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.

探究植物间的交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，尽管已有诸多研究揭示了直接交互沿气候梯度（climatic gradient）的变化规律，间接交互却鲜少受到关注。本研究旨在更全面地理解气候条件如何调控生物交互（biotic interactions）。我们针对法国阿尔卑斯山区的5个树种，探究了成年树冠（adult tree canopy）与地面植被（ground vegetation）对幼苗生长（seedling growth）及存活率（survival）的直接与间接效应。为探究环境条件的影响，本实验在与温度密切相关的气候梯度（climatic gradient）上的10个样地开展。

尽管幼苗生长受直接与间接交互的影响较小，但多个树种的幼苗存活率均呈现出显著的变化规律。在温度较高的生境中，地面植被对幼苗存活率具有强烈的直接竞争效应。该效应随温度降低而减弱，甚至在低温生境中转变为促进作用（facilitation）。尽管成年树冠效应的置信区间（confidence intervals）更宽，但其也呈现出相同的变化模式。微环境条件（micro-environmental conditions）监测结果显示，温度较高样地的地面植被竞争效应可能与水分可利用性（water availability）降低有关；而低温生境中成年树冠的微弱促进作用，则与抵御霜冻有关。对于不耐寒且耐阴的欧洲山毛榉（Fagus sylvatica）而言，在高温样地中，成年树冠通过抑制地面植被间接促进了幼苗存活率。其余更为耐寒的树种（Pinus uncinata、Larix decidua及Abies alba）则未呈现出这一间接效应。

本研究结果支持了学界广泛观测到的规律：在生产力更高的气候条件下，直接竞争效应更强。然而，对于耐阴树种而言，树冠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交互呈现出相反的变化趋势。