Requirement of vegetation gaps for seedling establishment of two unpalatable grasses in a native grassland of central Argentina

Abstract The objective was to determine the effects of root and shoot competition on seedling establishment of the unpalatable grasses Stipa gynerioides and S. tenuissima in a native grassland of central Argentina dominated by the palatable grass S. clarazii. Seeds of the two unpalatable species were sown in natural occurring microsites with shoot and root competition from the palatable species, and in artificially created microsites without either shoot competition or shoot and root competition. In addition, fresh seeds of the unpalatable species were subjected to daily alternating temperatures under laboratory and field conditions to determine the effect on seed dormancy and germination. Seedling establishment of S. gynerioides and S. tenuissima occurred only in microsites without shoot and root competition. Also, the fluctuation of temperature near the soil surface in these microsites reduced dormancy and promoted rapid germination in both species. Our results support the hypothesis that, in swards dominated by palatable grasses, vegetation gaps of low competitive pressure favour seedling establishment of unpalatable grasses. It is suggested that the creation of these gaps by overgrazing can be an important mechanism in the process of species replacement in native grasslands.     

Effect of competition intensity on recruitment of palatable and unpalatable grasses

In this study, we made an attempt to reveal how competition intensity from established plants impacts on palatable and unpalatable grass seedlings recruitment, in a natural mesic grassland of central Argentina. Our objective was to assess the seedling recruitment of a palatable species (Chascolytrum subaristatum) and an unpalatable species (Nassella trichotoma) in microsites differing in competition intensity from established plants. Identity (C. subaristatum and N. trichotoma) and defoliation severity were used as surrogate for competition intensity. In March 2017, we permanently marked established individuals of N. trichotoma and C. subaristatum and placed two circular plots adjacent to each individual. In one plot we added seeds of N. trichotoma and in the other seeds of C. subaristatum. After seeding, established plants were randomly assigned to one of three level of defoliation: without defoliation, low defoliation severity and high defoliation severity. From April to November 2017 (i.e. over a complete annual growing cycle), we measured seedling density, recruitment and growth. Our results supported the hypothesis that seedlings of palatable grasses are more competitive than seedlings of unpalatable grasses. Seedling of the palatable grass C. subaristatum recruited successfully regardless the intensity of competition from established plants, whereas seedlings of the unpalatable grass N. trichotoma recruited better under low competitive pressure from established plants. Our results suggest that the availability of microsites with low competitive pressure from the established vegetation, created by selective grazing of palatable grasses, promotes the recruitment of unpalatable grass seedlings. This mechanism may contribute to the species replacement process commonly observed in heavy grazed grasslands.     

Refuge effect of an unpalatable forb on community structure and grass morphology in a temperate grassland

The role of unpalatable plant species as biological antiherbivore refuges for palatable species is well-documented at community level particularly in harsh environments. In productive sub-humid temperate grassland subjected to domestic grazing, we examined the protective effect of Eryngium horridum on plant community structure and floristic composition, and evaluated whether these changes impacted on a number of morphological traits of grasses, related to grazing resistance. We also investigated, for a palatable grass species (Stipa neesiana) the existence of morphological differences between protected and unprotected plants and if this eventual variation was either plastic or genetic. The study consisted of a field survey where we compared paired patches, with and without E. horridum, and a greenhouse experiment where we evaluated individuals of S. neesiana coming from both patch types over a 11 months period. Patches dominated by E. horridum had lower richness and cover of forbs than patches without the forb, and similar richness but greater cover of cool-season tussock palatable grasses, which suggests a protective role on the latter. Grasses in these patches also had longer blades and sheaths and lower specific leaf area. The morphological differences of S. neesiana individuals collected from both patch types disappeared after 11 months growth in a common environment which revealed significant phenotypic plasticity in this species. These results suggest the existence of plant-to-plant facilitation in a productive ecosystem not only at community level, through changes in species richness and the promotion of palatable grasses, but also at population level, through plastic changes in aboveground morphological traits. Both facilitation and plasticity, would contribute to the persistence of threatened palatable grasses in the heavy grazed productive ecosystems.     

