Incipient Invasion of Urban and Forest Habitats in New Hampshire, USA, by the Nonnative Tree, Kalopanax septemlobus

We analyzed the recent (< 25 yr) spread in New Hampshire, USA, of the exotic tree Kalopanax septemlobus, native to Asia. The invasion was likely initiated by a single tree planted ca. 1972. Our objective was to assess the viability of the invasion, especially in light of the small propagule size. We tallied, mapped, aged, and measured the height and growth of K. septemlobus individuals at two sites, the University of New Hampshire campus (UC) and Thompson Farm (TF), both in Durham. We found over 3,800 plants at UC and 270 at TF in < 120 ha (296 ac) total area. Plant age ranged from 0 to 22 yr, and UC plants were as far as 775 m (2,543 ft) from the purported parent tree. Annual height growth was comparable to midtolerant native trees. Plants occurred in both open and forested habitats, and the mean level of photosynthetically active radiation incident on understory plants was 4 to 6% of full sun. The large population size, shade tolerance, rapid height growth, and ability to sprout from damaged stems suggest potential for K. septemlobus to invade and persist in forests, the most common natural ecosystem in the northeastern United States. We further suggest that small propagule size, likely a single tree, has not prevented K. septemlobus from initiating a spatially extensive and vigorous population. Kalopanax septemlobus has been planted as an ornamental in the northeastern United States, and prevention of region-wide invasion might depend on removal of these trees, even when they occur as single individuals. Nomenclature: Castor-aralia, Kalopanax septemlobus (Thunb.) Koidz. Management Implications: Castor-aralia (Kalopanax septemlobus) is capable of spreading via seed from planted individuals to urban and natural forest landscapes in southeastern New Hampshire. The invaded forest types, eastern white pine and sugar maple–beech–red oak, are common in the northeastern United States. Although the impact of castor-aralia on North American communities and ecosystem processes is not yet known, colonization of forest understories and rapid height growth there suggest it is capable of competing with economically important native plants. At present, early detection coupled with rapid removal is a sensible strategy. Because a single castor-aralia individual is capable of seed production, even small populations and isolated individuals should be eradicated. The large potential dispersal distance and success of seedlings in human-influenced habitats means that even isolated plants in urban environments, including those at urban arboretums, should be removed. Data on age and size of first reproduction are limited, but generally trees must be > 20 cm in diameter at breast height or in their second decade of life before they produce seed. Consequently, in populations of diverse age and size, trees exceeding these values should have first priority for removal. The long juvenile period also means that once all trees have been removed from a site, a second round of control need not occur for ca. 10 yr. No data on herbicide effects are available for castor-aralia, but plants can be killed by uprooting. The ability of cut plants to produce root and butt sprouts, however, means that removal of entire plants including major roots is essential.