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首页> 外文期刊>Oecologia >Nurse plants, tree saplings and grazing pressure: changes in facilitation along a biotic environmental gradient
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Nurse plants, tree saplings and grazing pressure: changes in facilitation along a biotic environmental gradient

机译：护士植物，树苗和放牧压力：沿生物环境梯度的便利变化

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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 field experiment in which tree saplings of four different 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 differing in grazing pressure (low and high) and in exclosures. During one grazing season we followed the survival of the different 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 differ between species. The relative facilitation of sapling browsing and sapling survival - i.e. the difference between saplings inside and outside the shrub canopy - decreased at high grazing pressure as the facilitative species became less protective. Interestingly, these findings do not agree with current conceptual facilitation-stress 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.
机译：当前的概念模型预测，压力的增加会使植物之间的相互作用从竞争性转变为促进性。因此，随着压力的增加，便利化在生态上的重要性有望提高。人们对植物之间的促进性相互作用如何随着生物胁迫的增加而变化知之甚少，例如由于消费者压力或食草引起的干扰（即sensen Grime干扰）。据报道，在放牧的生态系统中，难吃的植物的存在可保护树木的树苗免于放牧，并增强树木的生长。根据当前概念上的便利压力模型，我们假设便利与放牧压力之间存在正相关关系。我们在一项田间试验中验证了这一假设，在该试验中，将四个不同种类的树苗（落叶青豆，枫树针叶树和针叶树冷杉，云杉冷杉）种植在带刺护士灌木罗莎红花的冠层内部或外部。放牧压力（低和高）和排泄物。在一个放牧季节中，我们跟踪了不同树苗的存活情况以及在这些树苗上浏览的水平。我们还估计了浏览对护士灌木丛的破坏。在较高的放牧压力下，灌木丛的伤害最高。相应地，与低放牧压力相比，在高放牧压力下位于灌木冠层内部的树苗的浏览量增加而存活率降低。两种落叶树种的树苗均比常绿树具有更高的存活率，而树种之间的树苗浏览没有差异。幼树浏览和幼树生存的相对便利性-即灌木冠层内部和外部的幼树之间的差异-在高放牧压力下减少，因为便利性物种的保护性降低。有趣的是，这些发现与当前的概念性促进压力模型不一致，该模型预测非生物性压力会增加促进作用。我们使用我们的结果设计了沿着生物环境梯度进行促进的概念模型。需要进行实证研究以检验该模型的适用性。总之，我们建议当前的概念性促进模型至少应考虑在高压力水平下降低便利性的可能性。

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