On the border: Is emerald ash borer associated with ash trees exposed to higher light intensity?
Understanding the invasion patterns of non-native species is important to protect native communities, as preventing the introduction of invasive species becomes close to impossible in our globally interconnected world. Forest fragmentation could contribute to invasive species success through disrupting environmental conditions, such as light, and increasing the amount of forest edge, yet little empirical work has demonstrated the interaction between forest light regime patterns and pest invasion patterns. We investigated this relationship for the emerald ash borer (EAB, Agrilus planipennis), an invasive phytophagous insect that contributes to ash (Fraxinus spp.) mortality, in a heavily fragmented eastern deciduous forest site. The objectives of this research were to 1) investigate the density of EAB infestation in relation to distance from forest hard edge and 2) examine the relationship between light intensity and distance from forest edge. Large plots (60-m x 5-m) perpendicular to a maintained forest edge were partitioned into smaller subplots (10-m x 5-m), which were characterized for light intensity (canopy closure) and EAB intensity (number of EAB exit holes and ash canopy breakup).
We found no indication that light intensity decreases with distance from the border (F=1.28, p=0.28). However, indicators of EAB intensity showed opposing trends: number of EAB exit holes decreased with distance from the border (F=3.83, p=0.07), while ash canopy decay increased with distance from the border (F=3.11, p=0.10). These findings suggest that EAB invades forests from the edge, while other factors may be contributing to ash decline in the interior. Further investigation in a less fragmented habitat is needed to better understand the underlying drivers to infestation patterns of invasive insects, such as EAB, as well as gaining greater insight into edge effects, which can then be projected to the landscape scale.