Plant variation in nutrient concentrations encompasses two major axes. The first is connected to nitrogen (N) and phosphorus (P), reflects growth rate and has been designated as the leaf economics spectrum (LES) while the second follows the gradient in calcium (Ca) and magnesium (Mg) and mirrors cell structural differences. Here, we tested in grasslands whether the sum Ca + Mg concentrations is a better indicator of digestibility than LES constituents. Structural equation modelling revealed that the total effect size of N (0.30) on digestibility was much lower than that of Ca + Mg (0.58). The N effect originated predominantly from sampling date (biomass ageing), while the Ca + Mg effect largely from phylogenetic composition (proportion of monocots). Thus, plant variation in partially substitutable divalent cations seems to play a significant role in biomass digestion by ruminants. This finding contests, together with litter decomposition studies, the prominent role of the LES for understanding both fundamental ecological processes.

Summary1. Management of high-nature-value (HNV) grasslands follows agri-environmental schemes across large areas of Europe. Long-term agreements and restrictions of fertilizers cause soil nutrient impoverishment, but remarkably this quite often does not reduce biomass production. Therefore, we tested how species-rich vegetation copes with nutrient impoverishment under the most frequently used treatments, that is summer mowing and livestock grazing. 2. During 2011-2012 we studied, simultaneously, plant species composition, soil and biomass chemical properties in two equally designed experiments where mowing, grazing or leaving fallow have been applied since 2004. We asked whether soil-based (C org : N tot , plant-available P and K) and plant-based measures (N : P, N : K, K : P ratios and N-, P-, K-nutrition indices) indicate the same pattern of nutrient limitation as the observed productivity gradient. 3. Seven years of management application resulted in the lowest plant-available P under grazing and the lowest plant-available K under mowing, but neither grazed nor mown plots produced less biomass than fallow ones. Grazing supported dominance of grasses while mowing that of non-leguminous forbs. 4. Projection of nutrition indices to a common framework with nutrient ratios suggests that critical thresholds for diagnosis of nutrient limitation are a function of N deficiency. At biomass production of 2 t ha À1 a N-nutrition index of 50 yielded threshold N : P = 14Á0; hence, all our treatments with N : P of 9Á9-12Á5 should be N limited. 5. Inspecting the productivity gradient separately for each management, we found only soil C org : N tot negatively related to biomass production in mown plots indicating N limitation. However in grazed plots, positive association of biomass production with plant-available P and negative with biomass N : P and N : K suggested PK co-limitation. 6. Synthesis and applications. Mowing and grazing induced different patterns of soil nutrient impoverishment and nutrient limitation, but they did not reduce biomass production of highnature-value grasslands. Non-leguminous forbs prevailing under mowing precluded shortage of P, while grasses dominating under grazing efficiently captured N. We recommend designing agri-environmental measures that will encourage alternating mowing and grazing. This should promote coexistence of multiple forbs and grasses, balance nutrient limitation and ensure stable biomass production under future low-input scenarios.

BackgroundCurrent plant – herbivore interaction models and experiments with mammalian herbivores grazing plant monocultures show the superiority of a maximizing forage quality strategy (MFQ) over a maximizing intake strategy (MI). However, there is a lack of evidence whether grazers comply with the model predictions under field conditions.Methodology/FindingsWe assessed diet selection of sheep (Ovis aries) using plant functional traits in productive mesic vs. low-productivity dry species-rich grasslands dominated by resource-exploitative vs. resource-conservative species respectively. Each grassland type was studied in two replicates for two years. We investigated the first grazing cycle in a set of 288 plots with a diameter of 30 cm, i.e. the size of sheep feeding station. In mesic grasslands, high plot defoliation was associated with community weighted means of leaf traits referring to high forage quality, i.e. low leaf dry matter content (LDMC) and high specific leaf area (SLA), with a high proportion of legumes and the most with high community weighted mean of forage indicator value. In contrast in dry grasslands, high community weighted mean of canopy height, an estimate of forage quantity, was the best predictor of plot defoliation. Similar differences in selection on forage quality vs. quantity were detected within plots. Sheep selected plants with higher forage indicator values than the plot specific community weighted mean of forage indicator value in mesic grasslands whereas taller plants were selected in dry grasslands. However, at this scale sheep avoided legumes and plants with higher SLA, preferred plants with higher LDMC while grazing plants with higher forage indicator values in mesic grasslands.ConclusionsOur findings indicate that MFQ appears superior over MI only in habitats with a predominance of resource-exploitative species. Furthermore, plant functional traits (LDMC, SLA, nitrogen fixer) seem to be helpful correlates of forage quality only at the community level.

Questions Climbers do not invest in self‐supporting architecture, allowing them to grow quickly and search for light and nutrients in canopy gaps. The climbing strategy requires both disturbance and external support; however, in herbaceous systems, disturbances often homogeneously remove external support. As a result, there should be a negative relationship between light availability and the presence of external support. Here, we asked if the distribution of herbaceous climbers is positively or negatively associated with disturbance, light availability and plant‐available nutrients. Further, we tested if climbers differ in traits compared to co‐occurring herbs. Location Czech Republic. Methods We used observations from a phytosociological database, species‐rich grasslands and arable fields. First, we examined the presence of climbers in response to disturbance variables (frequency, severity and regime) and canopy cover (external support and light availability). Second, we examined the distribution of climbers along gradients of plant‐available nutrients in species‐rich grasslands. Finally, we compared seven functional traits between 31 climbers and 1,138 co‐occurring herbs growing in 18 Central European herbaceous habitats. Results We found no relationship between any disturbance variables and the occurrence of climbers; indeed, climbers tended to occur more frequently in sites with denser canopies. In species‐rich grasslands, the presence of climbers was positively related to plant‐available magnesium. Climbers tended to be taller, with greater seed mass and specific leaf area (SLA) than co‐occurring species. Conclusions Herbaceous climbers grow in plots with low light availability but plenty of external support and require magnesium for the formation of flexible stems. Compared to co‐occurring herbs, climbers allocate resources saved from their lack of self‐support to both shade‐avoidance (canopy height) and shade‐tolerance traits (seed mass, SLA). Compared to lianas, the distribution of herbaceous climbers is not affected by disturbance but is dependent on the availability of external support and magnesium.