Data from: Are variations of direct and indirect plant interactions along a climatic gradient dependent on species' strategies? An experiment on tree seedlings

Investigating how interactions among plants depend on environmental conditions is key to understand and predict plant communities’ response to climate change. However, while many studies have shown how direct interactions change along climatic gradients, indirect interactions have received far less attention. In this study, we aim at contributing to a more complete understanding of how biotic interactions are modulated by climatic conditions. We investigated both direct and indirect effects of adult tree canopy and ground vegetation on seedling growth and survival in five tree species in the French Alps. To explore the effect of environmental conditions, the experiment was carried out at 10 sites along a climatic gradient closely related to temperature. While seedling growth was little affected by direct and indirect interactions, seedling survival showed significant patterns across multiple species. Ground vegetation had a strong direct competitive effect on seedling survival under warmer conditions. This effect decreased or shifted to facilitation at lower temperatures. While the confidence intervals were wider for the effect of adult canopy, it displayed the same pattern. The monitoring of micro-environmental conditions revealed that competition by ground vegetation in warmer sites could be related to reduced water availability; and weak facilitation by adult canopy in colder sites to protection against frost. For a cold-intolerant and shade-tolerant species (Fagus sylvatica), adult canopy indirectly facilitated seedling survival by suppressing ground vegetation at high temperature sites. The other more cold tolerant species did not show this indirect effect (Pinus uncinata, Larix decidua and Abies alba). Our results support the widely observed pattern of stronger direct competition in more productive climates. However, for shade tolerant species, the effect of direct competition may be buffered by tree canopies reducing the competition of ground vegetation, resulting in an opposite trend for indirect interactions across the climatic gradient.探究植物间相互作用如何依赖环境条件，是理解并预测植物群落对气候变化响应的关键所在。尽管已有诸多研究揭示了直接相互作用（direct interactions）沿气候梯度的变化规律，但间接相互作用（indirect interactions）却长期未受到足够的关注。本研究旨在更全面地阐明气候条件如何调控生物相互作用（biotic interactions）。我们以法国阿尔卑斯山区的5个树种为研究对象，探究了成年树冠与地表植被对幼苗生长及存活的直接与间接效应。为探索环境条件的影响，本实验沿与温度密切相关的气候梯度布设了10个样地开展研究。 尽管幼苗生长受直接与间接相互作用的影响较小，但多个树种的幼苗存活均呈现出显著的变化规律。在温度较高的生境中，地表植被对幼苗存活具有强烈的直接竞争效应；而在温度较低的生境中，该效应会减弱，甚至转变为促进作用。尽管成年树冠效应的置信区间（confidence intervals）更广，但其变化模式与地表植被一致。对微环境条件（micro-environmental conditions）的监测显示，温度较高生境中地表植被的竞争作用可能与水分可利用性降低有关；而温度较低生境中成年树冠的弱促进作用，则与为幼苗提供霜冻防护有关。对于不耐寒且耐阴的欧洲山毛榉（Fagus sylvatica）而言，成年树冠可通过抑制高温生境中的地表植被，间接促进幼苗存活。其余耐寒性更强的树种（瑞士黑松Pinus uncinata、欧洲落叶松Larix decidua以及银冷杉Abies alba）则未呈现出这一间接效应。 本研究结果支持了"在更高产的气候中直接竞争作用更强"这一广泛观测到的规律。然而对于耐阴树种而言，树冠可通过削弱地表植被的竞争来缓冲直接竞争效应，进而使得间接相互作用沿气候梯度呈现出相反的变化趋势。