Large-scale facilitative effects of Vachellia caven

The importance of nurse plants structuring plant communities is well
appreciated at local scales, yet the effect of a single nurse on large
scales has been neglected in analyses. So far, studies only use
environmental gradients within one type of ecosystem and tend to
generalize the nurse effects. To assess how the effect of a single nurse
species is modulated by different environmental settings, interactions
between the shrub Vachellia caven and the surrounding plant communities
were evaluated at 481 paired plots (outside vs underneath the plant
crown), in 39 sites across two distribution ranges, the Mediterranean west
and the mostly subtropical east of the Andes mountains (covering ca. 2x10
6 km2). Cover, abundance, and richness of perennial plants underneath and
outside V. caven were used as response variables to estimate an index
indicative of plant interactions (RII) and tested how this was affected by
the rainfall gradient and distribution range. Overall, RII responses to
rainfall gradients had low conditional R2 (~0.25) at this large-scale of
analysis, but were significantly different between ranges: the RII
followed a quadratic trend across the rainfall gradient in the western
range, while this relationship was positive and close to linear at the
eastern range. Then, by projecting the RII models (i.e., for abundance,
cover, and richness) spatially through a consensus map, we show that most
positive effects of V. caven are geographically found in dissimilar areas:
the central part of Chile (western range) and across the Paraná river
(eastern range). When local fine-scale predictors (i.e., annual herbs
cover and height, and herbivores feces cover), were used to model each
response variable at the plot level (underneath or outside V. caven)
similar trends were kept as when considering only large-scale predictors.
Synthesis: Here, we show that the effect of the same nurse species on
neighbouring plant communities can be very different depending on ranges
of distribution, stressing that its ecological function cannot be
generalized and not only depends on local factors but also is large-scale
context-dependent.