Continuous grazing disrupts desert grass-soil seed bank composition under variable rainfall

Long-term field studies help unveil mechanisms of grass soil seed bank (SSB) persistence and resilience. We wonder whether grazing is a disruptive force that changes grass SSB composition and size, while heavy rainfall is a positive bottom-up force that restores the grass SSB in grazed areas of the Monte desert, Argentina. We used core sampling to study the grass SSB in cattle-free (17 year) and grazed (10 year) grasslands. The SSB size in cattle-free sites usually responded in a continuous positive fashion to rainfall, and the proportion of palatable grasses in the SSB was similar among years. Most grass species formed a transient SSB, but the presence of a stand of perennial plants would prevent their local extinction. In contrast, the SSB size in the grazed habitats showed a threshold response to rainfall (i.e. it did not increase under low-moderate rainfall), the seeds of palatable grasses were always scarce, and the proportion of seeds of less palatable grasses increased under extreme rainy conditions. Seed production from grasses that grew tillers from axillary buds during a drought was zero, suggesting that the bank of plants could have a role in the replenishment of the grass SSB only in mesic years. Within the time span assessed here, continuous heavy grazing together with scarce as well as heavy rainfall were disruptive forces that reduced the number and proportion of seeds from palatable grasses in the SSB, which are also the species preferred by seed-eating animals.

     

Grass-to-grass protection from grazing in a semi-arid steppe. Facilitation, competition, and mass effect

Plants of low palatability often serve as biotic refuges from grazing to palatable plants. Evidence for this facilitation comes from cases where the interacting species have different life form, which may minimize competition. Protected plants act as remnant seed sources that may maintain the palatable populations locally viable through mass effects. Here, we assess (1) the spatial association between a highly palatable Patagonian grass (Bromus pictus) and less preferred tussock grasses, (2) the role played by seed sources in maintaining the population in the face of heavy grazing by sheep, and (3) the facilitative and competitive components of the interaction. We quantified B. pictus density and its distance to nearest tussocks inside and outside a grazing exclosure. We also considered different distances from the exclosure, both leeward and windward, because strong westerly winds may be critical for dispersal. Additionally, we quantified several attributes of protected and unprotected B. pictus plants with and without grazing. Density of B. pictus was about 20 times greater inside the exclosure than outside. However, this difference was less pronounced in the leeward vicinity of the exclosure than in the windward one, which suggests a mass effect. B. pictus was significantly associated to less palatable tussocks, and the association became stronger under grazing and as distance from the exclosure edge increased. Protection under grazing was associated with a significant increase in plant biomass, height, tiller number, and panicle number, whereas protection in the absence of grazing, which could evidence competition, resulted in reductions of tiller number and panicle number, and an increase of height. These results suggest that in areas under grazing pressure on palatable grasses, other less palatable grasses may provide a protection from grazing that outweighs competitive effects. Such protection may generate small-scale mass effects that maintain the population at relatively high density.     

Soil nitrogen availability under grasses of different palatability in a temperate semi‐arid rangeland of central Argentina

Abstract Grazing by domestic livestock is frequently associated with the replacement of high‐nutrient palatable species with low‐nutrient unpalatable species, which may have a substantial effect on nutrient cycling. The objective of the present study was to compare soil N availability and net N mineralization in soils under Poa ligularis (palatable grass) with those in soils under Stipa tenuissima (unpalatable grass) in a temperate semi‐arid rangeland of central Argentina. Nitrogen availability and net mineralization under laboratory and field conditions were measured. Soil N availability under P. ligularis was higher than or similar to soil N availability under S. tenuissima. In situ net N mineralization in the soil under P. ligularis was lower than or similar to net N mineralization in the soil under S. tenuissima. Potential net N mineralization was greater in the soil under P. ligularis than in the soil under S. tenuissima. Our results suggest that the replacement of palatable grasses by unpalatable grasses in the temperate semi‐arid rangelands of central Argentina may imply a reduction in the rate of nutrient cycling.     

