In many parts of the world, invasions of woody plants into grassland communities have been attributed to an increase in grazing pressure, which is hypothesized to reduce competitive interactions between grasses and woody seedlings. However, few experimental studies have attempted to determine the importance of the effects of grass clipping (= simulated grazing) on biotic interactions between plant species in the field. In this study we tested the hypothesis that the establishment of Pinus sylvestris and Pinus nigra seedlings would vary with grass clipping and also soil nutrient level, owing to changes in the nature and intensity of biotic interactions. For both Pinus species, we found contrasting effects of the herbaceous cover on seedlings according to the life stage and the nutrient level. The net effects of the herbaceous cover on seedling recruitment rate switched from facilitation to competition with soil fertilization. The soil nutrient level also modified the impact of grass clipping on pine seedlings. On fertilized soils, clipping decreased competition intensity and increased the recruitment of pine seedlings. On non-fertilized soils, clipping decreased Pinus sylvestris recruitment but did not affect Pinus nigra recruitment. In the study area, under natural conditions, the herbaceous cover had a net positive effect on seedling recruitment owing to higher emergence. Grass clipping should not favour pine invasion in the absence of fertilization, but moderate grazing could favour the recruitment of Pinus nigra compared to Pinus sylvestris. Although an exotic species, Pinus nigra had a wider range of favourable conditions for recruitment than Pinus sylvestris. The effects of grazing on woody plant invasion are expected to differ strongly according to grazing intensity, the abiotic conditions, and the target species.
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Vol. 12 • No. 2