Current conceptual models predict that an increase in stress shifts interactions between plants from competitive to facilitative; hence, facilitation is expected to gain in ecological importance with increasing stress. Little is known about how facilitative interactions between plants change with increasing biotic stress, such as that incurred by consumer pressure or herbivory (i.e. disturbance sensu Grime). In grazed ecosystems, the presence of unpalatable plants is reported to protect tree saplings against cattle grazing and enhance tree establishment. In accordance with current conceptual facilitation-stress models, we hypothesised a positive relationship between facilitation and grazing pressure. We tested this hypothesis in a Weld experiment in which tree saplings of four diVerent species (deciduous Fagus sylvatica, Acer pseudoplatanus and coniferous Abies alba, Picea abies) were planted either inside or outside of the canopy of the spiny nurse shrub Rosa rubiginosa in enclosures diVering in grazing pressure (low and high) and in exclosures. During one grazing season we followed the survival of the diVerent tree saplings and the level of browsing on these; we also estimated browsing damage to the nurse shrubs. Shrub damage was highest at the higher grazing pressure. Correspondingly, browsing increased and survival decreased in saplings located inside the canopy of the shrubs at the high grazing pressure compared to the low grazing pressure. Saplings of both deciduous species showed a higher survival than the evergreens, while sapling browsing did not diVer between species. The relative facilitation of sapling browsing and sapling survival -i.e. the diVerence between saplings inside and outside the shrub canopy -decreased at high grazing pressure as the facilitative species became less protective. Interestingly, these Wndings do not agree with current conceptual facilitationstress models predicting increasing facilitation with abiotic stress. We used our results to design a conceptual model of facilitation along a biotic environmental gradient. Empirical studies are needed to test the applicability of this model. In conclusion, we suggest that current conceptual facilitation models should at least consider the possibility of decreasing facilitation at high levels of stress.

Summary1. According to the insurance hypothesis, more diverse plant communities are more likely to be resistant to drought. Whilst many experiments have been carried out to determine the effects of plant diversity on plant community insurance, the results are still contradictory. 2. Here, we conducted a drought experiment where we tested whether the presence of subordinate species increases plant community insurance. In Swiss Jura grassland, we combined a removal experiment of subordinate species with a summer drought event using rainout shelters. 3. Plant community composition was determined after the drought and based on biomass measurements; we estimated resistance, recovery and resilience of the plant community for each combination of treatments. Moreover, to assess drought impacts on water-use efficiency (WUE), we analysed carbon isotope ratios (d 13 C values) in plant leaves of two dominants and two subordinates collected at the end of the drought period. 4. We showed that subordinate species are more resistant to drought and increased community resistance by enhancing their above-ground biomass production during the imposed drought. These patterns were associated with decreased competitiveness of dominant species whose biomass decreased during drought. Significant increase in d 13 C values in plant tissue under drought indicated a better WUE for the measured species. Interestingly, the WUE was significantly higher in plots where subordinates were removed. Recovery and resilience were not affected by the summer drought, but the absence of subordinates reduced overall above-ground biomass in both watered and drought plots. 5. Synthesis. We demonstrated that, independent of plant diversity, the presence of drought-resistant subordinate species increases plant community insurance against drought and, hence, is important for the functioning of grassland ecosystems.