Morphological adaptation of a palatable plant to long-term grazing can shift interactions with an unpalatable plant from facilitative to competitive

Unpalatable plants can protect palatable neighbor plants from grazing pressure, but morphological evolution of a palatable species might change its interactions with unpalatable plants. We predicted that when a palatable species has locally adapted to grazing by expressing a dwarf phenotype that reduces grazer accessibility, the dwarf plants experience relatively more competitive effects than facilitative effects from large, well-defended, unpalatable species. We used a transplant experiment, in which both dwarf and large ecotypes of a palatable annual species, Persicaria longiseta, were transplanted outside and inside the canopy of an unpalatable nettle, Urtica thunbergiana, in a long-term deer grazing habitat of Nara Park, Japan. The dwarf ecotype of Persicaria has adapted to the grazing environments of the park by exhibiting inherently short shoots and small leaves, whereas the large ecotype is found in habitats with no grazing history. A previous common-garden study suggested that the phenotypic differences were genetically based and that phenotypic plasticity contributed little to the morphological difference. The large-phenotype of Persicaria experienced significantly increased morphological size, survival, and reproductive output under the Urtica canopy compared to outside the canopy, whereas these traits of the dwarf phenotype were reduced under the Urtica canopy compared to outside. These results indicate that the net effects of Urtica on Persicaria were positive for the large ecotype and negative for the dwarf ecotype. Thus, the morphological adaptation of a palatable species to avoid grazing altered its interactions with a large, well-defended neighbor.     

Effects of grazing on patch structure in a semi‐arid two‐phase vegetation mosaic

Abstract. Question: What are the grazing effects in the spatial organization and the internal structure of high and low cover patches from a two‐phase vegetation mosaic? Location: Patagonian steppe, Argentina. Methods: We mapped vegetation under three different grazing conditions: ungrazed, lightly grazed and heavily grazed. We analysed the spatial patterns of the dominant life forms. Also, in each patch type, we determined density, species composition, richness, diversity, size structure and dead biomass of grasses under different grazing conditions. Results: The vegetation was spatially organized in a two‐phase mosaic. High cover patches resulted from the association of grasses and shrubs and low cover patches were represented by scattered tussock grasses on bare ground. This spatial organization was not affected by grazing, but heavy grazing changed the grass species involved in high cover patches and reduced the density and cover of grasses in both patch types. Species richness and diversity in high cover patches decreased under grazing conditions, whereas in low cover patches it remained unchanged. Also, the decrease of palatable grasses was steeper in high cover patches than in low cover patches under grazing conditions. Conclusions: We suggest that although grazing promotes or inhibits particular species, it does not modify the mosaic structure of Patagonian steppe. The fact that the mosaic remained unchanged after 100 years of grazing suggests that grazing does not compromize population processes involved in maintaining patch structure, including seed dispersal, establishment or biotic interactions among life forms.     

More plant biomass results in more offspring production in annuals,or does it?

Competitive ability in plants has been previously measured almost exclusively in terms of traits related to growth (biomass) or plant size. In this study, however, we used a multi‐species competition experiment with six annuals to measure relative competitive ability in terms of reproductive output, i.e. the number of offspring produced for the next generation. Under greenhouse conditions, plants of each species were started in pots from germinating seeds and were grown singly (free of competition) and at high density in both monocultures and in mixtures with all study species. Several traits traditionally regarded as determinants of competitive ability in plants were recorded for each species grown singly, including: seed mass, germination time, early growth rate and potential plant size (biomass and height). Under competition, several traits were recorded as indicators of relative performance in both monocultures and mixtures, including: biomass of survivors, total number of survivors, number of reproductive survivors, and reproductive output (total seed production) of the survivors. As expected, species that grew to a larger biomass in isolation had higher seed production in isolation. However, none of the traditional plant growth/size‐related traits, measured either in isolation or under competition, could predict between species variation in reproductive output under competition in either monocultures or mixtures. In mixtures, 97% of this variation in reproductive output could be explained by between‐species variation in the number of reproductive survivors. The results indicate that traits measured on plants grown singly may be poor predictors of reproductive output under competition, and that species’ rank order of competitive ability in terms of the biomass of survivors may bear no relationship to their rank order in terms of the number of offspring produced by these survivors. This has important implications for the interpretation of mechanisms of species coexistence and community assembly within vegetation.     

Do life history traits predict responses to defoliation in co‐occurring prairie grasses?

