Data from: Foliar damage beyond species distributions is partly explained by distance dependent interactions with natural enemies

Plant distributions are expected to shift in response to climate change,
and range expansion dynamics will be shaped by the performance of
individuals at the colonizing front. These plants will encounter new
biotic communities beyond their range edges, and the net outcome of these
encounters could profoundly affect colonization success. However, little
is known about how biotic interactions vary across range edges and this
has hindered efforts to predict changes in species distributions in
response to climate change. In contrast, a rich literature documents how
biotic interactions within species ranges vary according to distance to
and density of conspecific individuals. Here, we test whether this
framework can be extended to explain how biotic interactions differ beyond
range edges, where conspecific adults are basically absent. To do so, we
planted seven species of trees along a 450 km latitudinal gradient that
crossed the current distributional range of five of these species and
monitored foliar disease and invertebrate herbivory over five years.
Foliar disease and herbivory were analyzed as a function of distance to
and density of conspecific and congeneric trees at several spatial scales.
We found that within species ranges foliar disease was lower for seedlings
that were farther from conspecific adults for Acer rubrum, Carya glabra,
Quercus alba, and Robinia pseudoacacia. Beyond range edges there was even
less foliar disease for C. glabra, Q. alba and R. pseudoacacia (A. rubrum
was not planted outside its range). Liriodendron tulipifera did not
experience reduced disease within or beyond its range. In contrast,
Quercus velutina displayed significant but idiosyncratic patterns in
disease at varying distances from conspecifics. Patterns of distance
dependent herbivory across spatial scales was generally weak and in some
cases negative (i.e., seedlings further from conspecific adults had more
herbivory). Overall, we conclude that differences in biotic interactions
across range edges can be thought of as a spatial extension to the concept
of distance dependent biotic interactions. This framework also provides
the basis for general predictions of how distance dependent biotic
interactions will change across range edges in other systems.