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核心所在。然而，尽管诸多研究已阐明直接相互作用沿气候梯度的变化规律，但间接相互作用受到的关注却远为不足。本研究旨在助力更全面地理解气候条件如何调控生物相互作用（biotic interactions）。我们针对法国阿尔卑斯山区的5个树种，探究了成年树木冠层与地面植被对幼苗生长及存活的直接与间接效应。为探究环境条件的影响，本实验在一条与温度密切相关的气候梯度上的10个样地开展。尽管幼苗生长几乎不受直接与间接相互作用的影响，但多个树种的幼苗存活均呈现出显著的变化模式。地面植被在温暖生境中对幼苗存活具有强烈的直接竞争效应；在温度更低的生境中，该竞争效应会逐渐减弱，甚至转变为促进作用。尽管成年冠层效应的置信区间（confidence intervals）更广，但其呈现出相同的变化模式。对微环境条件的监测显示，温暖生境中地面植被的竞争效应可能与水分有效性降低有关；而寒冷生境中成年冠层的微弱促进作用，则与幼苗获得霜冻防护有关。对于不耐寒且耐阴的欧洲山毛榉（Fagus sylvatica）而言，在高温生境中，成年冠层通过抑制地面植被间接促进了幼苗存活。其他更耐寒的树种则未呈现该间接效应，包括瑞士黑松（Pinus uncinata）、欧洲落叶松（Larix decidua）和欧洲冷杉（Abies alba）。我们的研究结果支持了一项被广泛观测到的规律：在生产力更高的气候条件下，直接竞争作用更强。然而对于耐阴树种而言，树木冠层可通过削弱地面植被的竞争来缓冲直接竞争效应，进而使得沿气候梯度的间接相互作用呈现出相反的变化趋势。