Stratified dispersion: tracking long-distance dispersal events in the forest understory

Background/Question/Methods: Understanding the dispersal mechanisms of invasive species is the key to predicting the speed and potential of their spread. Although most propagules disperse short distances, rare events and secondary dispersal can transport some great distances and strongly affect spread rates. The hemlock woolly adelgid Adelges tsugae is a passively-dispersed invasive hemipteran that infests and kills eastern hemlock (Tsuga canadensis) in the northeastern United States. Because of the adelgid’s small size and numerous possible source populations, it is difficult to identify the ‘true’ source population of dispersing A. tsugae and to model its true dispersal dynamics. We experimentally simulated adelgid dispersal by dusting mature hemlock trees with fluorescent powders made up of particles of similar size to the passively-dispersed crawler phase of the adelgid. We created three ‘source’ populations using different-colored powders and controlled propagule pressure by manipulating the amount of powder applied to each tree. We tracked particle movement using sticky cards located from 10 to 400 m away from the source trees. Powder dispersal onto each card was quantified by photographing cards in ultraviolet light and using image analysis software to assess the color and count the number of fluorescing pixels.

Results/Conclusions: Powder density was highest on sticky cards within 10-25 m of each source tree, and dropped off sharply (~83% lower) on traps from 33-100 m away. Powder from each tree was detected on the most distant cards (470 m) from each of the source trees. Insects facilitated the movement of powder throughout the trapping array, and powder-coated individuals were primarily responsible for powder movement from 200-400 m. This suggests that although wind is primarily responsible for particle dispersal throughout a forest understory, insects may also play an important role over longer distances. Although the powder density on each card tapered off with distance, its ability to travel upwards of 400 meters in the forest understory implies that wind-dispersed A. tsugae crawlers can spread rapidly even in closed-canopy settings.