Abstract. This study examines whether competition between the unpalatable grass Hilaria mutica and three co‐occurring, palatable grasses in a Texan mixed prairie is altered by non‐selective or selective defoliation. In this four‐year study, plants were grown in monoculture or in combination with the unpalatable Hilaria in a replacement design. Under no defoliation, the unpalatable Hilaria had a lower growth potential than Bouteloua curtipendula and Nassella leucotricha that were of equal stature, and produced only as much as the shorter grass, Buchloe dactyloides. Bouteloua had the highest growth potential under no‐defoliation and was defoliation tolerant, except when defoliated at ground level. Nassella was more productive than the unpalatable Hilaria, since the ability to grow earlier in the year enabled it to compete successfully with Hilaria. These results indicate that with adequate deferment Bouteloua and Nassella should compete successfully with Hilaria and Buchloe should be able to maintain itself in the presence of Hilaria. Under non‐selective defoliation, Hilaria was able to compete successfully only with Buchloe. Hilaria was sensitive to defoliation, despite being rhizomatous, and competed less successfully with Buchloe after non‐selective defoliation than it did when not defoliated. This indicates that the management practice of burning and stocking heavily with livestock until Hilaria is avoided, resulting in non‐selective defoliation, will not cause Hilaria to be more competitive with the more palatable Bouteloua, Buchloe or Nassella. Hilaria was able to compete most successfully under selective defoliation when it was not defoliated. Under selective defoliation, by avoiding herbivory, Hilaria is able to compete strongly with at least Buchloe and Nassella. The reaction of Nassella and Buchloe to selective defoliation indicates that they may have been displaced by Hilaria in the past. In contrast, under the short‐term and non‐limiting growth conditions of this study, Bouteloua competed successfully with Hilaria even under selective defoliation. These results do not rule out the possibility that, through selective defoliation, Hilaria may have displaced other grasses including Bouteloua in the past.     

Shifting Mosaics in Grazed Woodlands Driven by the Alternation of Plant Facilitation and Competition

Abstract: Free-ranging large grazers, such as cattle and horses, are increasingly reintroduced to former agricultural areas in Western Europe in order to restore natural and diverse habitats. In this review we outline mechanisms by which large grazers induce and maintain structural diversity in the vegetation (mosaics of grasslands, shrub thickets and trees). This variation in vegetation structure is considered to be important for the conservation of biodiversity of various plant and animal groups. The process of spatial association with unpalatable plants (as-sociational resistance) enables palatable plants to establish in grasslands maintained by large grazers. In this way, short unattractive (thorny, low quality or toxic) species facilitate taller unattractive shrubs, which facilitate palatable trees, which in turn outshade the species that facilitated their recruitment. Established trees can, therefore, not regenerate under their own canopy, leading to cyclic patch dynamics. Since this cyclic dynamic occurs on a local scale, this contributes to shifting mosaics. The mechanisms involved in creating and maintaining the resulting shifting mosaics are described for temperate flood-plain and heathland ecosystems, including the effects on nutrient transport within grazed landscapes. How grazing leads to shifting mosaics is described in terms of plant functional types, allowing potential generalisation to other ecosystems. The resulting interaction web of grasses, unpalatable forbs and shrubs, palatable light-demanding trees and shade-tolerant trees is discussed, and was found to contain various interesting direct and indirect effects. The key process contributing to spatial diversity in vegetation structure is the alternation of positive (facilitation) interactions between plant species at one life cycle stage, and competitive displacement at another stage. Grazing thus causes directional successional sequences to change to shifting mosaics. The implications of this theory for nature conservation are discussed, including the relevant management problems, possible choices and practical solutions. We conclude that the theoretical framework outlined in this review provides helpful insights when coping with nature conservation issues in temperate woodland habitats.     

Seasonality and edge effect determine herbivory risk according to different plant association models

