Halifax Tree Project

The GREEN ASH

Jordan Haughn, Halifax Tree Project

2020-07-06

As we reach mid-summer with high temperatures and plenty of sun, the broadleaf trees of the urban forest have reached their peak time to shine. Included in these tree species is the green ash (Fraxinus pennsylvanica Marsh), which is the feature for this week’s article.

Previously, green ash was distinguished in at least three varieties (red ash, green ash, and northern red ash). The common name green ash is now applied to the entire species, as these variants are no longer considered botanically valid. One feature of the green ash is that it is dioecious (translated “double house”), meaning some trees have all male flowers, while others have all female flowers. These flowers normally develop during mid-spring and are cross-pollinated largely by wind. The alternative strategy to this is monoecious (translated “single house”) which includes trees containing both male and female flowers on the same individual.

Green ash is a broadleaf tree that can be identified first by its compound leaves. As James mentioned in a previous post, compound leaves consist of distinct divisions forming blades known as leaflets. On the other hand, simple leaves are not divided into smaller units, such as red oak or red maple leaves. Green ash leaves usually consist of seven leaflets but can range from five to nine. These leaflets are ovular in shape and come to a tapered point. The edges are serrated above the middle. Mature green ash bark can be easily identified by its irregular, raised ridges that form a diamond pattern which is my personal favourite quality of the tree.

Although green ash is smaller than some of the magnificent and formidable trees previously featured such as the American elm, what the green ash lacks in size it makes up for in hardiness and adaptability. Green ash is the most widely distributed and one of the most common ash species, ranging naturally from central to eastern Canada to the northeastern United States. These trees are frequently planted in urban areas for landscape purposes and can reach up to 25 m high, 60 cm in diameter, and 100 years in age. The green ash is a fast-growing species that can tolerate a variety of soil types, compaction, temperatures, and moisture conditions. They can withstand both flood and mild drought conditions and are moderately shade-tolerant. The culmination of these characteristics allows this species to become an exceptional competitor and a tough tree! Green ash is considered by some to be invasive, as it can aggressively invade urban spaces.

As of 2018, according to Heba Jarrar’s undergraduate research, ash trees account for no more than 5% of the street tree inventory in Halifax, the majority of this being green ash, and an unknown population in parks. During my undergraduate degree, I conducted research surrounding the distribution of ash in the riparian zones (terrestrial land adjacent to a river or stream) of three parks in the Halifax Regional Municipality (HRM). Significant ash populations were found in two of the three parks. However, several parks I explored contain none or very small concentrations of ash. Although there is a definite presence of ash in the HRM, they are not hyper-abundant.

Apart from the general ecosystem services provided by trees such as shade and aesthetic beauty, green ash also has several other uses and commendable qualities. Primarily, the green ash has immense wildlife value. It boasts abundant seed crops which provide a source of food for various bird and mammal species. Furthermore, the twigs are a favourite food source for wildlife. The biodiversity value of green ash is emphasized by its presence in ecologically sensitive areas like riparian zones. Ash leaves were found to be either the first or second most preferred source for riparian organic matter consumers. Ash trees also support a community of almost three hundred arthropod species. These qualities make green ash a great tree to support biodiversity! Aside from the biodiversity benefits, green ash wood is commonly used in commercial forestry for tool handles, garden furniture, sports equipment, and oars. In fact, the green ash is the wood that most oars and canoe paddles are made of!

With all that being said, I would be remiss to exclude the devastating threat looming over the North American ash population, including green ash. This threat comes in the form of the emerald ash borer (EAB), an invasive insect native to east Asia. The EAB was first identified in Canada in 2002 and was first identified in Nova Scotia in 2018 at DeWolf Park. Sadly, ten trees along Waterfront Drive in the park were removed, and research traps were set up in various parks in the HRM. Unfortunately, the proximity of EAB ground zero to the urban forest does not bode well for the future of the green ash population in the HRM. The EAB feeds on all trees in the ash genus, boring through the bark and feeding on the tissue beneath the bark, impeding the tree’s ability to transport nutrients. As much as 99% of infected trees are dead after 8-10 years of EAB establishment. The EAB feeds preferentially on stressed trees, making individuals already stressed from the urban environment particularly vulnerable. My undergraduate study was driven by the need for further information on the ash distribution in HRM parks and ecologically sensitive areas. The EAB has already killed millions of ash trees in the USA and Canada and threatens millions more. You may have already heard of the damage that has been and will be caused by the EAB, but luckily the ash populations in Nova Scotia are much less significant than in other regions in Canada. Halifax Tree Project team members from Ontario have already observed the absence of ash trees in the Greater Toronto Area in the wake of EAB establishment.

Around the city you can find the odd large specimen of any ash species. There is a group of three large European ash trees on the east side of Preston St. south of Jubilee. A most fascinating green ash specimen is the tree growing in the stone fence in front of Howe Hall on Coburg Rd, just west of the intersection with Le Marchant St. If you examine the structure of the crown, you will see that the tree was severely pruned (one might say “topped”) some years ago, but the tree rebounded strongly and stands testament to the power some trees have to remain alive despite abundant adversities. By the way, the tree has a diameter at breast height of 135 cm, a diameter that is hard to beat anywhere in the city!

Although the green ash possesses some negative aspects in the urban forest context like less shade provided and aggressive growth, there is much to be admired in its biodiversity benefits and aesthetic beauty. The green ash serves as an inspiration and reminder to us humans to keep growing through whatever tough circumstances or environments we find ourselves in!