In preparation for a large-scale tree-planting project, Thomasville has recorded the location, species, and condition of every tree in its downtown area. The results will be compared with results of the last such tree census, taken 20 years ago. Planting trees primarily from the species that turns out to have the best record of survival will assure downtown Thomasville of an abundant tree population 20 years from now. Which of the following, if true, most seriously undermines the claim that the tree-planting proposal will assure an abundant tree population? ['Because of strict laws governing industrial development, environmental conditions in Thomasville are unlikely to become harsher for trees than they have been during the last 20 years.', 'A number of trees from one of the species widespread in Thomasville 20 years ago reached the end of their natural life span in the intervening period.', 'Thomasville has reliable records to show which trees in existence 20 years ago were cut down to permit new construction downtown.', 'A city with a high proportion of any one species of trees can have its tree population devastated by one outbreak of a disease.']