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.