The effect of multiple importations of pests and pathogens on a native tree

Climate changes in the past reshaped the North American native forests as some species increased and others decreased in number and distribution. Native American chestnuts were once abundant, but have been eliminated from the forest canopy by two imported pathogens, and are additionally threatened by an imported pest. The trees survive one of the pathogens, but rarely become larger than understory shrubs. These clumps of sprouts provide a reservoir of genetic diversity for use in reinstating this tree into its forest habitat. An imported parasite can be used to control one pathogen and allow the sprouts to grow large enough to flower. The trees can then be crossed with imported chestnuts of other species with good resistance to both pathogens, and resistant offspring selected. Good progress has been made in coping with these imported problems.