Complex biotic interactions mediated by shrubs: Revisiting the stress‐gradient hypothesis and consequences for tree seedling survival,Journal of Applied Ecology

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Complex biotic interactions mediated by shrubs: Revisiting the stress‐gradient hypothesis and consequences for tree seedling survival
Journal of Applied Ecology ( IF 5.0 ) Pub Date : 2020-05-06 , DOI: 10.1111/1365-2664.13641
Arthur Guignabert ₁ , Laurent Augusto ₁ , Maya Gonzalez ₁ , Christophe Chipeaux ₁ , Florian Delerue _{2,

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Affiliation

Using nurse shrubs to improve tree seedling establishment in stressed environments is a common practice in forestry. Recent refinements of the stress‐gradient hypothesis suggest that positive nurse effects occur under intermediate stress and decline in the harshest conditions. Additionally, indirect facilitation (e.g. protection from herbivory) is expected in low‐stressed/productive systems according to the initial stress‐gradient hypothesis. However, there have been fewer investigations into the use of nurse shrubs to decrease herbivore impacts in stressed systems compared with the role of nurses in stress alleviation. This raises the need to clarify conditions in which the use of nurse shrubs would be most appropriate.
We conducted a field experiment in the sand dune forest of southwest France characterized by severe summer water shortages and the presence of different types of wild herbivores. In 2015 and 2016, Pinus pinaster seedlings were planted under shrubs Arbutus unedo or in shrub‐removed locations, in fenced/unfenced areas. Survival, cause of mortality and water stress were monitored for 1 year after plantation.
Summer drought was the main cause of mortality. Water stress increased in both years as summer progressed. We found direct facilitation only in July 2016 at intermediate water stress, due to a decrease in vapour pressure deficit under shrubs. These positive effects declined in late summer when stress was at a maximum. Rodent damage occurred under shrubs in spring whereas ungulate browsing was observed in open areas in autumn/winter. Overall seedling survival was equivalent under or outside shrubs during the first year, but was better under shrubs in the second one.
Synthesis and applications. Our results partially validate the refinement of the stress‐gradient hypothesis, with a decline of positive interactions in the most water‐stressed conditions. This draws attention to the difficulty of using shrubs as nurses to improve tree regeneration in the most xeric systems. In addition, we show that indirect associational effects, both positive and negative, could be as important as abiotic stress alleviation in a stressful ecosystem, and using shrubs to facilitate seedling survival could be efficient depending on the type of herbivore involved. This highlights the fact that considering plant–plant interactions can help in the design of management options, providing that up‐to‐date ecological theories are considered and that the different biotic and abiotic constraints are accurately evaluated.

中文翻译：

灌木介导的复杂生物相互作用：重新审视应力梯度假说及其对树木幼苗存活的影响

在林业中，使用护士灌木改善树木在压力环境下的幼苗生长是一种常见的做法。对压力梯度假设的最新改进表明，积极的护士效应在中等压力下发生，并且在最恶劣的条件下下降。此外，根据最初的压力梯度假说，在低压力/生产系统中，间接的促进作用（例如免受草食的危害）是可以预期的。但是，与护士在缓解压力中的作用相比，使用护士灌木减少对食草动物在压力系统中的影响的研究较少。这就需要弄清使用护士灌木最合适的条件。
我们在法国西南部的沙丘森林中进行了野外试验，其特征是夏季严重缺水并且存在各种类型的野生食草动物。在2015年和2016年，松树樟树幼苗在灌木杨梅下或围栏/无围栏地区的灌木去除地上种植。种植后1年监测成活，死亡原因和水分胁迫。
夏季干旱是造成死亡的主要原因。随着夏季的进行，这两年的水分压力都增加了。我们仅在2016年7月的中等水分胁迫下发现了直接促进作用，这是由于灌木下的蒸气压亏缺减少了。这些积极影响在夏末压力最大时下降。春季在灌木下发生鼠害，而在秋/冬的空旷地区则观察到有蹄类动物的浏览。在第一年中，灌木下或外部的总体幼苗存活率是相同的，而在第二年中，在灌木下则更好。
综合与应用。我们的结果部分验证了应力梯度假说的细化，在大多数缺水情况下正向相互作用的下降。这引起了人们的注意，即在大多数干燥系统中，使用灌木作为护士来改善树木再生的难度。此外，我们表明，在胁迫性生态系统中，间接的正向和负向联合效应与非生物胁迫缓解同等重要，并且根据所涉及的草食动物的类型，使用灌木来促进幼苗存活可能是有效的。这突显了这样一个事实，即考虑到最新的生态学理论并准确评估不同的生物和非生物限制条件，考虑植物之间的相互作用可以帮助设计管理方案。

更新日期：2020-05-06

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