Changes in the Strength of Associations Between Tree Seedlings and Understory Shrubs Along a Regional Drought Gradient in Lebanese Coniferous Forests

Aims

Do the effects of an understory shrub on woody seedlings explain the distributions of relict conifer trees at a regional scale?

Location

Three natural Cedrus libani reserves from Lebanon.

Methods

Using an observational approach, we quantified associations of recruits of woody species at the center and periphery of the spiny shrub Juniperus oxycedrus in forests and gaps along a north–south climatic gradient of both winter rainfall continentality and summer drought. We quantified the associations of woody seedlings with shrubs using the Relative Interactions Index (RII) calculated for recruits of all species together (community scale) and for four groups of functionally different species. We then analyzed the variation of RIIs along different treatments. Vapor Pressure Deficit (VPD) was quantified in each treatment combination, and its relationship with species-group RIIs and abundance was evaluated.

Results

At the community-scale, shrubs showed likely higher negative effects on woody seedlings at their center than at their periphery. In contrast, there were contrasting responses at the species-group level, with overall negative associations for Cedrus, positive associations for a group including Abies cilicica, and intermediate responses for two other groups. Positive associations strongly varied along the drought gradient and with the canopy treatment, while negative associations only slightly increased from forests to gaps. Positive associations varied together with VPD, peaking at an intermediate position along the drought gradient coinciding with the southernmost limit of Abies and declining at the dry extreme of the gradient where Cedrus is still present.

Main Conclusions

VPD appeared as a crucial driver of tree species occurrence and likely facilitation. Although manipulative experiments are needed to confirm the real occurrence of facilitative and competitive effects of the shrub, our study highlights the potential role of facilitation in explaining tree species distribution at regional scales.