In its native environment and native climate, an ash tree is a species in perfect balance within its ecosystem. As one of the first trees to spring back after a forest fire, ash trees are very tolerant of the high alkali content of post-fire ashes, and in turn they help to moderate the post-fire pH for other tree species. They prefer deep, moist but well-drained soils, but can manage so long as the soil does not dry out entirely. Their leaves are food for several species of caterpillar, which will later fill the forest with butterflies and moths. Eastern North America is home to seven species of ash tree.
It takes generations for a species to adapt so perfectly to their environment. When rain patterns, temperature extremes, and soils change, that change is usually so gradual that the local species are able to adapt, or to spread their seeds into areas more suited to their original conditions. However, if climate change happens too rapidly, it can stress local species beyond their ability to adapt, increasing their susceptibility to predatory insects and disease. It can even bring about the extinction of the species.
The western parts of the Great Lakes region are gradually becoming drier. Lake Superior is the engine which drives the Great Lakes ecosystem. Since 1979, the surface temperature of Lake Superior has risen by 4.5 degrees Fahrenheit. As ice cover is reduced, the lake becomes even warmer. This in turn increases lake evaporation, resulting in a gradual lowering of lake level. A record low water level was recorded in September 2007. If current trends continue, Lake Superior could become completely ice-free by 2040. Although this would make winters in the region generally milder, it would also increase the amount of snow belt precipitation while shifting other forms of precipitation further east. As the air becomes drier, rainfall outside the belt regions would decrease and the region would become subject to greater extremes of temperature, which are not well tolerated by ash trees.
Ash trees which are stressed due to temperature extremes and drought become vulnerable to opportunistic insects such as the cottony psyllid, a sap-sucking insect originally native to Europe but which has now become well established across the Plains. The cottony psyllid thrives on warmer temperatures. Since they have no local predator capable of keeping them in check, the only thing that can slow their spread is a bitterly cold winter, with at least a week of temperatures lower than -30 degrees Fahrenheit.
The presence of the emerald ash borer in North America is not linked to climate change. Like the cottony psyllid, the emerald ash borer came as a hitchhiker on a boat from native climates similar to those of the Great Lakes region, where it quickly gained a foothold. However, the spread of the emerald ash borer is governed by climate tolerance, both for itself and for the ash trees on which it feeds. In traditionally cold winters, the emerald ash borer's life cycle lasts for two years. When the winter is warmer, the insect matures and is ready to fly on to new trees in only one year.
To find conditions similar to its previous environment, the ash tree would have to migrate further north and east, into the boreal forests and in places which, until recently, held permafrost. However, the soils in that part of the continent are much more acidic than what the ash tree comfortably tolerates, and cannot yet hold and drain moisture in the same way as the soils of the Midwest. For now, the future of the ash tree in eastern North America remains in doubt.