Summary1. Light-demanding trees and thorny shrubs in temperate plant communities may reflect adaptations to now-extinct large grazers, such as aurochs and tarpans, rendering these adaptations ecological anachronisms. 2. We explored the ecological functions of plant traits of Quercus robur and Prunus spinosa in areas grazed by cattle and horses, the domesticated descendants of aurochs and tarpans. Specifically, we tested the hypothesis that grazing induces a shifting mosaic of grassland, shrub thickets and woodlands through the key process of associational resistance: the protection of palatable young trees by thorny shrubs.3. An exclosure experiment with transplanted Q. robur seedlings revealed that Q. robur grew best in grassland exclosures and on the edge of thorny shrub thickets, which may be viewed as an optimal balance between sufficient protection from large herbivores and sufficient light availability. 4. A cross-site comparison of four floodplain woodlands in north-western Europe showed that Q. robur can regenerate in the presence of large herbivores through spatial association with P. spinosa . However, we found that expansion of P. spinosa shrubs and Q. robur coincided with periods of low rabbit abundance and not with livestock density. From this, it appears that the process of associational resistance does not work with rabbits. 5. Synthesis and applications . With extensive grazing by large (domesticated) grazers in temperate floodplains, a shifting mosaic of grassland, shrubs and trees may develop that has high conservation value. Palatable, light-demanding Q. robur seedlings can successfully regenerate in spiny P. spinosa shrubs through associational resistance. This process does not offer protection from abundant small herbivores, such as rabbits, that can inhibit the recruitment of shrubs and trees in this mosaic vegetation. In floodplain meadows frequent flooding may be an efficient way to reduce rabbit populations, with dry conditions in summer and wet in winter. When floodplain meadows are combined with adjacent higher grounds, large herbivores can escape the floods through migration.

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Browsing by livestock has been identified as an important factor preventing tree regeneration in wooded pastures. Two field experiments were performed to investigate the effects of cattle browsing on tree sapling growth in a mountainwooded pasture. Two size classes (ca. 12-17 cm and 41-59 cm) of each of 4 species (Picea abies, Abies alba, Acer pseudoplatanus and Fagus sylvatica) were exposed to zero, low and high grazing intensities. We measured the proportion of saplings browsed and the effect of browsing on growth. A mowing treatment within the zero grazing intensity treatment showed no significant effect on sapling growth. One percent of the large saplings but 25% of the small saplings escaped browsing. Saplings were better protected when surrounded by taller vegetation. The proportion of saplings browsed was not significantly different among species although evergreen tree saplings lost a larger proportion of biomass than deciduous species when browsed. Under grazing, average current-year shoot production and total aboveground biomass of all species were significantly reduced. Browsing effects tended to be smaller at the lower grazing intensity. Because the deciduous species were least reduced in aboveground growth, especially under the low grazing intensity, we conclude that at least in short-term, deciduous species are less affected by cattle browsing.

International audienceCattle activity greatly influences plant species composition and biomass production of grassland ecosystems. Dung deposition by cattle together with grazing and trampling can be considered as one of the important factors driving vegetation dynamics in pastures. The objective of this study was to investigate at 10-cm and 1-month resolution the plant community dynamics induced by dung deposition in two plant communities (a mesotrophic and an oligotrophic grassland) in a pasture of the Swiss Jura Mountains. Vegetation was sampled four or three times during the vegetation period in contiguous 10 cm x 10 cm quadrats from the centre of the dung pat to a distance of 60 cm. A lower grazing intensity near the dung pat was recorded for all observation periods. In the mesotrophic grassland the canopy was higher near the dung pat already one week after dung deposition. Vegetation around dung pats was submitted to two opposite fertilizing and grazing gradients, which induced changes in vegetation texture and structure at fine scale and short term. We observed a positive rank correlation between species turnover and distance to the dung for both communities, suggesting a seasonal stabilizing effect of dung on the plant composition of their direct surroundings (0–10 cm) likely due to cattle avoidance. Since dung pats are dropped every year in different locations, they create in the pasture a shifting mosaic of nutrient availability and grazing intensity inducing at seasonal scale microsuccessions in plant communities

