provide multiple ecosystem services, including fodder production and carbon sequestration. The provision of these services depends on the control exerted on the biogeochemistry and plant diversity of the system by the interplay of biotic and abiotic factors, e.g., grazing or mowing intensity. Biogeochemical models incorporate a mechanistic view of the functioning of grasslands and provide a sound basis for studying the underlying processes. However, in these models, the simulation of biogeochemical cycles is generally not coupled to simulation of plant species dynamics, which leads to considerable uncertainty about the quality of predictions. Ecological models, on the other hand, do account for biodiversity with approaches adopted from plant demography, but without linking the dynamics of plant species to the biogeochemical processes occurring at the community level, and this hampers the models’ capacity to assess resilience against abiotic stresses such as drought and nutrient limitation. While setting out the state-of-the-art developments of biogeochemical and ecological modelling, we explore and highlight the role of plant diversity in the regulation of the ecosystem processes underlying the ecosystems services provided by multi-species grasslands. An extensive literature and model survey was carried out with an emphasis on technically advanced models reconciling biogeochemistry and biodiversity, which are readily applicable to managed grasslands in temperate latitudes. We propose a roadmap of promising developments in modelling

Modelling the root system architecture of Poaceae. Can we simulate integrated traits from morphological parameters of growth and branching?

Our objective was to calibrate a model of the root system architecture on several Poaceae species and to assess its value to simulate several 'integrated' traits measured at the root system level: specific root length (SRL), maximum root depth and root mass. We used the model ArchiSimple, made up of sub-models that represent and combine the basic developmental processes, and an experiment on 13 perennial grassland Poaceae species grown in 1.5-m-deep containers and sampled at two different dates after planting (80 and 120 d). Model parameters were estimated almost independently using small samples of the root systems taken at both dates. The relationships obtained for calibration validated the sub-models, and showed species effects on the parameter values. The simulations of integrated traits were relatively correct for SRL and were good for root depth and root mass at the two dates. We obtained some systematic discrepancies that were related to the slight decline of root growth in the last period of the experiment. Because the model allowed correct predictions on a large set of Poaceae species without global fitting, we consider that it is a suitable tool for linking root traits at different organisation levels

Within and among species diversity as insurance against extreme drought in a coordinated distributed mesocosm experiment across Europe

International audienc

Effects of extreme drought on grasslands Evaluation of the buffering effect of plant diversity using an experimental approach

The frequency and magnitude of extreme drought events are expected to increase with climate change. Consequently, it is important to assess the ability of temperate grassland to resist and recover from more frequent and intense drought stress. To study this topic, we established a mesocosm experiment in autumn 2012: large pots (100 L) containing combinations of grassland species (1, 2 or 5 species) were placed on weighing scales to continuously measure the actual evapotranspiration of the plant canopy. Species selected (four grasses and one legume) were representative of temperate upland grasslands on fer- tile soils. An extreme summer drought was applied to half the pots and plant recovery following rewetting was monitored. Total biomass and evapotranspiration were analyzed to test the effects of species diversity on the resistance and recovery of grasslands to extreme drought. As expected, drought dramatically reduced by 79% biomass of all mixtures. The 5 species mixtures showed higher drought avoidance by maintaining inte- grated water-use efficiency and took up water at deeper soil layers than the less diverse mixtures during drought. These findings indicated that buffering effect of diversity during drought occurs for the 5-species mix- tures, possibly due to below-ground niche differentiation. This study emphasizes the high capacity of grass species mixtures to recover after an extreme drought

Leaf gas exchange and carbohydrate concentrations in Pinus pinaster plants subjected to elevated CO2 and a soil drying cycle

Plants of maritime pine (Pinus pinaster Ait.) were acclimated for 2 years under ambient (350 μmol mol-1) and elevated (700 μmol mol-1) CO2 concentrations ([CO2]). In the summer of the second growing season, the plants were subjected to a soil drying cycle for 6 days. Drought reduced plant transpiration rate and net CO 2 assimilation rate (A) by about 80 %. Elevated [CO,] induced a substantial increase of A (+105 % and +229 % in well-watered and in droughted plants, respectively) and of the needle starch (+145 %) and sucrose (+20 %) concentrations, whatever the watering regime. Drought did not significantly affect starch and sucrose concentrations, while hexose concentrations were slightly increased in the most severe drought condition (predawn water potential value equal to -1.5 MPa). The stimulating effect of elevated [CO,] on A was maintained along the drying cycle, whereas no significant CO2 effect was observed on the soluble carbohydrate concentration. These compounds did not contribute to an enhancement of osmotic adjustment under elevated [CO2] in P. pinaster. (© Inra/Elsevier, Paris.)Échanges gazeux foliaires et concentrations en glucides de plants de Pinus pinaster soumis à un enrichissement en CO2 de l'air et à un dessèchement du sol. Des semis de pin maritime (Pinus pinaster Ait.) ont été acclimatés pendant deux ans à 350 et à 700 μmol mol -1 de concentrations en CO2 atmosphérique [CO,]. Au cours de l'été de la deuxième saison de croissance, les plants ont été soumis à un dessèchement du sol pendant 6 j. La sécheresse a réduit d'environ 80 % la transpiration de la plante entière et l'assimilation nette de CO 2 (A). L'enrichissement en CO, de l'air a induit une augmentation marquée de l'assimilation nette de CO, (+105 % et +229 % en conditions de bonne alimentation hydrique et de sécheresse, respectivement), ainsi que des concentrations en amidon (+145 %) et en saccharose (+20 %), quelle que soit l'alimentation hydrique. Le traitement sécheresse n'a pas significativement affecté les concentrations en amidon et en saccharose, tandis que les concentrations en hexoses ont légèrement augmenté en condition de sécheresse sévère (valeur du potentiel hydrique de base égale à -1.5 MPa). L'effet stimulant de la [CO 2] sur A était maintenu au cours du dessèchement du sol, alors que cela n'était pas observé pour la concentration en glucides solubles. Ces composés ne contribuent pas à une augmentation de l'ajustement osmotique par l'enrichissement en CO2 de l'air chez P. pinaster. (© Inra/Elsevier, Paris.