Protecting our native forests through urban policies

by

Hunter Gosda

Have you ever seen the supposed iconic American chestnut tree?  Probably not, because the species was nearly wiped out by an introduced pathogen.  Before the introduction of the pathogenic fungus, American chestnut made up to 50% of eastern hardwood forests, it had a population of about 4 billion and was the largest growing tree as well as one of the most beloved and utilized trees among people (TACF). We have seen this happen to a number of trees since then including but not limited to American elm, hemlock and ash species. Currently, ash species are threatened by an Asian beetle called the emerald ash borer, elms are still dying from Dutch elm disease (originated from Asia) and oaks on the west coast are beginning to be threatened by sudden oak death which is caused by a fungus also believed to have originated from Asia (Martin and Tooley).  Many tree species, both widely distributed and not, have or are being affected by an exotic pathogen.  Throughout the time that people have lived in North America, we have seen many tree species come and go due to the introduction of exotic pests and mismanagement of tree population distributions. 

Although American chestnut was widely distributed naturally, its unusually large population distribution and relatively high density levels made it quite prone to a disease outbreak.  It was not until around 1900 that the disease was introduced but it only took 40 years for the disease to spread throughout the range and kill approximately 3.5 billion trees.  Dutch Elm disease was introduced in 1928 most likely from a Dutch source (Spooner and Roberts).  American elm was an absolute favorite tree to be planted in cities and along roadsides.  Its vase shaped crown and attractive shape made it an ideal street tree candidate.  Unfortunately it was planted at such high levels that it provided a hotbed for an infestation.  Just as with chestnut blight, it only took about 40 years for Dutch elm disease to spread throughout the range and kill millions of trees both in our forests and in our cities.  More recently in the 1990’s, emerald ash borer, a metallic wood boring beetle native to Asia was introduced and is now infesting ash trees at a high rate.  Ash trees have been a popular choice for urban forest managers because of its tolerance to compacted soils and air pollutants while being an attractive, northern hardy, large stature tree.  Because of these reasons, it was planted at high levels in cities throughout the eastern United States.  The unusually high population level once again provided an excellent habitat and food source for the emerald ash borer which it is not accustomed to in its native range.  As managers have stopped planting ash and elm trees, they are looking to other non-native alternatives; surely this cannot be our best option.  Maples have been planted in high volumes and that has continued especially with the absence of several very popular species.

As we look into the future and try to make predictions of possible changes that may occur, we must acknowledge the threat of global climate change and how that may affect pathogen population dynamics.  The effect of global climate change surely would not make our native forests any less susceptible to exotic pest invasions and we must make management decisions accordingly.  Since most exotic pests were likely transported by humans engaged in international trade, the pests were surely introduced into an urban environment before they reached the forest.  If urban foresters can better manage trees in cities, the better chance we have of stopping or slowing the next devastating introduced tree pathogen and its associated damage.  No longer should we allow ourselves to overplant a single species of tree simply because it lacks a pathogen or is a cheaper alternative to the safer choice of planting a diverse mixed species urban forest.