Trees are capable of rapid evolution in response to pollution. For example, white pine, a species native to North America is highly sensitive to sulphur dioxide and ozone. A field test established using seed collected before and after heavy pollution became widespread in the central US demonstrated both the sensitivity of many trees and the rapid response of the population. More than 40% of the trees had died within just 10 years of air contamination, but the offspring of the surviving trees were genetically resistant to the contaminant. Seedlings grown from seed collected from white pines before pollution-intensive factories were built in the area had high mortality while seedlings grown from seed collected from white pines that had survived had very low mortality. Similar before and after effects of air pollution have been observed in poplar species, in response to high levels of ozone. Without genetic diversity, tree species would not be able to adapt to such assaults.
Genetic diversity of trees is also necessary for their adaptation to climate change. Although trees have long generation times and each individual takes up a lot of space, under the right conditions they can adapt surprisingly quickly to changes in environmental conditions. This is because of the enormous quantity of diverse seed produced by each individual tree and the high level of genetic diversity among trees of the same species (Petit and Hampe 2004). In fact, most tree species have a lot in common with humans, which also have high genetic diversity within and among populations and experience negative effects of inbreeding when population size is too small.
In a resilient population of trees of a given species, a severe drought may kill 90% of the trees; but the 10% with the right genes to survive will produce offspring that are better adapted to drought, so the resilient population bounces back after a severe stress. A population of trees that lacks diversity in genes important for surviving climatic extremes lacks resiliency and is much more likely to die out.