Disentangling short‐ and long‐term neighbour effects, using both removal and observational methods within a single experiment, has strongly improved our understanding of the driving mechanisms of plant–plant interactions. However, there has been no attempt to assess two important underlying processes of their changes along gradients, either environmental‐severity (changes in target performance without neighbours) or neighbour‐traits (changes in performance with neighbours) effects, the former previously shown in alpine communities to be involved in competition and the latter in facilitation. We addressed this goal in an experiment conducted in continental saline depressions (sebkhas) from the Mediterranean arid climate of central Tunisia. We quantified short‐ and long‐term effects of dominant shrubs, transplanting three target grass species in open, nurse and removed‐nurse microhabitats of two habitats of different salinity levels in height sebkhas. The design extended greographically from central Tunisia to the Libyan border, 500 km southeastward. We used the relative interaction index to calculate short‐ and long‐term effects before and after the dry summer seasons and environmental‐severity and neighbour‐trait effects. Short‐term effects were slightly negative and long‐term effects strongly positive before the dry summer season in the two habitats. Short‐term effects switched to positive with increasing drought stress, due to an environmental‐severity effect, whereas long‐term effects decreased due to a neighbour‐trait effect. Salinity did not affect neither short‐ nor long‐term shrub effects. Soil moisture measurements showed that both changes were due to vanishing shrub soil engineering‐effects during the summer drought. We conclude that an increase in short‐term facilitation with increasing drought stress through time, apparently supporting the stress gradient hypothesis, might be due to a decrease in long‐term facilitation. Thus, we recommend using, as much as possible, both the removal and observational methods in experiments assessing changes in plant–plant interactions along stress gradients to avoid wrong conclusions.

中文翻译：

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从短期竞争向干旱压力促进的转变是由于长期促进的减少

在单个实验中同时使用去除和观察方法，可以消除短期和长期邻居效应，从而极大地增进了我们对植物-植物相互作用的驱动机制的理解。但是，没有尝试评估其沿梯度变化的两个重要的潜在过程，即环境严重程度（无邻国时目标绩效的变化）或邻域特征（有邻国时绩效的变化）影响，前者先前在高山地区显示参与竞争的社区，后者参与便利化。我们在来自突尼斯中部地中海干旱气候的大陆盐洼（sebkhas）中进行的一项实验中实现了这一目标。我们量化了优势灌木的短期和长期影响，在海拔高的塞布哈斯州两个盐度不同的生境中，在开放，哺乳和裸露的微生境中移植三种目标草种。该设计在地理上从突尼斯中部延伸到东南500公里的利比亚边界。我们使用相对相互作用指数来计算夏季干旱前后的短期和长期影响，以及环境严重程度和邻居性状影响。在两个生境的夏季干燥之前，短期影响是轻微的负面影响，长期影响是强烈的正面影响。由于对环境的严重影响，随着干旱压力的增加，短期影响转为正，而由于邻居性状影响，长期影响下降。盐度不影响短期或长期灌木效应。土壤水分测量结果表明，这两种变化都是由于夏季干旱期间灌木土壤工程效应的消失所致。我们得出的结论是，随着时间的推移，干旱压力随时间的推移而增加，短期促进作用的增加，显然支持了应力梯度假设，这可能是由于长期促进作用的减少。因此，我们建议在实验中尽可能使用去除和观察方法，以评估植物-植物相互作用沿应力梯度的变化，以避免错误的结论。