A fungal infection affecting some of Halifax's trees is a sign of big changes to come in Halifax's tree canopy.

Tar spot affects the Norway maple, and while it's not harmful to trees in the short term, it's just one of the issues limiting the species' lifespan.

"At the moment we have 35 per cent of Norway maple crown cover on the [Halifax] peninsula. That number has come down in the last five to seven years probably three to five percent," said Kevin Osmond, supervisor of urban forestry with the Halifax Regional Municipality.

"We are losing Norways. They are starting to end their lifespans, but they're also starting to show these other issues that are causing decline."

'Kind of unsightly'

Tar spot starts with yellowish spots on the leaves early in the summer and results in brown or dark grey foliage by the end of the season.

"It makes the tree look kind of unsightly, and the leaves start coming off early.… It can be a completely healthy tree and look like it's declining."

Osmond says the fungus has proliferated in the municipality this year because of the wet spring. If seasonal conditions aren't similarly wet next year, tar spot won't be as widespread.

Inherent issues

But Norway maples face a host of problems apart from tar spot, and those issues could significantly change Halifax's tree cover in the next one to two decades.

"Norway maples were brought over from Europe years ago because they grow fairly fast, they provided us with crown cover, a fairly fast crown cover on the peninsula. We didn't really know what the inherent issues were with Norway maples at that time," said Osmond.

Apart from tar spot, Norway maples have issues with susceptibility to breaking apart in high winds or strong rain, and girdling root systems, which essentially choke the tree to death.

Leaves affected by tar spot in Halifax. (Moira Donovan/CBC)

"[The root system] will throw a root around the base of the tree that causes the tree to get strangled off on one side or choke off and slowly die back that way," Osmond said.

Norway maples are also among the trees in the municipality that continue to show damage from Hurricane Juan, which hit the area in September 2003.

"In the next 15 to 25 we're going to see quite a bit of decline, quite a bit of dieback and we are prepared for that."

Adapting for the future

Osmond said the municipality is preparing by replacing and planning to replace Norway maples with species of trees that have fewer inherent issues, are well-suited to the environment and are resistant to insects.

In the meantime, he said residents noticing tar spot on the trees in their area can combat it by raking up and burying, burning or composting the leaves — and by informing the city when they notice other issues.

"I think it's important, if anybody notices something that's going on differently with the tree in front of their house or the tree in their yard, you can at any time give HRM a shout. It could impact the whole community of HRM if we don't keep each other informed."