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.

探究植物间的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。然而，尽管诸多研究已阐明直接相互作用沿气候梯度（climatic gradient）的变化规律，间接相互作用却尚未得到足够关注。本研究旨在更全面地解析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的5个树种，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，实验沿一条与温度密切相关的气候梯度设置了10个样地开展。
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空间格局。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；该效应在温度较低的生境中会减弱，甚至转变为促进作用。虽然成年树冠效应的置信区间（confidence interval）更宽，但其呈现出相同的变化模式。对微环境条件（micro-environmental condition）的监测显示，温度较高样地中地表植被的竞争效应可能与水分有效性（water availability）降低有关；而寒冷生境中成年树冠的微弱促进作用，则与幼苗免受霜冻胁迫相关。对于一个不耐寒但耐阴的树种（欧洲山毛榉 *Fagus sylvatica*），成年树冠通过在高温样地抑制地表植被，间接促进了幼苗存活。其余耐寒性更强的树种（瑞士黑松 *Pinus uncinata*、欧洲落叶松 *Larix decidua* 与欧洲冷杉 *Abies alba*）则未表现出这一间接效应。
我们的研究结果支持了“在更具生产力的气候中直接竞争更强”这一广泛观测到的格局。然而，对于耐阴树种而言，树冠层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。