We report evidence of hierarchical resource selection by large herbivores and plant neighbouring effects in a Mediterranean ecosystem. Plant palatability was assessed according to herbivore foraging decisions. We hypothesize that under natural conditions large herbivores follow a hierarchical foraging pattern, starting at the landscape scale, and then selecting patches and individual plants. A between- and within-patch selection study was carried out in an area formed by scrubland and pasture patches, connected by habitat edges. With regard to between-patch selection, quality-dependent resource selection is reported: herbivores mainly consume pasture in spring and woody plants in winter. Within-patch selection was also observed in scrub habitats, influenced by season, relative patch palatability and edge effect. We defined a Proximity Index (PI) between palatable and unpalatable plants, which allowed verification of neighbouring effects. In spring, when the preferred food resource (i.e. herbs) is abundant, we observed that in habitat edges large herbivores basically select the relatively scarce palatable shrubs, whereas inside scrubland, unpalatable shrub consumption was related to increasing PI. In winter, a very different picture was observed; there was low consumption of palatable species surrounded by unpalatable species in habitat edges, where the latter were more abundant. These outcomes could be explained though different plant associations described in the literature. We conclude that optimal foraging theory provides a conceptual framework behind the observed interactions between plants and large herbivores in Mediterranean ecosystems.     

Tradeoff between height and relative growth rate in a dominant grass from the Serengeti ecosystem

We determined the relationship between plant height and whole-plant relative growth rate (g g^-1 day^-1) for ten genotypes of Sporobolus kentrophyllus collected from an intensively grazed site on the Serengeti Plains, Tanzania. Plants were grown for 7 weeks in a greenhouse in Syracuse, N.Y., and harvested weekly. Plants that received simulated bovine urine showed a negative relationship between plant height and growth rate, suggesting a genetic tradeoff between competitive ability if ungrazed (height) and ability to recover from grazing (growth rate). There was no height-growth rate relationship under nitrogen addition rates similar to field mineralization rates. In addition, faster-growing, shorter plants tended to have relatively higher above-ground growth rates than slower-growing, taller plants. These results suggest that natural selection has maintained a gradient of morphologies within this species ranging from short, rapidly growing genotypes adapted to intense grazing conditions to tall, slow-growing, grazer-susceptible genotypes that are superior light competitors in absence of herbivory.     

Effects of grazing on soil seed bank dynamics: An approach with functional groups

Abstract. The relationship between intensity and timing of cattle grazing on changes in the size and composition of the soil seed bank were investigated in a 3‐yr study in a Mediterranean grassland in northeastern Israel. Treatments included manipulations of stocking rates and of grazing regimes, in a factorial design. The retrieved soil seed bank community was rich in species, with 133 species accounting for 80% of the 166 species recorded at the site. Within the seed bank, 89% of the species were annuals. Seed bank dynamics was analysed in terms of plant functional groups and germination strategies. In terms of total seed bank density and including all functional groups, 42% of the seeds present in the soil did not germinate under watering conditions. The dormancy level differed greatly among functional groups. The seed bank of annual legumes, crucifers, annual thistles and annual forbs had a large fraction of non‐germinated seeds and characterized areas grazed early in the growing season under high and very high grazing intensity. These functional groups were considered to have a higher potential for persistent seed banks production. In contrast, short and tall annual grasses and tall perennial grasses, that were dominant in ungrazed or moderately grazed paddocks, generally had seed banks with a very small fraction of non‐germinated seeds. Seed bank densities varied widely between grazing treatments and years. Under continuous grazing, heavy grazing pressure reduced seed bank densities of grasses and crucifers in comparison to moderate grazing. The greatest reduction on the seed bank densities resulted from heavy grazing concentrated during the seed‐set stages.     

Size traits and site conditions determine changes in seed bank structure caused by grazing exclusion in semiarid annual plant communities

1. Contrasting patterns of change in the seed bank of natural grasslands are frequently found in response to grazing by domestic herbivores. Here, we studied the hypotheses that a) patterns of change in seed bank density and composition in response to grazing depend on spatial variation in resource availability and productivity, and b) that variation among species in patterns of seed bank response to grazing is linked to differences in species size traits (i.e. size of plant, dispersal unit and seed). 2. Effects of sheep grazing exclusion on the seed bank were followed during five years in a semiarid Mediterranean annual plant community in Israel. Seed bank density and composition were measured in autumn, before the rainy season, inside and outside fenced exclosures in four neighboring topographic sites differing in vegetation characteristics, soil resources and primary productivity: Wadi (dry stream terraces, high productive site), Hilltop, South‐ and North‐facing slopes (less productive sites). 3. Topographic sites differed in seed density (range ca 2500–18000 seed m^?2) and in seed bank response to grazing exclusion. Fencing increased seed density by 78, 51 and 18% in the Wadi, South‐ and North‐facing slopes, respectively, but had no effect in the Hilltop. At the species level, grazing exclusion interacted with site conditions in determining species seed bank density, with larger or opposite changes in the high productive Wadi compared to the other less productive sites. 4. Changes in seed bank structure after grazing exclusion were strongly related to species size traits. Grazing exclusion favored species with large size traits in all sites, while seed density of tiny species decreased strongly in the high productive Wadi. Species with medium and small size traits showed lesser or no responses. 5. The size of plants, dispersal units and seeds were strongly correlated to each other, thus confounding the evaluation of the relative importance of each trait in the response of species to grazing and site conditions. We propose that the relative importance of plant size vs seed size in the response to grazing changes with productivity level.     

