Data from: Fine nurse variations explain discrepancies in the stress-interaction relationship in alpine regions

Despite a large consensus on increasing facilitation among plants with increasing stress in alpine regions, a number of different outcomes of interaction have been observed, which impedes the generalisation of the ‘stress-gradient hypothesis’ (SGH). With the aim to reconcile the different viewpoints on the stress-interaction relationship in alpine environments we hypothesized that fine nurse variations within a single life form (cushion) may explain this pattern variability To test this hypothesis, we compared the magnitude of the stress-interaction relationship in a single study area with that observed in existing studies involving cushions, worldwide. We characterized the nurse effects of cushions on the whole plant community at inter-specific, intra-specific and intra-individual levels along a stress gradient in the dry, alpine tropics of Bolivia (4400 m, 4700 m and 4900 m a.s.l). Using a relative index of interaction (RII) we included our data in a meta-analysis on the nurse effects of cushions along alpine gradients, worldwide. At inter-specific level, the loose cushion Pycnophyllum was a better nurse than the compact Azorella compacta. However, at intra-individual level facilitation was higher at the periphery than at the centre of cushions, exceeding in magnitude the variation observed at inter-specific level. This pattern was associated with higher minimum temperature and lower mortality at the periphery of cushions. The net effects of cushions on plant communities became more positive at higher elevation, corroborating the SGH. Within our single site in Bolivia, fine morphological nurse variations captured a similar variability in the stress-interaction relationship as that observed in a subset of studies on cushions on a worldwide scale. This suggests that fine variations in nurse traits, in general those not considered in protocols dealing with facilitation or in restoration/conservation management plans, explain in part the current discrepancies among SGH studies in alpine regions.

尽管学界普遍认为高寒地区植物间的促进作用会随胁迫程度升高而增强，但已有研究观测到多种不同的交互作用结果，这阻碍了“胁迫梯度假说（stress-gradient hypothesis, SGH）”的普适性推广。为调和高寒环境中胁迫与交互作用关系的不同观点，我们提出假说：单一生活型（垫状植物，cushion）内部的细微保育性状差异，或许可以解释上述模式的变异性。为验证该假说，我们将单一研究区域内的胁迫-交互作用关系强度，与全球范围内已发表的垫状植物相关研究结果进行了对比。我们在玻利维亚的干旱热带高山区域（海拔4400米、4700米与4900米 a.s.l.）的胁迫梯度上，从种间（inter-specific）、种内（intra-specific）以及个体内（intra-individual）三个层面，表征了垫状植物对整个植物群落的保育效应。我们采用相对交互作用指数（Relative Interaction Index, RII），将本研究数据纳入全球范围内沿高山梯度的垫状植物保育效应元分析（meta-analysis）中。在种间层面，松散型垫状植物Pycnophyllum的保育效应优于致密型的Azorella compacta。但在个体内层面，垫状植物外围的促进作用强于其中心区域，且该差异的幅度超过了种间层面的观测变异。该模式与垫状植物外围更高的最低温度以及更低的死亡率相关。垫状植物对植物群落的净效应随海拔升高愈发偏向正向，印证了胁迫梯度假说。在玻利维亚的单一研究站点内，细微的保育性状差异所带来的影响，与全球尺度下垫状植物相关研究子集所观测到的胁迫-交互作用关系变异性一致。这表明，保育性状的细微差异——通常是那些在促进作用研究方案或恢复/保护管理计划中未被纳入考量的差异——在一定程度上解释了当前高寒地区胁迫梯度假说研究间的分歧。