SummaryDeterministic niche-based processes have been proposed to explain species relative abundance within communities but lead to different predictions: habitat filtering (HF) predicts dominant species to exhibit similar traits while niche differentiation (ND) requires that species have dissimilar traits to coexist.Using a multiple trait-based approach, we evaluated the relative roles of HF and ND in determining species abundances in productive grasslands. Four dimensions of the functional niche of 12 co-occurring grass species were identified using 28 plant functional traits. Using this description of the species niche, we investigated patterns of functional similarity and dissimilarity and linked them to abundance in randomly assembled six-species communities subjected to fertilization/disturbance treatments.Our results suggest that HF and ND jointly determined species abundance by acting on contrasting niche dimensions. The effect of HF decreased relative to ND with increasing disturbance and decreasing fertilization. Dominant species exhibited similar traits in communities whereas dissimilarity favored the coexistence of rare species with dominants by decreasing interspecific competition. This stabilizing effect on diversity was suggested by a negative relationship between species over-yielding and relative abundance.We discuss the importance of considering independent dimensions of functional niche to better understand species abundance and coexistence within communities.

Summary1. By comparing plant species under the same experimental field conditions, the direct effects of plant traits on ecosystem processes can be studied. We have analyzed the role of leaf traits (leaf lamina dry matter content, LDMC; leaf lamina N content, LNC and specific leaf lamina area, SLA) for the annual above-ground primary productivity (ANPP) and quality (pepsin-cellulase digestibility, crude protein content) for herbivores of 13 perennial C 3 pasture grass species. 2. These relationships were investigated over 2 years with monocultures grown in a fully factorial block design crossing the plant species, the cutting frequency and the N supply factors. 3. The within species variation in leaf traits, ANPP, digestibility and protein content was less than between species variation. Species ranks for leaf traits were conserved among N supply and cutting frequency levels. Highly significant ( P < 0·001) between species allometric relationships were found for LNC × SLA and SLA × LDMC, with common slopes but differences in intercept and shifts among factor levels. 4. The between species variation in ANPP was strongly ( P < 0·001) and negatively correlated with the fresh-matter based leaf N content (i.e. LDMC × LNC) and was not affected by SLA, apparently because of a trade-off between SLA and leaf lamina fraction. Digestibility increased with SLA and declined with LDMC. Protein content increased with both fresh and dry-matter based LNC. 5. N supply increased LNC and SLA but reduced LDMC. Cutting frequency increased LDMC and reduced LNC. In response to cutting frequency, changes in digestibility and in fresh-matter based LNC were positively correlated. 6. We conclude that the between species variation in the annual production of digestible energy and of proteins by pasture grasses is controlled in an additive way by two leaf traits: LNC and LDMC.

Summary 1.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. 2. 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. 3. 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. 4. 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. 5. 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.

Monocultures of thirteen perennial C 3 grass species that co-occur in temperate semi-natural grassland communities in Europe were compared in a factorial field design of two levels of N supply and two levels of cutting frequency. Above-ground yield of dry matter (DM), crude protein (CP) concentration and pepsincellulase DM digestibility of herbage were measured in two successive years. Species was the largest source of variability in yield of DM and DM digestibility of herbage, while CP concentration of herbage responded more to management factors. The highest mean DM yields and values of DM digestibility of herbage were achieved in spring for Festuca arundinacea and in autumn for Phleum pratense. Poa trivialis and Festuca rubra had the lowest DM yield and DM digestibility values, respectively, regardless of seasons and treatments. For all species CP concentration in herbage increased in response to an increase in cutting frequency and N supply by an average of 46 and 34 g kg )1 DM respectively. Differences between years and seasons indicated the importance of plant phenology on nutritional variables and the influence of environmental factors on species performance. Species ranking was compared according to their annual digestible DM and CP yields. The results show that some grasses have a nutritive value which is comparable to that of forages selected for high yields.

RESUMO -Variáveis morfogênicas e estruturais foram quantificadas a partir da técnica de "perfilhos marcados" no período de agosto a outubro de 1999 numa pastagem de azevém anual, mantida em quatro diferentes alturas (5, 10, 15 e 20 cm) através de lotação contínua em carga variável com ovinos. O delineamento experimental foi o de blocos completos casualizados com três repetições. Utilizaram-se 50 perfilhos marcados por unidade experimental, distribuídos ao longo de transectas. As observações foram efetuadas em dois períodos: no primeiro, da entrada dos animais até a obtenção das alturas pretendidas, não se observaram diferenças entre os tratamentos. No segundo (fase de estabilização das alturas pretendidas), a taxa de elongação foliar, o comprimento da lâmina verde total por perfilho e o tamanho das folhas apresentaram resposta positiva frente a maiores alturas de manejo, enquanto o tempo de elongação da lâmina foliar diminui com a altura. As variáveis taxa de surgimento de folhas, tempo de vida das folhas, densidade populacional de perfilhos e número de folhas vivas por perfilho não foram afetadas pelos tratamentos.Palavras-chave: perfilhos marcados, altura, azevém, pastejo, ovinos Morphogenetic and Structural Traits of Ryegrass (Lolium multiflorum Lam.) Managed under Different Sward HeightsABSTRACT -Morphogenetic and structural variables were obtained from measurements performed on marked tillers and were quantified during August to October 1999 in a ryegrass pasture grazed by sheep under continuous stocking and variable stocking rate in order to maintain the sward at four different heights (5, 10, 15 and 20 cm). The experimental design was a randomized complete block with three replicates. Fifty marked tillers per experimental unit were assessed and distributed along transects lines. Sward measurements were performed in two periods: the first one, from the start of grazing until the point where the intended grazing heights were established, with no differences among treatments. In the second (after achieving the intended grazing heights), leaf elongation rate, total green leaf lamina length per tiller and leaf size showed a positive response to increasing sward heights, while the time to leaf lamina elongation decreased. The variables leaf appearance rate and leaf life span, tiller population density and number of live leaves per tiller were not affected by treatments.