Exotic herbivores directly facilitate the exotic grasses they graze: mechanisms for an unexpected positive feedback between invaders

The ability of an exotic species to establish in a system may depend not only on the invasibility of the native community, but also on its interactions with other exotic species. Though examples of mutually beneficial interactions between exotic species are known, few studies have quantified these effects or identified specific mechanisms. We used the co-invasion of an endangered island ecosystem by exotic Canada geese (Branta canadensis) and nine exotic annual grasses to study the effects of an invading herbivore on the success of invading grasses. On our study islands in southwestern Canada, we found that geese fed selectively on the exotic grasses and avoided native forbs. Counter to current theory suggesting that the grasses should be limited by a selective enemy, however, the grasses increased in proportional abundance under grazing whereas forbs showed declining abundance. Testing potential mechanisms for the effects of grazing on grasses, we found that the grasses produced more stems per area when grazing reduced vegetation height and prevented litter accumulation. Forming dense mats of short stems appeared to be an efficient reproductive and competitive strategy that the Eurasian grasses have evolved in the presence of grazers, conferring a competitive advantage in a system where the native species pool has very few annual grasses and no grazers. Germination trials further demonstrated that selective herbivory by geese enables their dispersal of exotic grass seed between heavily invaded feeding areas and the small islands used for nesting. In summary, the exotic geese facilitated both the local increase and the spatial spread of exotic grasses, which in turn provided the majority of their diet. This unexpected case of positive feedback between exotic species suggests that invasion success may depend on the overall differences between the evolutionary histories of the invaders and the evolutionary history of the native community they enter. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Soil phytoliths as evidence for species replacement in grazed rangelands of central Argentina

Rangeland utilization for livestock production in central Argentina has apparently resulted in the replacement of preferred mid-grasses ( Poa ligularis and Stipa clarazii ) by preferred short-grasses ( Piptochaetium napostaense and Stipa tenuis ) and avoided grasses ( Stipa tenuissima and Stipa eriostachya ). However, the botanical composition in the pristine condition is unknown due to the lack of historical data and relict areas. The objective of this study was to analyze soil phytolith assemblages from a representative site of central Argentina rangelands to test the presumed floristic changes. Soil phytolith assemblages (SPAs) were compared with three plant phytolith assemblages (PPAs), each of it made up of two species. One PPA was integrated by the preferred mid-grasses P. ligularis and S. clarazii , another by the preferred short-grasses P. napostaense and S. tenuis , and a third one by the avoided grasses S. eriostachya and S. tenuissima . For each PPA, the proportion of species representing the best fit with SPAs was estimated by considering all possible lineal combinations between the phytolith assemblage of the two integrating species, and selecting that representing the minimum distance to SPA. The highest degree of similarity between SPAs and PPAs corresponded to the PPA integrated by P. ligularis and S. clarazii . Our results support the hypothesis of the dominance of preferred mid-grasses in the pristine condition in rangelands of central Argentina, and that a shift towards the dominance of preferred short-grasses and avoided grasses has occurred in its present disturbed condition.     

