One Sentence Summary: Empirical evidence from grasslands around the world demonstrates a humped-back relationship between plant species richness and biomass at the 1 m 2 plot scale.Abstract: One of the central problems of ecology is the prediction of species diversity. The humped-back model (HBM) suggests that plant diversity is highest at intermediate levels of productivity; at low productivity few species can tolerate the environmental stresses and at high productivity a small number of highly competitive species dominate. A recent study claims to have comprehensively refuted the HBM. Here we show, using the largest, most geographically diverse dataset ever compiled and specifically built for testing this model that if the conditions are met, namely a wide range in biomass at the 1 m 2 plot level and the inclusion of plant litter, the relationship between plant biomass and species richness is hump shaped, supporting the HBM. Our findings shed new light on the prediction of plant diversity in grasslands, which is crucial for supporting management practices for effective conservation of biodiversity. 4Main Text: The relationship between plant diversity and productivity is a topic of intense debate (1-6). The HBM states that plant species richness peaks at intermediate productivity, taking above-ground biomass as a proxy for annual net primary productivity (ANPP) (7-9). This diversity peak is driven by two opposing processes; in unproductive and disturbed ecosystems where there is low plant biomass, species richness is limited by either stress, such as insufficient water and mineral nutrients, or high levels of disturbance-induced removal of biomass, which few species are able to tolerate. In contrast, in the low disturbance and productive conditions that generate high plant biomass it is competitive exclusion by a small number of highly competitive species that is hypothesized to constrain species richness (7-9). Other mechanisms proposed to explain the unimodal relationship between species richness and productivity include disturbance (10), evolutionary history and dispersal limitation (11,12), and density limitation affected by plant size (13).Different case studies have supported or rejected the HBM, and three separate meta-analyses reached different conclusions (14). This inconsistency may indicate a lack of generality of the HBM, or it may reflect a sensitivity to study characteristics including the type(s) of plant communities considered, the taxonomic scope, the length of the gradient sampled, the spatial grain and extent of analyses (14,15), and the particular measure of net primary productivity (16). Although others would argue (6), we maintain that the question remains whether the HBM serves as a useful and general model for grassland ecosystem theory and management. 5 We quantified the form and strength of the richness-productivity relationship using novel data from a globally-coordinated (17), distributed, scale-standardized and consistently designed survey, in which plant richness and biomass were m...

Topics discussed in this review, mainly of the major forage grasses include tiller morphology, canopy leaf area optimization for continuously and rotationally grazed swards, and tiller population demography including its manipulation and the effects of mixed species swards. It is considered that the primary driving principle for tiller dynamics is the optimization of leaf canopy area in relation to defoliation intensity and available resources, such as light and water. The concept of a multiphase size-density compensation relationship along an environmental boundary may rationalize otherwise conflicting observations on tiller density and on tiller appearance and death rate and are relevant to issues such as carrying capacity or sustainability. There remain substantive complex differences in tillering behaviour, often unique to a particular species, which are best explained from a tiller demography basis. A well-directed understanding of tiller demography may result in significant improvements in productivity in some situations.

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Bacteria Shoots Roots Fungi Mycorrhizae Bacterialfeeding nematodes Fungal-feeding nematodes Collembola Root-feeding nematodes Predacious nematodes Mites * Net plant assimilate was calculated as the amount of C in the plant biomass at harvest plus the amount of C collected as rhizodeposition throughout the growth period.

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.

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RESUMO -Em pastagem de azevém anual sob lotação contínua com ovinos, quantificaram-se os fluxos de biomassa no período de 02 a 20 de outubro de 1999. Os tratamentos consistiram de quatro alturas (5, 10, 15 e 20 cm) de manejo, mantidas pelo uso de lotação variável. O delineamento experimental utilizado foi o de blocos casualizados com três repetições. Os fluxos foram determinados por meio de variáveis morfogênicas e estruturais obtidas a partir de medições feitas em afilhos marcados. No período de estabilização das alturas pretendidas, diferenças significativas na estrutura da pastagem resultaram em alterações nos fluxos de biomassa. Os fluxos de crescimento e de consumo apresentaram resposta quadrática às alturas de manejo, com valores máximos entre 10 e 15 cm, enquanto o fluxo de senescência aumentou linearmente com a altura. Máximos ganhos médios diários e ganhos por hectare foram alcançados nesse intervalo, indicando que essa faixa é apropriada para se manejar a pastagem. O período de avaliação coincidiu com a fase pós-indução floral, determinando diminuição na massa de lâminas e balanço negativo no fluxo de biomassa para todos os tratamentos.Palavras-chave: afilhos marcados, consumo, crescimento, morfogênese, ovinos, senescência Biomass Flows in Italian Ryegrass Pastures (Lolium multiflorum Lam.) Managed under Different Sward HeightsABSTRACT -In an italian ryegrass pasture under continuous stocking with sheep, biomass flows were estimated from 2 to 20 October 1999. Treatments corresponded to four different sward heights (5, 10, 15 and 20 cm) maintained by variable stocking. The experimental design was the randomized complete block design with three replicates. The fluxes were defined by morphogenetic and structural variables obtained from measurements performed on marked tillers. During the period in which the intended sward heights were established, significant effects in sward structure resulted in alterations on the biomass fluxes. The growth and intake fluxes showed a quadratic response with maximum values between 10 and 15 cm grazing heights, while the senescence flux increased linearly with sward height. Maximum daily live weight gain and animal yield per hectare were obtained within this interval, indicating this interval being the most appropriate to pasture management. The evaluation period was coincident with the floral induction phase, so the leaf lamina mass was progressively smaller and biomass fluxes showed a negative balance for all treatments.Key Words: intake, growth, marked tillers, morphogenesis, senescence, sheep R. Bras. Zootec., v.33, n.3, p.529-537 IntroduçãoO conhecimento do processo de morfogênese (dinâmica de geração e expansão da forma da planta no espaço) possibilita modelar o fluxo de biomassa foliar em nível de um afilho individual, através do crescimento, morte e colheita dos diferentes componentes vegetais (Davies, 1993). As taxas de alongamento foliar e de surgimento de folhas e o tempo de vida das folhas constituem os fatores morfogênicos do afilho que determinam o ritmo de...