First-stage invasion of urban and forest habitats by the exotic tree Kalopanax septemlobus (Araliaceae)

Background/Question/Methods

Little research has been done on the earliest stages of exotic plant invasions and yet much may be learned from them. We present observations on a recent (<25 yr) invasion in New Hampshire USA by the exotic tree Kalopanax septemlobus (KS), native to northeastern Asia. Our objective was to assess the viability of the invasion. We tallied, mapped, aged (via wood rings), and measured height and annual growth of all KS individuals encountered in comprehensive censuses and samples of two invaded sites, the University of New Hampshire campus (UC) and Thompson Farm (TF), both in Durham. We evaluated KS shade tolerance by estimating exposure of forest understory plants to light, measured with quantum sensors and canopy photos.

Results/Conclusions

We estimated >3700 plants at UC and censused 270 at TF in a total area of ca. 100 ha. Plants ranged in age from first year seedlings to 22 yrs, and UC plants were as far as 775 m from the purported parent tree. Over 200 plants, including the oldest ones, occurred in understories of white pine – mixed hardwood forest where they withstood light levels down to 5% full sun. Over 25% of understory KS plants added ≥ 30 cm of height annually, and 8% added ≥ 50 cm, comparable to fast-growing natives. While none of the tallied plants (except the purported parent) were as yet reproductive, the large population size, shade tolerance, and rapid height growth rates suggests potential for KS to invade forests in the region. As the entire campus population (and perhaps the Thompson Farm population) likely arose from seed derived from a single tree planted as an ornamental ca. 1980, our results suggest that large propagule size and high propagule diversity may not be required to initiate invasion.