Testing the limits of resistance: a 19‐year study of Mediterranean grassland response to grazing regimes

A synthesis of a long‐term (19 years) study assessing the effects of cattle grazing on the structure and composition of a Mediterranean grassland in north‐eastern Israel is presented, with new insights into the response of the vegetation to grazing management and rainfall. We hypothesized that the plant community studied would be resistant to high grazing intensities and rainfall variability considering the combined long history of land‐use and unpredictable climatic conditions where this community evolved. Treatments included manipulations of stocking densities (moderate, heavy, and very heavy) and of grazing regimes (continuous vs. seasonal), in a factorial design. The effect of interannual rainfall variation on the expression of grazing impacts on the plant community was minor. The main effects of grazing on relative cover of plant functional groups were related to early vs. late seasonal grazing. Species diversity and equitability were remarkably stable across all grazing treatments. A reduction in tall grass cover at higher stocking densities was correlated with increased cover of less palatable groups such as annual and perennial thistles, as well as shorter and prostrate groups such as short annual grasses. This long‐term study shows that interannual fluctuations in plant functional group composition could be partly accounted for by grazing pressure and timing, but not by the measured rainfall variables. Grazing affected the dominance of tall annual grasses. However, the persistence of tall grasses and more palatable species over time, despite large differences in grazing pressure and timing, supports the idea that Mediterranean grasslands are highly resistant to prolonged grazing. Indeed, even under the most extreme grazing conditions applied, there were no signs of deterioration or collapse of the ecosystem. This high resistance to grazing intensity and interannual fluctuation in climatic conditions should favor the persistence of the plant community under forecasted increasing unpredictability due to climate change.     

Sawdust Addition Reduces the Productivity of Nitrogen‐Enriched Mountain Grasslands

Anthropogenic nutrient enrichment of mountain grasslands has boosted grasses and fast‐growing unpalatable plants at the expense of slow‐growing species, resulting in a significant loss in biodiversity. A potential tool to reduce nutrient availability and aboveground productivity without destroying the perennial vegetation is carbon (C) addition. However, little is known about its suitability under severe climatic conditions. Here, we report the results of a 3‐year field study assessing the effects of sawdust addition on soil nutrients, aboveground productivity, and vegetational composition of 10 grazed and ungrazed mountain grasslands. Of particular interest was the effect of C addition on grasses and on the tall unpalatable weed Veratrum album. After 3 years, soil pH, ammonium, and plant‐available phosphorus were not altered by sawdust application, and nitrate concentrations were marginally higher in treatment plots. However, the biomass of grasses and forbs (without V. album) was 20–25% lower in sawdust‐amended plots, whereas the biomass of V. album was marginally higher. Sawdust addition reduced the cover of grasses but did not affect evenness, vegetation diversity, or plant species richness, although species richness generally increased with decreasing biomass at our sites. Our results suggest that sawdust addition is a potent tool to reduce within a relatively short time the aboveground productivity and grass cover in both grazed and ungrazed mountain grasslands as long as they are not dominated by tall unpalatable weeds. The technique has the advantage that it preserves the topsoil and the perennial soil seed bank.     

Competition for nitrogen between Australian native grasses and the introduced weed Nassella trichotoma

BACKGROUND: and Aims Nassella trichotoma is an unpalatable perennial grass weed that invades disturbed native grasslands in temperate regions of south-eastern Australia. This experiment investigated whether elevated N levels, often associated with disturbance, increases the competitiveness of N. trichotoma relative to C3 and C4 native Australian grasses. METHODS: A pot experiment investigated competitive interactions between four native grasses, two C3 species (Microlaena stipoides and Austrodanthonia racemosa) and two C4 species (Themeda australis and Bothriochloa macra), and N. trichotoma at three different N levels (equivalent to 0, 60 and 120 kg ha-1) and three competing densities (zero, one and eight neighbouring plants), using an additive design. KEY RESULTS: All native grasses were competitive with N. trichotoma at low N levels, but only M. stipoides was competitive at high N. High densities of native grasses (8:1) had a major competitive effect on N. trichotoma at all N levels. The competitive ranking of native grasses, across all N levels, on N. trichotoma was: M. stipoides>A. racemosa>B. macra>T. australis. The C3 species were generally more competitive than the C4 species and C4 grasses were not inherently more productive at low N levels, in contrast to the results of other studies. CONCLUSION: To resist invasion from N. trichotoma, these native grasses need to be maintained at a high density and/or biomass. The results do not support the theory that species such as N. trichotoma, with high tissues density, are always less competitive than those of low tissue density; in this case competitiveness depended on N levels. The ability of N. trichotoma to accumulate biomass at a higher rate than these native grasses, helps to explain why it is a major weed in disturbed Australian native grasslands.