The fate of alien conifers in long-term plantings in the USA

For more than 100 years, non-native conifers have been introduced into habitats in the USA that already support native conifers. These introductions have yielded few naturalizations and even less evidence of invasions. We investigated the specific fates of nine non-native conifers in an array of introduction sites across the USA (Priest River, Idaho, Wind River, Washington, Cedar Creek, Minnesota, and Nantucket Is. and Martha's Vineyard, Massachusetts) through tree-ring analyses, comparisons of growth with adjacent native conifer populations, and surveys for regeneration and spread. Most of the original non-native tree plantings have died (e.g. Abies veitchii, Pinus densiflora, and Pinus halepensis at Wind River, WA); a few have survived but display low vigour and are not regenerating (e.g. Larix decidua, Pinus mugo, and Picea abies stands at Priest River, ID). Pinus sylvestris recruitment is apparent at all sites examined. Pinus thunbergii appears to be invasive on Nantucket Is., although the native Bursaphelenchus xylophilus (pinewood nematode) causes high mortality in mature trees. Non-native Pinus spp. at the Eddy Arboretum, California and Pack Forest, Washington also experienced high mortality. Dendroclimatic analyses revealed no difference in the effect of climate on the annual growth of native and non-native conifers. Plantations of introduced conifers in the south-eastern USA have died en masse (e.g. Harrison Experimental Forest, Mississippi, Olustee Arboretum, Florida). Such widespread extirpations are in sharp contrast to the fate of native conifers in adjacent stands as well as the multiple cases of large-scale conifer invasions in the Southern Hemisphere. Given the diversity of alien plant species that have invaded the USA, the circumstances surrounding the lack of persistence of introduced conifers becomes an important line of inquiry for understanding the factors and circumstances that facilitate or thwart biological invasions.