Dataset for: Patterns of understory invasion in exotic, invasive timber stands

Current climate and land cover change threaten global mountaintops with
increased spread of invasive species. Long-established plantations of
exotic and invasive trees on these mountaintops can alter their
surroundings, further increasing invader-facilitated invasion. Identifying
the ecological conditions that promote such specific associations can help
develop better management interventions.  The Western
Ghats's Shola Sky Islands (>1400m MSL) host vast stretches of
exotic and invasive tree plantations that sustain colonisation of other
invasive woody, herbaceous and fern species in their understories. Here we
analysed vegetation and landscape variables from 232 systematically-placed
plots in randomly selected grids using NMDS and Phi Coefficient
approaches, to examine patterns of association (positive interactions)
between understory invasive species with specific exotic and invasive
overstory species. We also conducted GLMM with zero inflation to determine
the influence of environmental variables where such associations
occur.  We find that understory invasion of multiple species
under the canopy of other exotic invasives is widespread across the Shola
Sky Islands. Stands of Eucalyptus host the colonisation of 70% of
non-native invasive species surveyed across the Shola Sky Islands. In
particular, Lantana camara invasion is strongly associated with Eucalyptus
stands.  We also found that climatic variables affect the
colonisation of understorey woody invasive species, while invasion by
exotic herbaceous species is associated with the density of road networks.
Canopy cover impacts all invasives negatively, while incidence of fire was
negatively associated with invasion by Lantana spp and the Pteridium spp.
 While the restoration of natural habitats largely targets the
highly invasive Acacia, less invasive Eucalyptus and Pine are often not
included. Our study suggests that retaining such exotic species in natural
habitats, particularly protected areas, can hinder ongoing restoration
efforts by facilitating further invasions by multiple woody and herbaceous
species.