Conceptual models of how interactions with native species influence invasions emphasize competition, but recent evidence suggests facilitation can promote invasion in stressful environments. However, how nurse-plants with contrasting growth-forms and distribution interact with invaders remains unexplored, although it could offer insights on nurse/exotic interaction mechanisms. We asked whether shrub and cushion nurses differed in their effects on the exotic Rumex acetosella in sites at four elevations in the high tropical Andes (4100–4400 m), shrubs dominating the lowest sites and cushions the highest sites. During the dry season, we measured soil organic matter (SOM) and water content (SWC) under the shrub Hypericum laricifolium, the cushion Azorella julianii, and adjacent areas outside. We compared Rumex’s performance under each situation, measuring midday leaf temperatures (T_leaf), vapor pressure deficit (VPD), minimum water potentials (Ψ_min) and leaf nitrogen (N_leaf) and compared the number, size and proportion of fruiting ramets within sampling rings in each situation. SOM and SWC were higher at all elevations under cushions, then under shrubs and lower outside. Rumex’s density was generally reduced under shrubs but increased on cushions. However, both nurses had positive effects along the gradient on Rumex’s size, reproduction, water balance and N_leaf, shrubs having stronger effects on T_leaf and VPD and cushions on N_leaf. Our results indicate that alternating nurses influenced an invader’s physiological performance to different extents via contrasting effects on shading and soil resources, leading to mixed competitive/facilitative effects of shrubs on the exotic’s demography, while cushions had more consistent facilitative effects across elevations.

中文翻译：

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沿着热带高山海拔梯度，护士植物与外来入侵者之间的相互作用：生长形式很重要

与本地物种的相互作用如何影响入侵的概念模型强调了竞争，但最近的证据表明，便利化可以促进压力环境中的入侵。但是，尽管生长方式和分布不同的护士植物如何与入侵者相互作用，但仍可以提供关于护士/异国相互作用机制的见解，目前尚待探索。我们询问灌木和垫层护士对热带高海拔安第斯山脉（4100–4400 m）四个海拔处的外来酸模酸小球藻的影响是否不同，灌木在最低位点占主导地位，在最高位点起垫作用。在干旱季节，我们测量了灌木金丝桃（Hypericum laricifolium），坐垫（Azorella julianii），土壤中的土壤有机质（SOM）和水分含量（SWC），和外面的邻近区域。我们比较羊蹄每一种情况下的性能，测量正午叶温度（Ť_叶），水汽压亏（VPD），最小水势（Ψ_分钟）和叶片氮（Ñ_叶）和比较的数量，大小和结果的比例每种情况下采样环内的分母。在所有高度下，垫层下的SOM和SWC都较高，然后是灌木下和较低的外部。羊蹄“小号密度一般在灌木减少，但在垫子增加。但是，两位护士都对Rumex的尺寸，繁殖力，水分平衡和N_叶片，灌木对T_叶片和VPD的作用较强，对N_叶片的缓冲作用。我们的结果表明，交替护士通过对阴影和土壤资源的对比影响在不同程度上影响入侵者的生理表现，从而导致灌木对异国人口的混合竞争/促进作用，而靠垫在整个海拔上具有更一致的促进作用。