Grazing intensity drives plant diversity but does not affect forage production in a natural grassland dominated by the tussock-forming grass Andropogon lateralis Nees.

Andropogon lateralis is a tall and highly plastic tussock-forming grass native from southern South America. It is a frequent component of Campos and Subtropical highland grasslands that often becomes dominant under lax grazing regimes. The aim of this work was to analyze the response of species diversity and forage production of a natural grassland dominated by A. lateralis to a wide range of grazing intensity. We hypothesized that species diversity and forage production would both peak at the intermediate canopy heights determined by grazing regimes of moderate intensity. A grazing experiment was conducted in a highland grassland with mesothermal humid climate at 922 masl (Atlantic Forest biome, Santa Catarina state, Brazil) that comprised 87 species from 20 families but had 50% of its standing biomass accounted by A. lateralis. Four pre-/post-grazing canopy heights-12/7, 20/12, 28/17, and 36/22 cm (measured on A. lateralis)-were arranged in a complete randomized block design with four replications, and intermittently stocked with beef heifers from October 2015 to October 2017. Andropogon lateralis cover decreased (from 75 to 50%), and species richness increased (15-25 species m-2) as canopy height decreased. Grazing intensity did not affect annual forage production (4.2 Mg DM ha-1). This natural grassland dominated by A. lateralis had a high capacity to adjust to grazing regimes of contrasting intensity, maintaining forage production stable over a wide range of canopy heights. However, to prevent losses in floristic diversity, such grassland should not be grazed at canopy heights higher than 28 cm.

The allocation of assimilated carbon to shoot growth: in situ assessment in natural grasslands reveals nitrogen effects and interspecific differences.

In grasslands, sustained nitrogen loading would increase the proportion of assimilated carbon allocated to shoot growth (A shoot), because it would decrease allocation to roots and also encourage the contribution of species with inherently high A shoot. However, in situ measurements of carbon allocation are scarce. Therefore, it is unclear to what extent species that coexist in grasslands actually differ in their allocation strategy or in their response to nitrogen. We used a mobile facility to perform steady-state (13)C-labeling of field stands to quantify, in winter and autumn, the daily relative photosynthesis rate (RPR~tracer assimilated over one light-period) and A shoot (~tracer remaining in shoots after a 100 degree days chase period) in four individual species with contrasting morpho-physiological characteristics coexisting in a temperate grassland of Argentina, either fertilized or not with nitrogen, and either cut intermittently or grazed continuously. Plasticity in response to nitrogen was substantial in most species, as indicated by positive correlations between A shoot and shoot nitrogen concentration. There was a notable interspecific difference: productive species with higher RPR, enhanced by fertilization and characterized by faster leaf turnover rate, allocated ~20% less of the assimilated carbon to shoot growth than species of lower productivity (and quality) characterized by longer leaf life spans and phyllochrons. These results imply that, opposite to the expected response, sustained nitrogen loading would change little the A shoot of grassland communities if increases at the species-level are offset by decreases associated with replacement of 'low RPR-high A shoot' species by 'high RPR-low A shoot' species.

13C-labeling shows the effect of hierarchy on the carbon gain of individuals and functional groups in dense field stands.

Measurements of resource capture by individuals, species, or functional groups coexisting in field stands improve our ability to investigate the ecophysiological basis of plant competition. But methodological and technical difficulties have limited the use of such measurements. Carbon capture, in particular, is difficult to asses in heterogeneous, dense field stands. Here we present a new approach to measure in situ daily gross carbon gain of individuals. It is based on measuring the 13C content of shoots after a few hours of continuous labeling of all assimilated CO2. The technique is simple and has few assumptions. A new, fully mobile facility was developed, capable of providing a labeling environment with a CO2 concentration close to atmospheric air and known, constant 13C-enrichment, while maintaining temperature and relative humidity within ambient values. This facility was used in seminatural grasslands of Germany and Argentina to explore the relationship between size and carbon gain of individuals of coexisting species growing in contrasting hierarchical positions, and to analyze the carbon gain of functional groups. In general, carbon gain per unit shoot mass increased with increasing size among small individuals, but it became independent of size among the largest ones. In consequence, competition appeared to be size asymmetric between subordinate individuals but size symmetric between dominant individuals. When comparing functional groups, the carbon gain per unit shoot mass of rosette dicots vs. grasses reflected not their relative contribution to stand biomass, but their hierarchical position: irrespectively of mass or growth form, being taller than neighbors was most important in determining carbon gain per unit shoot mass. We believe these results show that in situ measurements of carbon gain can provide valuable insight in field studies of plant competition.