Elevated CO\u3csub\u3e2\u3c/sub\u3e and Extreme Climatic Events Modify Nitrogen Content and Ruminal Protein Digestion of Temperate Grassland

This study was aimed at analyzing changes in nitrogen (N) content and in vitro protein rumen digestion of an upland grassland exposed to climate changes in controlled conditions. Monoliths of grassland were inserted in 12 macrocosms in which climatic conditions for the 2050s were simulated (i.e., +2.3°C and 33 mm less precipitation compared to the current climatic conditions). Six of them were subjected to ambient CO2 (390 ppm) while the other six were subjected to elevated CO2 (520 ppm). After four months, an extreme climatic event (ECE) consisting of four weeks of reducted precipitation (-50%) followed by two weeks without irrigation combined with a heat wave (+6°C) were applied in three macrocosms at ambient CO2 and three macrocosms at elevated CO2. Then, all the macrocosms were irrigated to allow the vegetation to recover. The N content and in vitro parameters of rumen protein digestion were measured on plant samples collected before the extreme event (two cuts) and after recovery. Our results indicate that, irrespective of the sampling date, elevated CO2 results in a decrease in plant N content (P \u3c 0.01). Inversely, the application of the extreme event resulted in a large increase in N content (P \u3c 0.001) without a significant interaction with the CO2 effect. These changes significantly impacted ruminal protein digestion as evidenced by changes in the production of the fermentation end-products indicators of the proteolysis, namely ammonia and iso-volatile fatty acids. We conclude that several components of climate change can impact the nitrogenous quality of the forage and its use by ruminants

Herbaceous angiosperms are not more vulnerable to drought-induced embolism than angiosperm trees

The water transport pipeline in herbs is assumed to be more vulnerable to drought than in trees due to the formation of frequent embolisms (gas bubbles), which could be removed by the occurrence of root pressure, especially in grasses. Here, we studied hydraulic failure in herbaceous angiosperms by measuring the pressure inducing 50% loss of hydraulic conductance (P50) in stems of 26 species, mainly European grasses (Poaceae). Our measurements show a large range in P50 from 20.5 to 27.5 MPa, which overlaps with 94% of the woody angiosperm species in a worldwide, published data set and which strongly correlates with an aridity index. Moreover, the P50 values obtained were substantially more negative than the midday water potentials for five grass species monitored throughout the entire growing season, suggesting that embolism formation and repair are not routine and mainly occur under water deficits. These results show that both herbs and trees share the ability to withstand very negative water potentials without considerable embolism formation in their xylem conduits during drought stress. In addition, structure-function trade-offs in grass stems reveal that more resistant species are more lignified, which was confirmed for herbaceous and closely related woody species of the daisy group (Asteraceae). Our findings could imply that herbs with more lignified stems will become more abundant in future grasslands under more frequent and severe droughts, potentially resulting in lower forage digestibility.

A starting guide to root ecology: strengthening ecological concepts and standardising root classification, sampling, processing and trait measurements

In the context of a recent massive increase in research on plant root functions and their impact on the environment, root ecologists currently face many important challenges to keep on generating cutting-edge, meaningful and integrated knowledge. Consideration of the below-ground components in plant and ecosystem studies has been consistently called for in recent decades, but methodology is disparate and sometimes inappropriate. This handbook, based on the collective effort of a large team of experts, will improve trait comparisons across studies and integration of information across databases by providing standardised methods and controlled vocabularies. It is meant to be used not only as starting point by students and scientists who desire working on below-ground ecosystems, but also by experts for consolidating and broadening their views on multiple aspects of root ecology. Beyond the classical compilation of measurement protocols, we have synthesised recommendations from the literature to provide key background knowledge useful for: (1) defining below-ground plant entities and giving keys for their meaningful dissection, classification and naming beyond the classical fine-root vs coarse-root approach; (2) considering the specificity of root research to produce sound laboratory and field data; (3) describing typical, but overlooked steps for studying roots (e.g. root handling, cleaning and storage); and (4) gathering metadata necessary for the interpretation of results and their reuse. Most importantly, all root traits have been introduced with some degree of ecological context that will be a foundation for understanding their ecological meaning, their typical use and uncertainties, and some methodological and conceptual perspectives for future research. Considering all of this, we urge readers not to solely extract protocol recommendations for trait measurements from this work, but to take a moment to read and reflect on the extensive information contained in this broader guide to root ecology, including sections I–VII and the many introductions to each section and root trait description. Finally, it is critical to understand that a major aim of this guide is to help break down barriers between the many subdisciplines of root ecology and ecophysiology, broaden researchers’ views on the multiple aspects of root study and create favourable conditions for the inception of comprehensive experiments on the role of roots in plant and ecosystem functioning

Réponses morphologiques et physiologiques de jeunes plants de hêtre (Fagus sylvatica) à la compétition souterraine exercée par l'herbe

