Summary: The effects of plants on the biosphere, atmosphere and geosphere are key determinants of terrestrial ecosystem functioning. However, despite substantial progress made regarding plant belowground components, we are still only beginning to explore the complex relationships between root traits and functions. Drawing on the literature in plant physiology, ecophysiology, ecology, agronomy and soil science, we reviewed 24 aspects of plant and ecosystem functioning and their relationships with a number of root system traits, including aspects of architecture, physiology, morphology, anatomy, chemistry, biomechanics and biotic interactions. Based on this assessment, we critically evaluated the current strengths and gaps in our knowledge, and identify future research challenges in the field of root ecology. Most importantly, we found that belowground traits with the broadest importance in plant and ecosystem functioning are not those most commonly measured. Also, the estimation of trait relative importance for functioning requires us to consider a more comprehensive range of functionally relevant traits from a diverse range of species, across environments and over time series. We also advocate that establishing causal hierarchical links among root traits will provide a hypothesis‐based framework to identify the most parsimonious sets of traits with the strongest links on functions, and to link genotypes to plant and ecosystem functioning.

Summary: Although plant nitrogen (N) strategies may play an important role for community structure and ecosystem functioning, there is not a clear understanding of the link between N acquisition by roots and N utilization by shoots. Particularly, it is unclear how the co‐variations between size‐ and physiology‐related traits determine N acquisition and N utilization at the plant scale. We used 13 co‐occurring temperate pasture grasses to study inter‐specific variations in above‐ground N yield and in root N acquisition and shoot N utilization traits. N acquisition traits concerned root influx capacities for and , root mass and specific root area in ingrowth cores. N utilization traits concerned leaf life span, leaf N content, leaf N resorption, mean residence time of N and leaf N use efficiency. We found evidence for three trade‐offs across species concerning root N acquisition: (i) root mass increased when specific root area declined; (ii) an increase in root area was observed when total N influx capacity decreased; and (iii) root influx capacity increased when capacity declined. High total root uptake capacity gave rise to high leaf N content and was associated across species to low leaf N use efficiency. Tall grasses were characterized by high shoot N yield, high root biomass and high leaf N use efficiency. Physiology‐related traits and size‐related traits were generally found independent. Our study demonstrates how size and N uptake related root traits are associated to major axes of plant specialization ((i) plant size and (ii) conservation vs. exploitation of N) which were previously identified based on shoot traits. Contrasted N strategies were segregated across species according to four combinations along these two axes.

To gain a better understanding of beech growth requirements and assess the competition with the surrounding vegetation at two successional stages after agricultural land abandonment, we introduced two-year-old beech seedlings (i) in a recently abandoned meadow (one half weeded) and (ii) in an old meadow colonised by 25-year-old natural Scots pine, with one part thinned to increase light availability at ground level. Beech seedlings presented significantly different stem diameter growth rates according to vegetation composition (grasses or dicotyledon species) and light availability for both successional stages. Grass species, which developed efficient strategies to extract soil water, competed strongly with beech seedling compared with dicotyledon species. Water competition led to a strong reduction of beech diameter growth. For a given vegetation composition, increasing light availability improved beech growth.Afin de mieux comprendre les conditions de croissance du hêtre et d’évaluer l'impact de la compétition par la végétation à deux stades de la succession végétale après abandon de terres agricoles, des plants de hêtre de deux ans ont été introduits : (i) dans une prairie récemment abandonnée (dont la moitié était désherbée) et (ii) dans une prairie abandonnée plus anciennement et colonisée par un boisement naturel de pin sylvestre de 25 ans (en partie éclairci pour obtenir différents niveaux d’éclairement au niveau du sol). Le taux de croissance en diamètre des jeunes plants de hêtre a été significativement différente en fonction de la composition de la végétation (graminées ou dicotylédones) et de l'éclairement. Les graminées ont la capacité de puiser l'eau très efficacement dans le sol en comparaison des dicotylédones, ce qui les rend très compétitives vis-à-vis des jeunes hêtres. Cette compétition a diminué la croissance en diamètre des jeunes plants. Par contre, à composition égale de la végétation, la croissance des plants de hêtre augmente avec le niveau d'éclairement