Facilitation of tree regeneration by nurse shrubs that offer protection against large herbivores is an important driver of wood-pasture dynamics. Here we asked whether the response to facilitation by nurse shrubs depends on the grazing resistance of the prote´ge´saplings. We experimentally tested the protective effects of the thorny Rosa rubiginosa on browsing frequency, survival, and biomass change of saplings of two species-groups, presumably differing in grazing resistance: the coniferous Abies alba and Picea abies and the deciduous Acer pseudoplatanus and Fagus sylvatica saplings. The saplings were planted under and outside (1.5 m) planted nurse shrubs, under zero, low and high grazing intensity. In total, 1920 young saplings were transplanted to 60 blocks and followed for 1 year.Although the number of saplings browsed did not differ between species-groups, the coniferous saplings showed lower resistance to cattle browsing (i.e. lower survival and growth rates) than the deciduous saplings. The less resistant coniferous saplings benefited significantly more from nurse shrubs than the more resistant deciduous species in terms of growth of the surviving saplings, but not in terms of overall survival. This was likely due to herbivory on the nurse shrubs causing incidental browsing on prote´ge´saplings and differences in biomass off-take. At high grazing intensity facilitative effects of the nurse shrubs decreased, especially for the coniferous species.These results have important management implications for the endangered wood-pastures in Western Europe. For a sustainable management and conservation that allows tree recruitment, grazing intensity should remain low to best promote facilitation processes for all tree species, but in particular for the less resistant conifer saplings. die Fo¨rderung durch Ammengebu¨sch von der Beweidungsresistenz der geschu¨tzten Scho¨sslinge abha¨ngt. Wir untersuchten den schu¨tzenden Effekt der dornigen Rosa rubiginosa auf die Beweidungsfrequenz, die Ü berlebensrate und die Vera¨nderungen in der Biomasse fu¨r Scho¨sslinge aus zwei Artengruppen experimentell, von denen wir annehmen, dass sie sich in der Beweidungsresistenz unterscheiden: die Nadelba¨ume Abies alba und Picea abies und die Laubba¨ume Acer pseudoplatanus und Fagus sylvatica. Die Scho¨sslinge wurden unter die und außerhalb (1,5 m) der gepflanzten Ammengebu¨sche bei fehlender Beweidung, geringer und hoher Beweidungsintensita¨t gepflanzt. Insgesamt wurden 1920 junge Scho¨sslinge in 60 Blo¨cken verpflanzt und u¨ber ein Jahr beobachtet. Obwohl sich die Anzahl der beweideten Scho¨sslinge nicht zwischen den Artengruppen unterschied, zeigten die Nadelbaumscho¨sslinge eine geringere Resistenz gegenu¨ber der Rinderbeweidung (d. h. geringere Ü berlebens-und Wachstumsraten) als die Laubbaumscho¨sslinge. Die weniger resistenten Nadelbaumscho¨sslinge profitierten signifikant mehr von den Ammengebu¨schen als die resistenteren Laubbaumscho¨sslinge in Bezug auf das Wachstum der u¨berlebenden Scho¨sslinge, aber nicht in Bezug ...

Question:What is the effect of gap size on the seedling emergence, growth and survival of four common tree species in wooded pastures? Location: A pasture in the Jura mountains, Switzerland. Methods: Seeds were sown in a complete three-way factorial design with eight blocks in May 2003. Each block consisted of a competition treatment (four gap sizes including zero) and a mowing treatment (mown and unmown). Emergence, survival and total biomass of tree seedlings of three species (Picea abies, Acer pseudoplatanus and Fagus sylvatica) were measured. A fourth species (Abies alba) failed to germinate. Results: Gaps had a positive influence on the early stages of tree development for all species. Larger gaps favoured growth and survival more than small gaps. Seedling growth was higher when vegetation around the openings was mown. Mowing the vegetation at gap size zero enhanced both growth and survival compared to unmown vegetation. Mown gaps larger than zero had increased seedling desiccation but decreased seedling predation. Species showed similar trends in their emergence and growth responses to gap size and mowing treatments but for Picea emergence rate was higher and survivorship was lower than for Acer and Fagus. Conclusions: Gap size does matter for tree seedling success but even in more favourable large gaps only a small percentage of seedlings emerged and survived. The effects of gaps on tree seedling establishment are complex as a result of interactions between biotic and abiotic changes caused by gaps.