International audienceWe examined the morphological and physiological response of beech (Fagus sylvatica L.) seedlings to grass-induced belowground competition in full light conditions. Two-year-old beech seedlings were grown during two growing seasons in 160 L containers in bare soil conditions and with a mixture of five grasses widely represented in semi-natural meadows of central France. At the end of the second growing season, beech growing with grass presented significant reductions on diameter and height growth, annual shoot elongation and stem, root and leaf biomasses, but the root to shoot biomass ratio increased in such conditions. Grasses highly reduced soil water availability which was positively correlated with daily seedling diameter increment. Beech seemed to respond to water deficit by anticipating stomatal closure. Evidence of nitrogen competition by grasses was found, but its effect on seedlings development could not be separated from that derived from water competition. By labelling the plants with 15N we showed that nitrogen absorption by beech seedlings was very low when growing with grasses, since grasses took up more than 97% of the total nitrogen provided in the container. We conclude that, even if beech presents signs of morphological and physiological adaptation to belowground competition, beech development in full light conditions may be strongly restricted by the high competitiveness of typical full light grass species.Nous avons examiné les réponses morphologiques et physiologiques de jeunes hêtres à la compétition souterraine exercée par de l'herbe dans des conditions non limitantes en lumière. Des hêtres de 2 ans ont été cultivés en container de 160 l pendant deux ans, en sol nu ou en présence d'un mélange de 5 graminées. A la fin de la deuxième saison, les hêtres en présence d'herbe montraient une réduction significative de leur croissance en diamètre et en hauteur, de l'allongement des pousses et de la tige, des racines et de la biomasse foliaire mais à l'opposé une augmentation du rapport-racine:tige. Ces réponses ont été reliées à la disponibilité en eau et en azote du sol, grandement modifiée par la présence des graminées et à la réponse physiologique des hêtres à ces modifications

Rôle de la compétition souterraine pendant les premiers stades des successions secondaires : cas de pin sylvestre de 3 ans en prairie abandonnée

International audienceIn abandoned or extensively managed grasslands, the mechanisms involved in pioneer tree species success are not fully explained. Resource competition among plants and microclimate modifications have been emphasised as possible mechanisms to explain variation of survivorship and growth. In this study, we evaluated a number of mechanisms that may lead to successful survival and growth of seedlings of a pioneer tree species (Pinus sylvestris) in a grass-dominated grassland. Threeyear-old Scots pines were planted in an extensively managed grassland of the French Massif Central and for 2 years were either maintained in bare soil or subjected to aerial and below-ground interactions induced by grass vegetation. Soil temperatures were slightly higher in bare soil than under the grass vegetation, but not to an extent explaining pine growth differences. The tall grass canopy reduced light transmission by 77% at ground level and by 20% in the upper part of Scots pine seedlings. Grass vegetation presence also significantly decreased soil volumetric water content (Hv) and soil nitrate in spring and in summer. In these conditions, the average tree height was reduced by 5% compared to trees grown in bare soil, and plant biomass was reduced by 85%. Scots pine intrinsic water-use efficiency (A/g), measured by leaf gas-exchange, increased when Hv decreased owing to a rapid decline of stomatal conductance (g). This result was also confirmed by d13C analyses of needles. A summer 15N labelling of seedlings and grass vegetation confirmed the higher NO3 capture capacity of grass vegetation in comparison with Scots pine seedlings. Our results provide evidence that the seedlings' success was linked to tolerance of below-ground resource depletion (particularly water) induced by grass vegetation based on morphological and physiological plasticity as well as to resource conservation

Réponses morphologiques et physiologiques de jeunes plants de hêtre (Fagus sylvatica) à la compétition souterraine exercée par l'herbe

International audienceWe examined the morphological and physiological response of beech (Fagus sylvatica L.) seedlings to grass-induced belowground competition in full light conditions. Two-year-old beech seedlings were grown during two growing seasons in 160 L containers in bare soil conditions and with a mixture of five grasses widely represented in semi-natural meadows of central France. At the end of the second growing season, beech growing with grass presented significant reductions on diameter and height growth, annual shoot elongation and stem, root and leaf biomasses, but the root to shoot biomass ratio increased in such conditions. Grasses highly reduced soil water availability which was positively correlated with daily seedling diameter increment. Beech seemed to respond to water deficit by anticipating stomatal closure. Evidence of nitrogen competition by grasses was found, but its effect on seedlings development could not be separated from that derived from water competition. By labelling the plants with 15N we showed that nitrogen absorption by beech seedlings was very low when growing with grasses, since grasses took up more than 97% of the total nitrogen provided in the container. We conclude that, even if beech presents signs of morphological and physiological adaptation to belowground competition, beech development in full light conditions may be strongly restricted by the high competitiveness of typical full light grass species.Nous avons examiné les réponses morphologiques et physiologiques de jeunes hêtres à la compétition souterraine exercée par de l'herbe dans des conditions non limitantes en lumière. Des hêtres de 2 ans ont été cultivés en container de 160 l pendant deux ans, en sol nu ou en présence d'un mélange de 5 graminées. A la fin de la deuxième saison, les hêtres en présence d'herbe montraient une réduction significative de leur croissance en diamètre et en hauteur, de l'allongement des pousses et de la tige, des racines et de la biomasse foliaire mais à l'opposé une augmentation du rapport-racine:tige. Ces réponses ont été reliées à la disponibilité en eau et en azote du sol, grandement modifiée par la présence des graminées et à la réponse physiologique des hêtres à ces modifications

Resilience after extreme drought: the role of cavitation in herbaceous species

National audienc

Root structure-function relationships : evidence of a “root economics spectrum”?

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